LECTURES ON HUMAN PHYSIOLOGY THE ABERDEEN UNIVERSITY PRESS. OUTLINE COURSE OF LECTURES HUMAN PHYSIOLOGY BY ERNEST ALBERT PARKYN,; M.A. LATE SCHOLAR OF CHRIST'S COLLEGE, CAMBRIDGE feon^on ALLMAN & SON, LIMITED 67 NEW OXFORD STREET, W.C. 1889 " Man know thyself." " The proper study of mankind is man." ^f I ' / PS PREFACE. THIS little book contains an outline of two short Courses of Lectures on Human Physiology delivered to many thousands of persons at large towns in the North of England and the Midlands, and also in the Metropolis. It embodies an attempt I have made to excite an interest in, and give some sound elementary knowledge of, a branch of science which surpasses all others in its practical bearing upon the maintenance and improvement of the health of the community. It is not intended as a text-book, but as an adjunct to the Lecture-room. Although written chiefly to help the numerous students who attend my own Lectures, it may also possibly prove of service to medical and other students who are engaged in attending Lectures on the same subject. E. A. P. CONTENTS. LECTURE I. The Human Body Its Build and General Structure ... LECTURE II. Lowest Forms of Life Cells LECTURE III. Tissues The Skin J 3 LECTURE IV. The Connective Tissues Cartilage and Bone 20 LECTURE V. The Skeleton 27 LECTURE VI. The Muscles 34 LECTURE VII. Movement Nervous Tissue ... ... ... ... 39 LECTURE VIII. The Blood and Lymph 44 LECTURE IX. The Circulation of the Blood 5 2 LECTURE X. The Circulation of the Blood (Continued) 60 LECTURE XL Respiration The Lungs 68 Vlll 1'AGE LECTURE XII. Respiration 73 LECTURE XIII. The Organ of Digestion 78 LECTURE XIV. Foods 85 LECTURE XV. Digestion ... ... ... ... ... ... ... ... ... 92 LECTURE XVI. Nutrition 99 LECTURE XVII. The Nervous System Nerves ... ... ... ... ... ... 106 LECTURE XVIII. Nerve-Centres The Spinal Cord 112 LECTURE XIX. Nerve-Centres The Brain .. 119 LECTURE XX. Nerve-Centres Functions of the Brain 127 LECTURE XXI. Sensation Touch ... ... ... ... ... ... ... ... 134 LECTURE XXII. Smell Taste Voice ... ... ... ... ... ... ... 139 LECTURE XXIII. Hearing 147 LECTURE XXIV. The Eye Light 154 LECTURE XXV. Vision . 161 LECTURE I. THE HUMAN BODY ITS BUILD AND GENERAL STRUCTURE. Physiology defined. The branch of science which treats of the functions of living things is termed Physiology. It is divisible into Animal and Vegetal Physiology, according as animals or plants are dealt with. That portion of Animal Physiology dealing with the body of man is further distinguished as Human Physiology. By the word functions is simply meant the changes and pro- cesses going on within the body making up its life. Physiology, in a phrase, is the science of life. Physiology, therefore, treats of the human body as a living organism. It thus differs from Anatomy, or the study of the form and construction of the body independently of its life. Microscopic Anatomy is often called Histology. Importance of Physiology. The study of the human body as a living organism, or Physiology, is of great importance, for it is only by the aid of such knowledge as an acquaintance with it conveys that we can understand the proper relations of our bodies to the world in which we live. It teaches the true relation of our bodies to the air we breathe, to the water we drink, to the food we eat, to the house we live in, and to the work of our daily life. It thus has a most practical bearing upon the health of the individual, and upon the preservation and improvement of the health of the community of which he is a member. The Problems of Physiology. Life is the result of, or at least invariably accompanied by, perpetual change. The living human body is the seat of a series of perpetual changes. These changes involve a loss of substance and an expenditure of energy. Matter and force must, accordingly, be supplied to the body, to make good this loss. Matter is supplied in the form of food and oxygen ; force as potential energy in the food, and as heat resulting from oxidation. Matter in chemical combination much simpler than that taken in is continually being passed away from 2 LECTURE I. the body by the excretory organs. The chain of connection between this gain of matter in the form of complex food and loss of matter in the form of the much simpler compounds excreted, and between the gain and loss of energy, and the rela- tions of this gain and loss to the activity and growth of the various constituent parts of the body, and of the body as a whole, constitute the fundamental problems of Physiology. Man a Yertebrate Animal. Man is a member of the great family of backboned animals, or sub-kingdom Vertebrata. By this is meant that his body, in common with all members of the five great classes of animals Mammals, Birds, Reptiles, Amphibia, and Fishes has an internal bony framework or skeleton. This skeleton, in the main, determines the shape of the body, and is characterised by the presence of a central axis or backbone. The word Vertebrata is derived from the fact that the so-called backbone is not one bone, but a series of similar small bones, piled up one above the other in the form of a column. Each of these bones is a Vertebra, and the column formed by them is called the vertebral column. Vertebral Column is the anatomical expression for what in ordinary language is called the backbone. The vertebral column with the bones directly attached to it determine the shape of the trunk of the body ; the bones in- directly attached to it (excepting those of the skull) determine the shape of the limbs. Head and Trunk considered as Two Tubes. The head and trunk may be regarded as consisting of two tubes, placed one behind the other. There is a large anterior tube, made up of the cavity of the mouth, of the gullet in the neck, the cavities of the chest, abdomen, and pelvis. A smaller posterior tube is enclosed within the vertebral column and skull. The Posterior Tube How formed. The posterior tube in the Trunk is formed by the peculiar structure of the vertebral column. A vertebra is an irregularly shaped bone, consisting of a solid mass of bone in front the body and of an arch behind, from which project processes. The arches of contiguous vertebrae correspond both above and below. In this way the arches form a tube the Vertebral Canal. The posterior tube in the Skull is formed by the bones of the skull, and is here much larger than in the trunk. Its Contents. The posterior contains the principal parts of the Central Nervous System, viz., the Brain in the skull, and the Spinal Cord in the vertebral canal. THE HUMAN BODY. 3 The Anterior Tube How formed. The anterior tube is formed above by the bones of the skull surrounding the cavity of the Mouth. In the neck it is represented by the Gullet, lying immediately in front of the vertebral column. In the trunk it is composed above of the Chest or Thorax a bony box, bounded behind by twelve verte- brae in the region of the back, on either side by twelve ribs, and in front by the Breastbone or Sternum and pieces of gristle joining it to the ribs. The chest is separated from the anterior tube below by a great fleshy muscle the Midriff or Diaphragm its upper surface being convex. Below the diaphragm are the Abdomen and Pelvis. The shape of this part is determined by two large bones the Hipbones joined posteriorly to five vertebrae near the bottom end of the vertebral column, and connected to one another in front. The five vertebras thus forming a support for the hipbones behind are joined together in adult life to form one bone called the Sacrum. The basin-like cavity thus formed by the sacrum behind and the hipbones at the sides and in front is the Pelvis. The upper edges of the hipbones are joined to the ribs by strong fleshy muscles. The portion of the anterior tube thus enclosed between the pelvis below and the chest above is the Abdomen. Its Contents In the Chest. The lungs, heart, large bloodvessels (aorta, pulmonary artery, venae cavae, and pulmonary veins), gullet. In the Abdomen. Stomach, small and large intestine, liver pancreas, spleen, kidneys, bloodvessels, and nerves sup- plying these organs. In the Pelvis. The urinary and generative organs, termina- tion of alimentary canal. Extending from the neck to the pelvis in the anterior tube is also the Sympathetic Nervous System. Contents of Two Tubes connected. The contents of the anterior tube are connected with those of the posterior, i.e., with the brain and spinal cord, by fine, smooth, whitish cords of varied thickness. These are Nerves. The Limbs. There are no tubes in the limbs comparable to those in the trunk just considered. The shape and length of the limbs are determined by the bones forming their innermost part. These bones are indirectly connected to the vertebral column. Those of the lower limbs through the hipbones ; those of the upper through the shoulder-blade, collar-bone, breastbone, and 4 LECTURE I. ribs. Attached to and surrounding the bones of the limbs are muscles. The muscles are more or less enveloped in fat, and the whole is enclosed by the skin. Amongst and supplying the different parts are nerves and bloodvessels (arteries and veins). Analysis of the Human Body a. Chemical. About 70 per cent, of the body consists of Water. Of the 30 per cent, solid matter the most important and characteristic constituents are highly complex, but imperfectly understood, chemical compounds containing nitrogen, known by the generic term of Proteids. There are then non-nitrogenous organic bodies, of which Fat is the most widely distributed. In much smaller but most constant quantities there are Salts of such metals as potassium, sodium, calcium, and magnesium, some of which contain Phosphorus. b. Anatomical i. Head, trunk, and limbs. Regarded from the exterior, the human body exhibits a striking Bilateral Symmetry ; that is to say, the parts of the body on either side of a vertical median line are similar. Indications of a Serial Symmetry are visible in the two pairs of limbs and the twelve pairs of ribs. 2. Organs (as heart, liver, lungs, brain, &c.) and tissues (as muscle, bone, nerve, blood, &c.). 3. Microscopic anatomical elements, or Cells. The applica- tion of the microscope to the constituent tissues and organs of the body shows that, however different in their appearance they may be, they all agree in being composed of minute anatomical elements called cells. So universal is this that the generalisation has been made that the whole body is composed, or is an aggrega- tion, of cells (Cell Theory). Certain exceptions are observable in the form of fibres, but the study of development points to these fibres having arisen from cells, or being modified cells. There may thus be said to be an underlying Unity of Structure throughout all parts of the body. This unity of structure applies only to the cellular nature of the constituent elements. The cells differ widely from one another in their shape, appearance, structure, and constitution in the different tissues and organs, e.g., the cells of muscle, nerve, and blood ; or the liver-cells and those of the lung and kidney. c. Physiological, Correlated with these differences in the structure of the component cells of the tissues and organs are differences of function or use. The muscle-cells, especially THE HUMAN BODY. 5 endowed with the power of contraction, have a very different use from the nerve-cells, the special function of which is to evolve, receive, or transmit nervous energy. Whilst the bone-cells are embedded in a hard passive substance, especially adapted to protect and support, the blood-cells are in a state of perpetual motion, enabling them to play an active part in the dynamical changes of the body. Whilst the cells of the salivary glands have the power of secreting saliva, the liver-cells are endowed with the function of producing bile, and so on through the whole body. In fact, when we make a physiological analysis we see that there is a most evident division of labour in the human body. Each and every organ and tissue is endowed with one particular function or use more than any other. It performs a definite, more or less restricted, duty. In so doing it benefits itself, and conduces to the welfare of the whole organism. There is, indeed, a physio- logical division of labour in the human body comparable to that which obtains in a social community. The organs and tissues may be compared to the individuals, or groups of individuals, who pursue some special calling, which by their education and situa- tion they can perform better than any other. Any particular work is thus done better and more thoroughly, to the advantage both of the individual who accomplishes it and of the whole community to which he belongs. This physiological division of labour is possible, and is carried on in a harmonious manner, because the organs and tissues are brought into relation with one another, and their activity regulated, modified, and harmonised by a complex mechanism called the Nervous System. In the cellular nature of its constituent parts the human body resembles the bodies of all other animals, from the highest to the lowest. This remarkable similarity in fundamental microscopic structure throughout the animal world is not the least striking of the many facts pointing to a common law of development. LECTURE II. LOWEST FORMS OF LIFE CELLS. In the last lecture the cellular nature of the body was illustrated, and the similarity in this respect of the human body to all other animals referred to. As we descend the scale of life we find that the variety in the nature and structure of the constituent cells becomes less and less as the organism becomes less complex. The division of labour correspondingly diminishes. At last, when the lowest forms are reached, they are found to consist of only one cell, which, living an independent existence, carries on all the processes necessary to its life, and, in fact, all division of labour has disappeared. Such cells are not only anatomical, but also physiological, units. In their cellular structure there is thus a connection between the highest and the lowest, the most complex and the simplest, forms of animal life. These remarks apply equally to plants. In the lowest* forms of animal and plant life we therefore have organisms consisting of a single cell, of a very minute mass of living matter or protoplasm, the life-changes and life-history of which can be brought under observation by the microscope. Any knowledge such observation and experiment can reveal to us concerning the functions or vital actions of these simple organisms is so much information regarding the nature of living matter in its simplest form. From the study of these simple organisms something may, therefore, be learnt regarding the fundamental properties of living matter which can- not fail to throw light upon the great problem of life, even of life as manifested in man himself. Two unicellular organisms will be considered : one a plant (Yeast), the other an animal (Amoeba). YEAST (Saccharomyces Cerevism). A drop of the brown-coloured liquid called yeast, when examined under the microscope, is found to consist of two distinct parts, viz., a colourless liquid in which float enormous numbers of minute globular bodies termed yeast-cells. The cells CELLS. 7 are rounded or oval in shape, -0^17 to y^^th inch in diameter. They exist singly or joined together as strings or masses. Structure of the Cell. Each cell is an independent organism, a minute plant, sometimes called Torula. The cell is comparable to a sac, and consists of two parts : (i.) The Cell-wall^ an external, enclosing, colourless, trans- parent membrane, consisting chiefly of Cellulose, a compound of carbon, hydrogen, and oxygen. (ii.) The Cell-contents, or semi-fluid substance within, called Protoplasm. The protoplasm encloses one or more spaces, or Vacuoles, filled with liquid. Usually plain and clear but sometimes granular in appearance, the protoplasm is of the utmost importance, for it is the essential part of the cell. It is the part alone endowed with life. On it depend all the vital functions exhibited by the little yeast plant. If then, by obser- vation or by experiment, any insight into the life of the plant can be gained, it will be perfectly justifiable to attribute the functions and vital activities observed to the simple living matter, or protoplasm. Nature of the Protoplasmic Contents. The protoplasm is not a definite chemical compound such as cellulose, but has an exceedingly complex constitution. It appears to be rather a mixture of substances (or in only a very loose chemical combina- tion), some of which are of a very complicated nature. It is largely composed of the three elements Carbon, Hydrogen, and Oxygen. In addition there is Nitrogen, and probably always Sulphur. These five elements are united to form a highly complex chemical body which goes by the name of Proteid a generic term for a number of substances all containing the fore- going elements in the same percentage composition (within narrow limits), one or more of which are invariably present in all forms of living matter. Besides proteid the protoplasm contains Water, and Salts of Calcium, Potassium, and Magnesium. In addition, in some cases e.g., in yeast Fat may be present, composed of Carbon, Hydrogen, and Oxygen. Reproduction. Under favourable conditions of temperature and medium, the yeast-cell not merely lives an independent life, but lives an extremely active one, for it reproduces itself at a rapid rate. This multiplication or Reproduction is effected by a process of Gemmation or budding, thus producing the strings of cells previously alluded to. In such circumstances Torula must manufacture a considerable amount of protoplasm, and it has the 8 LECTURE II. remarkable power of manufacturing this exceedingly complex substance out of comparatively simple bodies. This is proved by growing yeast in a liquid (Pasteur's fluid) in which are dissolved substances the chemical composition of which is exactly known. All these dissolved substances are of simpler composition than proteid, the essential constituent of the protaplasm. The yeast- cell can obtain the nitrogen necessary to form the proteid of the protoplasm from chemical compounds much simpler than the proteid itself. In this respect it profoundly differs from all animals, for they are able to obtain the necessary nitrogen only from some form of proteid. Assimilation. The process of the manufacture of protoplasm out of simpler substances forming the food may take place quite independently of reproduction. It obviously must occur when- ever growth of a yeast-cell takes place. To this important and fundamental process, viz., the conversion of the dead matter of the food into the living matter of the organism, the term Assimilation is applied. Respiration. The yeast-cell absorbs oxygen, and gives out carbonic acid in about equal volumes. This exchange of gases is called Breathing, or Respiration. Under favourable circumstances this function is performed with considerable activity. Neverthe- less the yeast-cell can live and grow without a supply of free oxygen, in which case it is assumed that it obtains oxygen from some compound containing it (e.g., sugar). Inasmuch as the carbonic acid given out contains as much oxygen as has been taken in, it is obvious that respiration must entail a waste of the organism. If yeast be placed in distilled water instead of in Pasteur's fluid, the result of this becomes obvious in the gradual diminution in substance of the cells. . We may infer, then, that under ordinary conditions there are two opposite kinds of processes going on in a yeast-cell one tending to its growth and reproduction (Assimilation), and, since it involves the building up of the materials of the cell, may be described as of a constructive character the other tending to its diminution or decrease (Respiration), and may, conversely, be said to be of a destructive nature. The cell is, therefore, the seat of a perpetual conflict between these two opposing influences. If the former is in the ascendency, the cell grows and ultimately reproduces ; if the latter gets the upper hand, the cell decreases, and, if continued long enough, decays, and ultimately dies. If the two forces are equal, there is neither increase nor decrease : there CELLS. 9 is a condition of physiological equilibrium. Every living organism is the seat of two great opposing forces constructive on the one hand, and destructive on the other of this kind. The Yeast-cell is a Plant for two reasons 1. Its protoplasm is enclosed within a cell-wall of cellulose. 2. It can manufacture living matter out of chemical sub- stances, all of which are of simpler composition than proteid. Fermentation. When yeast is put into a saccharine solu- tion it produces certain changes, viz., an evolution of carbonic acid gas, the formation of alcohol and small quantities of glycerin and succinic acid. These changes constitute the phenomenon of fermentation. The above-mentioned sub- stances apparently result from the decomposition of the sugar, for 99 per cent, of the sugar which disappears is thus accounted for. How the yeast-cell causes this decomposition of the sugar has been a question of much dispute. According to Pasteur, the yeast-cell, by its vital activity, takes oxygen from the sugar, this being the i per cent, unaccounted for. In fact the yeast is able to obtain its supply of oxygen from this chemical compound in place of the gas in the free condition. It is thus able to live independently of a supply of air, and has in consequence been termed by Pasteur anaerobic. According to this view, fermenta- tion is essentially a vital phenomenon. AMCEBA. The Amoeba is an irregularly-shaped mass of protoplasm found in liquids containing decomposing organic matter. Differences between the Yeast-cell and Amoeba 1. It is usually irregular in shape and has no cell-wall. Al- though there is no cell-wall, yet a slight difference (differentiation} is to be observed between the outer and inner parts of the cell. The outer portion, or Ectosarc, is clearer and denser than the granular, more fluid central part, or Endosarc. 2. It exhibits movements of a curious and distinctive character, consisting in the protrusion of a process, or pseudopodium^ which may be soon retracted, or draw the rest of the cell after it, thus causing a distinct locomotion of the amoeba. Hence its old name of Proteus animalcule. 3. Within its protoplasm is a round body, the nucleus, in the centre of which is a round granule, the nucleolus. The absence of a nucleus from yeast may be apparent rather than real, due to our imperfect means of observation. 2 10 LECTURE II. 4. In the Ectosarc is sometimes seen a clear space which rhythmically appears and disappears. This structure, known as the Contractile Vesicle, has been regarded by some as a primitive heart. 5. It is Irritable and Contractile. The properties of irritability and contractility are intimately connected with one another, the presence of the former becoming known through an exhibition of the latter. Contractility probably causes movement solely by a rearrangement of the ultimate particles of the protoplasm, and not by their approach ment nearer to each other. It is there- fore not synonymous with contraction as applied to inorganic bodies under the influence of variation of temperature. The contractility of the protoplasm occurs in consequence of its response to external stimuli, or of its irritability. Such stimuli are : (a) Heat ; (b] Oxygen ; (c) Electricity. Of all such stimuli it may be said that they are quite inadequate to produce by direct application that change of form of the protoplasm which consti- tutes its movement. And it may be said generally that living matter which exhibits movement, when acted upon by some stimulus which is of itself quite incommensurate with that move- ment, is irritable. Here again it must be noted that this difference between the yeast-cell and amoeba is probably apparent rather than real. The protoplasm of the yeast-cell being completely surrounded and bounded by the non-living cell-wall has no opportunity of exhibiting these properties, should it possess them. If,, as from analogy, we may perhaps assume contractility and irritability are fundamental properties of all protoplasm, that of yeast will be endowed with them. Resemblances between the Yeast-cell and Amoeba. They agree in exhibiting the three fundamental functions of 1. Respiration. Like the yeast-cell, the amoeba absorbs oxygen and evolves carbonic acid in about equal volumes, or respires. 2. Reproduction. The amoeba reproduces by division, or fission. The protoplasm becomes cleft into two portions, its division being preceded by a division of the nucleus. 3. Assimilation. Whilst this is fundamentally the same in both cases, viz., the conversion of the dead matter of the food into the living protoplasm, yet there is a striking difference in the conditions necessary for the transformation. The amoeba requires in its food the complex proteid. It can obtain nitrogen only from CELLS. II this. In this respect all animals resemble it. The yeast-cell can obtain nitrogen from ammonia or some salt of ammonia or from nitrates, and in thus being able to obtain this essential element from such simple combinations resembles all other plants . Another difference is that the amoeba can make use of solid food, whereas yeast, like all other plants, requires its food in a fluid condition. Automatism. Movements of the amoeba appear some- times to take place independently of any external stimulus. Its movements are of that kind called Spontaneous. The amoeba on this account is said to be endowed with automatism, or to exhibit automatic movement. This is a somewhat unfortunate term, for it is often used in a sense the opposite to spontaneous. The doctrine that animal bodies are automata asserts that all actions (i.e., all movements) are the result of external influences direct or indirect. In the case of amoeba it is impossible to be sure that no external stimulus is acting upon it, or that some change due to a previous external stimulus is not taking place internally in other words, the effect of an external stimulus having been stored internally for a time. HYDRA VIRIDIS. Organisms next above the preceding in the scale of life con- sist of aggregations of simple cells. An example of such is the freshwater polype Hydra Viridis. Structure of Hydra. Tubular in shape, Hydra is usually attached at one end. At the free extremity is a circular row of tentacles surrounding an orifice, or mouth, opening into a central space, the body cavity. The body-wall is composed of cells arranged in two layers 1. An outer layer, or Ectoderm. 2. An inner layer, or Endoderm. The cells of this layer con- tain chlorophyll. Reproduction of Hydra. This occurs either (i) Asexually, by budding, or (2) Sexually, by the action of uniciliated motile cells, or spermatozooids (formed in projections from the body near the fixed end), on a large cell, the Ovum or Egg-cell, enclosed within a projection, the ovary, near the free end. As the result of the action of the spermatozooids on the egg-cell a series of remarkable changes take place in the latter. It divides and subdivides until a mass of similar cells is produced. This be- 12 LECTURE II. coming surrounded with a chitinous coat passes into a resting condition, and by the rearrangement and alteration of the cells a new Hydra is developed. The reproduction of this organism is interesting, for both in its origin from a single cell, and in the series of changes the egg-cell undergoes in the first stages of development, the reproduction of Hydra resembles in a striking manner the earliest stages in the reproduction of the higher animals. LECTURE III. TISSUES-THE SKIN. The Blastoderm. The higher animals originate from a single egg-cell, or ovum. As the result of fertilisation of the ovum, it divides into two cells. These again divide. Further division and subdivision produce a mass of cells from which a germ-membrane, or Blastoderm, is formed. The blastoderm, consisting at first of two layers, soon has a third. These three layers are known as the Epiblast (upper), Hypoblast (lower), and Mesoblast (middle). From these three layers the several tissues and organs of the body are developed, viz. : from the Epiblast the nervous system, includ- ing the organs of sense, the epidermis, and the epithelium of the mouth ; from the Hypoblast the epithelial lining of the air-tubes and of the alimentary canal, and also the proper substance of the glands communicating with these passages ; from the Mesoblast all the remaining parts of the body. The development of the different structures of the body con- sists essentially in the alteration, modification, and development of the comparatively simple cells of the blastoderm, so as to form the many varied forms of cells which go to build up the tissues and organs of the body, briefly referred to in the first lecture, and which will be further explained hereafter. Cells of similar struc- ture are aggregated together in masses. Such cells not only resemble each other in structure, but also in the use they subserve in the economy. Such masses of cells form what are known as the tissues of the body. Definition of Tissue. A tissue may be defined as an aggre- gation of similar cells, having a common embryonic origin, endowed with one particular function or use more than any other. Simple Tissues. That the human body is largely composed of tissues is seen from the following list : 1. Epidermal, or epithelial (epidermis, cellular lining of mucous membrane.) 2. Connective (fat, tendon). 14 LECTURE III. 3. Cartilaginous (gristle). 4. Osseous (bone). 5. Muscular (muscles). 6. Nervous. 7. Blood. Compound Tissues. Several of the above may be combined together to form what are called compound tissues ; e.g. : 1. The Bloodvessels (connective, muscular, nervous). 2. Lymphatics (connective, muscular). 3. The Skin (connective, muscular, nervous, epidermal). 4. Serous, Synovial, and Mucous Membranes (connective, epi- thelial, nervous, muscular). Organs. Several tissues may be combined together so as to form in one particular definite position in the body a special struc- ture which performs some particular and peculiar function. Such a structure is termed an organ. The Heart is a very good ex- ample of such an organ. In other cases the special function of the organ is due to its being chiefly composed of cells peculiar to it ; i.e., not found in any tissue or other organ. The Liver is a striking instance. Division of Labour. All the actions, processes, changes, and functions which in the aggregate constitute the life of a human being are therefore carried out and performed by a number of different and separate agents the afore-mentioued tissues and organs. There is thus a most elaborate and beautifully-adjusted physiological division of labour, each organ and tissue doing the work best suited to it, for which it is specially adapted, both to its own advantage and to that of the body as a whole. I. The Epidermal Tissue. This tissue comprises the outer skin or Epidermis, with the parts developed from it, and the cellular lining of the alimentary canal and air-tubes, or Epithelium. The Epidermis. This is composed of a number of layers of cells separable into two strata. i. The Horny layer, or outer stratum, consisting largely of cells modified to scales. ii. The Rete mucosum, or Malpighian layer. This inner stratum is much softer, the cells being highly vital, and capable of multiplication. The innermost part, viz., that next to the dermis, consists of a single layer of columnar-shaped cells. The rest is made up of many layers of cells having a ridged outline, hence called " ridged " or " prickle " cells. The ridges meeting, channels are left between the cells by which nutriment is probably conveyed THE SKIN. 15 by the plasma exuded from the bloodvessels of the dermis, for no bloodvessels pass into the epidermis. Fine nerve-fibrils are said to penetrate into the epidermis. The thickness of the Epidermis varies in different parts of the body. It is thinnest on the back and neck, thickest on the palms of the hands and soles of the feet. The black colour of the negro's skin is due to the deposition of pigment granules in the rete mucosum, especially in the layer of columnar cells. The Dermis, True Skin, or Corium is situated immediately beneath the epidermis. It does not belong to the epidermal tissue, but is composed of connective tissue (vide Lecture IV.). It is dealt with here for convenience' sake. It is composed of a mass of fibres in which two layers can be distinguished : i. The Outer or Papillary layer, so dense that the fibres are scarcely distinguishable in it. It is raised up into a great number of conical or finger-like projections, termed Papillae. The epi- dermis is moulded over the papillae of the dermis, from which it can be separated by maceration. The papillae are in some parts arranged in a very definite manner, in rows producing a ridged appearance of the epidermis. This is well seen in the Curvilinear markings on the palm of the hand. The papillae are sometimes cleft, forming Compound Papilla. The papillae are largest and most numerous in parts most sensitive to touch. Length yj-^th inch ; in some cases much less. Nerves pass into many of them, there terminating in end-organs. The papillae are very vascular. ii. The Inner or Eeticular layer, in which the fibrous tissue is much looser. It passes gradually into the fatty tissue beneath. Modifications of the Epidermis are Nails, Hairs, and Glands. A Nail consists of three parts : 1. The Root, or part under the skin. 2. The uncovered part, or Body, terminating in 3. The Free-edge. In structure the nail agrees essentially with that of the epi- dermis. The horny layer has become much thickened and hardened. The dermis beneath the nail is called the matrix, or bed of the nail. It is raised up into a great number of very vascular papillae, arranged in rows slightly diverging from the posterior part. The series of ridges, or lamina, thus formed fit into corresponding depressions on the under surface of the nail. The nail grows by new cells being continually formed at the root and on the bed. l6 LECTURE III. A Hair may be said to be composed of three parts : i. The Root ; 2. the Shaft or Stem ; 3. the Point. The depression of the skin containing the hair is called the Hair-follicle. It consists of an inner part continuous with the dermis, and an outer continuous with the epidermis. The follicle becomes closely connected with the root, forming the Root-sheath. The portion continuous with the dermis is called the Outer Root- sheath, that with the epidermis the Inner Root-sheath. At the bottom of the root is a conical projection of cells known as the Papilla of the Root. Growth and regeneration of the hair is effected by this papilla, which is a modified papilla of the dermis. The stem or shaft consists of three parts i. The Cuticle, composed of finely imbricating scales causing the appearance of fine waved transverse lines. It completely sur- rounds the shaft, and is found in all hairs. ii. The Fibrous substance inside the cuticle is resolvable into straight rigid longitudinal fibres. In some hairs the whole internal part consists of this. Within it are cavities containing pigment granules, giving a dark colour to the hair, or air when it is light coloured or white. iii. The Medulla, or Pith, innermost of all, does not exist in all hair; e.g., woolly hair. It appears to be composed of little clusters of cells of different shapes containing granules or air. Muscles of the Hairs (arrector pili) are small bundles of in- voluntary muscular tissue passing from the upper part of the dermis to the hair follicle near the root. In their course they, to some extent, surround the sebaceous glands. By their contraction, the upper end being fixed, they necessarily cause the hairs to become erect. They also facilitate the passage out of the secre- tion of the sebaceous glands by compressing them. Glands. These are of two kinds 1. Sebaceous Glands are small racemose glands, the ducts of which open into the hair follicles near their mouths. They pour out a thick fatty secretion which moistens the hair. There are usually two glands to each hair. 2. Sudoriparous, or Sweat Glands, are fine tubes terminating below at various depths in the subcutaneous tissue in a coiled, blindly ending knob. In its passage through the epidermis the tube is coiled like a corkscrew. The tube, except in the epidermis, is lined by layers of cubical cells resting upon a basement membrane. The coiled knob has only a single layer of cells. In the larger glands the duct may divide. They are most numerous where there are THE SKIN. 17 no hairs. As many as 3000 in a square inch have been counted on the palm of the hand, where the orifices are on the ridges formed by the lines of papillae. It is estimated that there are between two and three millions on the surface of the whole body, or a sur- face of more than 1000 square yards. Surrounding the knob is a network of capillaries, and nerves are distributed to the glands. The Epithelium. The mouth and oesophagus are lined by layers of delicate flattened cells (Stratified Epithelium) ; the rest of the alimentary canal by a single layer of columnar cells (Colum- nar Epithelium}. The air tubes, including the nostrils (except the olfactory part) and nasal part of pharynx, are also lined with a columnar epithelium, the cells of which are provided with minute vibratile filaments (^Vtf to HToo^ tn inc ^ l n g) or c i ua (Ciliated Epithelium). These cilia are, during life, in a state of incessant movement of a peculiar and definite nature. Their motion con- sists in a quick bending towards the mouth, followed by a slower return to the vertical. Although unknown to be co-ordinated by the nervous system, all the cilia act in unison, thereby gradually passing up the air tubes to the mouth any substances e.g., mucus accumulated on the surface. Ciliated epithelium is also found in the central canal of the spinal cord and the ventricles of the brain, and in various tubes and ducts. In the embryo they are found also in the oesophagus and stomach. The motion of the cilia is affected by temperature, water, alkalis, chloroform, carbonic acid. Functions of the Skin. Apart from its being the end-organ for the sense of touch (vide Lecture XX.), and Protective, the great function of the Skin is Excretory. By it the body loses i. Water. The amount of water given off from the body by the skin is very considerable (2 Ibs. in 24 hours), being nearly double that passing out from the lungs. The quantity has been determined by enclosing the body, or a limb, in an air-tight chamber. In the former case respiration changes are carried out independently of the chamber. The greater portion of the water passes off as invisible aqueous vapour. This is known as Insensible Perspiration in contradistinc- tion to that which appears as drops upon the skin when the loss of water is copious, or Sensible Perspiration. The perspiration is secreted by the sweat glands, and probably little or none passes through the epidermis directly. Perspiration contains about i per cent, of solids, amongst which are common salt, acids of the fatty series, neutral fat, and cholesterin. These latter are probably wholly, or in part, derived from the secretion of the sebaceous l8 LECTURE III. glands. To this small extent the skin is therefore an organ for the excretion of these substances. The amount of water given off is affected by the nature and quantity of food, amount of Liquid drunk, by Exercise, and by Poisons (increased by strychnin, nico- tin, calabar bean, pilocarpin, and camphor ; decreased by atropin and morphia). 2. Carbonic Acid. This gas is given off by the skin to the ex- tent of about o^yo~th that evolved by the lungs. Oxygen is absorbed in slightly less volume. There is thus, to a very slight degree, a breathing function carried on by the skin, or Cutaneous Respiration. Compared with the Pulmonary Re- spiration it is insignificant. It is more marked in some of the lower animals ; e.g., the frog. Respiration by the skin is estimated by a method similar to that used for determining the loss of water. The amount of carbonic acid evolved by the skin is increased by strong exercise, and by rise of temperature of the surrounding air. 3. Heat. Heat is lost from the surface of the body by con- duction, evaporation, and radiation. This loss necessarily follows from the physical conditions present, viz., a thin membrane (the epidermis) having on one side a hot liquid (the blood) and on the other side a cooler fluid (the air). Heat in consequence passes through the membrane from the hot blood to the cooler air. The skin in thus permitting a loss of heat from the blood, becomes a Regulator of the Temperature of the body. When, for example, in consequence of violent exercise, much extra heat is produced in the body, it enters the blood, which, on passing through the skin, parts with the surplus to the air, and thus prevents an accumulation of heat and consequent rise of temperature. This regulation is further assisted by a coincident dilation of the arteries of the skin (flushing of the skin) causing a much larger quantity of the blood than usual to be brought into close proximity to the colder atmosphere and thereby a correspondingly greater loss of heat. Clothing, by diminishing the loss of heat, warms the body. The worse conductor of heat the material composing it the warmer the clothing is. The looseness and denseness of its texture must, however, be taken into account. Washing the Skin is of importance for the removal of the outer scales of the epidermis continually being shed ; also to remove the solid constituents of the sweat and sebaceous secre- tion. These, if not removed, soon decompose, become offensive, and injurious. THE SKIN. ig Influence of the Nervous System on the Secretion of Sweat. Although usually associated with increased blood supply, yet that the secretion of sweat may be dependent on direct nervous action is seen from the fact that profuse perspiration may occur when the skin is pale through want of blood. From ex- periments on animals the nerves to the sweat glands affecting their secretion appear to arise from definite Sweat Centres, one for the lower limbs being situated in the lower part of the dorsal spinal cord, another for the upper limbs in the cervical spinal cord. A general centre for the whole body is said to be present in the medulla oblongata. LECTURE IV. THE CONNECTIVE TISSUES CARTILAGE AND BONE. II. Connective Tissue. This is the most widely diffused of all the tissues, being found in almost all parts of the body. Microscopic examination shows it to be composed largely of fibres of which there are two kinds white and elastic. In addi- tion, there are large much-branched cells, or connective tissue corpuscles. Finally, there is a homogeneous cementing material, or ground substance. According to the proportion and arrange- ment of these constituents, connective tissue has different names. The most common variety is Areolar Tissue situated beneath the skin, the mucous and serous membranes. It is also found in nearly all the tissues and organs in fact is continuous throughout all parts of the body. It has the appearance of consisting of little interlacing bundles forming a network. The meshes or spaces the areolce thus resulting communicate. The bundles are made up of numbers of excessively fine-waved White Fibrils, 0-5- J^y to -^ Joijth inch thick. The bundles join and interlace in all directions, but the individual fibrils neither branch nor join. The Elastic Fibres are much larger than the white, measuring on an average about -n^nyth inch across. They branch and join with one another, forming a network. Their elasticity is shown by their free ends curling up. They resist boiling water, and for a time even the action of potash. Connective Tissue Corpuscles are also present. They are irregular cells with branches by which neighbouring cells seem to be connected. Fat, in the form of drops of oil, may be deposited within the cells. When this occurs to any extent what is usually called fat is produced, and the tissue is then distin- guished as adipose. Adipose Tissue, or Fat. The cells measuring -^th to 7nro tri inch across are more numerous in this form of connective tissue, and are much distended with the contained oil. The nucleus is pressed up against the side of the cell, and usually has a little CARTILAGE AND BONE. 21 granular protoplasm around it. The cells are aggregated in lumps, or lobules, which are well supplied with bloodvessels. The cells are now called fat-cells. Uses of Fat. Fat is useful in several ways 1. It acts as Padding around the different parts of the body. Deposited around and between the organs, it supports them, per- mits easy movement, and protects them from pressure. Beneath the skin it forms a considerable layer. In this situation it consti- tutes the Panniculosus adiposus, and gives the exterior of the body its smooth rounded contour. 2. Being a bad conductor of heat, the layer of fat beneath the skin retains heat, and serves to maintain the temperature of the body. Accordingly, warm-blooded animals living in water have this layer of fat especially thick. 3. It is a Store of Nutriment. Consisting chiefly of carbon and hydrogen, its oxidation within the body to form carbonic acid and water contributes to the maintenance of its heat. It is supposed that excess of carbon and hydrogen, when introduced into the body, may be stored up as fat for use at some subsequent period when the expenditure is greater than the supply. From this would be expected what actually happens, viz., that muscular exertion, by increasing oxidation, prevents the accumulation of fat, and may even cause a diminution of the amount stored up. Also when the supply of oxidative material is diminished by want of food, or failure of digestion, recourse is had to the fat already accumulated, for, in wasting through starvation or disease, the fat of the body is the part first used up. Fibrous Tissue. The Ligaments, Tendons, and fibrous expan- sions found on the surfaces of flat muscles known as Aponeuroses or Fascice, are composed of this form of connective tissue. It is made up of the pure white fibrils collected in bundles which join, intersect, and decussate with one another. In tendons, in addi- tion to the fibrils, there are oblong-shaped cells arranged in rows. Elastic Tissue. This variety of connective tissue is found in certain ligaments of the vertebral column called ligamenta subflava, in the ligaments of the larynx, and composing the vocal chords. It is composed solely of elastic fibres of a yellowish tinge : hence the light yellow colour of the tissue. The largest fibres measure ^ou^h mcn across, but others are much smaller. The fibres branch and join to form a network which may be so close as to become quite a membrane ; e.g., the fenestrated mem- brane in the coats of the arteries. 22 LECTURE IV. Retiform and Lymphoid Tissue. Sometimes fibres and ground substance are absent. The cells are then much branched, and, joining one another by their branches, form a fine network. This is termed Retiform or Adenoid Tissue. The meshes of this cellular network may be filled with lymph corpuscles, and the tissue is then known as Lymphoid. It is found largely in the mucous membrane of the alimentary canal, and in lymphatic gl-ands. Jelly-like, or Mucous Tissue. In this form only the ground substance remains. All the other elements, excepting perhaps a few cells, have disappeared, as in the vitreous humour of the eye. III. Bone or Osseous Tissue is distinguished from other tissues by its hardness, due to the large amount of mineral matter in it. Composition of Bone. Two-thirds of bone consists of mineral matter, as the following analysis by Berzelius shows : Animal matter, - 33-3 Mineral matter Phosphate of Lime, 5 1 -04 Carbonate of Lime, 1 1 -30 Fluoride of Lime, 2-00 Magnesic Phosphate, - i'i6 Soda and Sodic Chloride, - 1*20 667 By the action of dilute hydrochloric acid all the mineral matter can be dissolved out, when the animal matter remaining still retains the original form of the bone. By the action of heat all the animal matter can be burnt away, leaving the mineral alone. The animal matter when boiled yields gelatin. Bone is tough and elastic. Its specific gravity is i'g. The bones are covered externally by a fibrous membrane called the Periosteum \ and they contain a fatty substance, or Marrow. Different Kinds of Bones. There are four 1. Long Bones, as the Humerus and Femur. A long bone consists of a central shaft at each end of which is an enlargement. 2. Flat Bones, as the Parietal and Frontal bones of the skull. 3. Short Bones, as the Metacarpal and Metatarsal bones. 4. Irregular Bones, as the Vertebrae, and the Ethmoid and Sphenoid bones of the skull. Minute Structure of Bone. The hard substance of bone is of two kinds : CARTILAGE AND BONE. 23 (i) Dense or Compact, and (2) Spongy or Cancellated. These are differently distributed in the different sorts of bones. A Long Bone, e.g., the Humerus, when cut down the middle lengthwise is seen to contain an elongated cavity extending throughout the length of the shaft the medullary canal in which is the marrow. The bone surrounding this canal is of the dense or compact variety, no spaces being visible to the naked eye. Very little magnification is sufficient, however, to show that it is traversed longitudinally by very fine canals the Haversian Canals which communicate with one another by side branches, with the exterior, and with the medullary canal. The enlarged extremities of the bone consist chiefly of spongy or cancellated bone, i.e., with an open texture, in which are spaces discernible to the naked eye. The mass of spongy bone is invested by a thin layer of compact substance. A transverse section of the shaft shows the medullary canal as a central orifice circular in shape. A hand-glass is sufficient to show in the bone surrounding it a number of small holes, which are the Haversian canals cut across. A thin transverse section of this compact bone more highly magnified shows each Haversian canal to be surrounded by a concentric series of alternate dark and light rings, due to the bone being deposited around each Haversian canal in layers or lamellce. At intervals on the dark rings are irregular markings. These are little flattened fusiform-shaped cavities called lacunce. From the lacunae pass excessively fine channels canaliculi which communicate with one another, and with the Haversian canals. There is thus a network of fine channels ramifying throughout the bone substance. In the living state each lacuna contains a cell a bone-cell ^3x\& it gives off fine branches into the canaliculi. A Haversian canal with the lamellae surrounding it constitutes a Haversian System. Lamellae are also found disposed between the Haversian system, and also parallel to the surface of the bone. Structure of the Lamella. A lamella is composed of a great number of very fine fibres which cross one another decussating fibres so as to give the appearance of a fine network. In the lamellae minute round holes are seen which are the points of passage of the canaliculi. There are often seen other larger holes where have been situated elongated pointed fibres the perforating fibres of Sharpey which possibly are of use in binding together the superposed lamellae. A Flat Bone. In a flat bone of the skull e.g., the Parietal 24 LECTURE IV. bone there are thin external and internal layers of compact sub- stance the tabula and between these a spongy substance, here known as the diploe. Within the spaces of the diploe is marrow. The spongy bone has essentially the same structure as that just described. The thin plates of bone bounding the spaces are composed of lamellae with lacunae. Short Bones consist internally of spongy bone enclosed by a thin layer of the compact kind. Comparison of Bone and Connective Tissue. Both bone and cartilage have been classified with areolar tissue, &c., under the head of connective tissue. The grounds for this are that the bone-cells are regarded as similar and analogous to the connective tissue corpuscles, that the decussating fibres of the lamellae resemble the white fibrils of areolar tissue, and that, on boiling, the animal basis of bone yields, like areolar tissue, gelatin. The fact that bone differs in being permeated with mineral substance, and, further, that the uses of the tissues are so widely different, suffice to make their separate treatment highly desirable. The Periosteum. The surfaces of bones except those parts entering into the formation of joints, where they are covered with cartilage, are closely invested with a fibrous membrane called the Periosteum. It is composed of white and elastic fibres, and serves as a support for bloodvessels and nerves going to the bone. It also assists in giving firm attachment to the tendons and ligaments connected to the bone. The inner surface immediately adjacent to the bone contains a layer of cells which are supposed to be intimately connected with the development and growth of bone. The importance of the Periosteum is seen from the facts : (i) that when removed the bone decays; (2) a portion removed and placed in skin or muscle may develop bone in those tissues. The Marrow is composed chiefly of fat in the long bones, where it extends into the larger Haversian canals. In spongy bone it is of a reddish colour ?nd contains few fat-cells. Other cells are present in marrow, some of which are believed to be of considerable im- portance in the development of the blood corpuscles (i) The proper marrow cells somewhat similar to white blood corpuscles and endowed with amoeboid movement; (2) smaller reddish nucleated cells, supposed to be transitional forms between (i) and red blood corpuscles ; (3) cells containing red corpuscles ; (4) cells containing reddish granules; (5) multi-nucleated cells, or myeloplagues, probably connected with the absorption of bone. IY. Cartilage, or Gristle, is a soft elastic substance covering CARTILAGE AND BONE. 25 the surfaces of bones where they enter into the formation of joints (Articular Cartilage) or connecting bones together, as the ribs and sternum (Costal Cartilage), or the hipbones to the sacrum, or the vertebrae to one another. Hyaline Cartilage is applied to articular and costal cartilage. In thin layers it is translucent. The microscope applied to such thin layers reveals a homogeneous groundwork or matrix, scattered throughout which are cells collected in 2's or 4/3 or groups of larger numbers. Fibro-cartilage. When the matrix is permeated with fibres the cartilage is called Fibro-cartilage, which is of two kinds : 1. White Fibro-cartilage, when the fibres are white. The Intervertebral, Interarticular, Circumferential, and Sesamoid Cartilages are formed of this variety. 2. Elastic Fibro-cartilage, when the fibres are of the elastic or yellow kind. It forms the Epiglottis, the Cartilages of the Ear, and of the Eustachian tube. Cartilage differs from bone in yielding Chondrin, instead of Gelatin, on boiling. Cartilages, with the exception of the Articular, are covered with a fibrous membrane called the Perichondrium. Ossification and Growth of Bone. The intimate connec- tion between bone and cartilage is seen when the development of the former is studied. In the embryo the long bones are preceded by similarly shaped masses of cartilage. These cartilaginous models are gradually replaced by bone through a process of ossification which begins in an individual bone at several different points, known as " centres of ossification ". The bone thus formed, however, is merely temporary, to* growth in thickness takes place by layers being deposited on the outside of the shaft, and whilst this thickening proceeds a corresponding absorption takes place within, probably effected by large multi- nucleated cells, or osteodasts. Consequently in the fully developed bone the whole shaft has been formed by the periosteum, or in mem- brane, and the cartilage must therefore be regarded as merely a temporary model on which the lines of the bone can be laid down. Growth in length takes places by new layers being deposited at the ends of the shaft. For this purpose the shaft and extremi- ties, when they are ossified, remain separated by a layer of cartilage, in which the new bone is formed. When the full size of the bone is attained these connecting pieces of cartilage become transformed into bone and the ossification is complete. 3 26 LECTURE IV. That growth of bone does actually take place in this way is shown by numerous experiments e.g., (a) boring two holes in the shaft of a growing bone at a certain distance apart, in the fully developed bone they are not more separated ; (b) putting a silver ring round a young bone : it becomes enclosed by the new bone and may be found in the medullary canal, proving a simultaneous absorption ; (c] feeding young animals on madder, which colours the bone formed pink ; the exact mode in which the new bone is deposited is thus beautifully shown. Ossification in Membrane. The flat bones are not preceded by cartilage, but by membrane. The deposit of bone (e.g., in the parietal bone) begins in the middle of the membrane as a centre of ossification, and gradually spreads in all directions until the whole bone is formed. The active agent in the deposit of the bone is probably certain cells termed Osteoblasts. Some of these, becoming involved in the process of ossification, remain as bone-cells inclosed in lacunae. The bone-cells and lacunae in the long bones probably have a similar origin. There is much uncertainty regarding the destiny of the cartilage-cells. They probably either disappear altogether as the result of absorption, or by division form osteoblasts. Alteration in shape of fully developed bones results from deposi tion and absorption in a manner precisely similar to that by which the bone is formed in the first instance. Regeneration of bone is brought about chiefly, if not wholly, through the instrumentality of the periosteum. LECTURE V. THE SKELETON. Vertebrate Type. All the bones of the body, with the cartilages and ligaments connecting them, form together an internal framework, or skeleton. The form of the skeleton determines the shape of the body. All vertebrate animals are characterised by the presence of such a skeleton, built up on the same fundamental plan, viz., of a central or axial portion and an appendicular portion. It is the varied modification of this fundamental plan which is the underlying cause of the different shapes of the bodies of vertebrate animals. To this fundamental plan, in which the body of man agrees with that of all the five classes of vertebrate animals Mammals, Birds, Reptiles, Amphibia, and Fishes the term Vertebrate type is often applied. Classification of the Bones of the Skeleton. The expression " Vertebrate type v is derived from the fact that all the animals forming the sub-kingdom Vertebrata have a long, central, bony axis, or " backbone," at one extremity of which is the skull, at the other the coccyx, or tail. This central axis is the Vertebral column, and is composed of a number of small bones, or vertebra, of similar construction, placed one above, or in front of the other. All the other bones may be regarded as appendages to this central axis. Of these bones those directly attached to the vertebral column determine the shape of the trunk, whilst those indirectly attached determine the shape of the limbs. The former, together with the vertebral column, constitute the axial portion, and the latter the appendicular portion of the vertebrate skeleton previously alluded to. Homology and Analogy. Those bones which in different animals have a similar position and relation in the skeleton are termed homologous. To those which, whether homologous or not, subserve a similar use or function is applied the term analogous. Exoskeleton and Endoskeleton. The skeleton of Inverte- brate animals differs from that of Vertebrata in being external, in 28 LECTURE V. the form of a calcareous or chitinous covering. This hard invest- ment is termed an Exoskeleton, in contradistinction to the internal bony framework, or Endoskeleton, of Vertebrata. In some Verte- brata there is an additional exoskeleton e.g. 9 scales of Fishes, and scales and plates of Reptiles. Number of Bones. In the adult there are 200 distinct bones, omitting the auditory ossicles (6) (vide Lecture XXIII. ), and several masses of bone, termed sesamoid, situated in the tendons. Pairs. Total. Vertebral column, 26 o 26 Skull, - 6 8 22 Ribs and Sternum, i 12 25 Shoulder-blade, Collar-bone, Hipbone,- 036 Hyoid-bone, - TOT Upper Limb, - o 30 60 Lower Limb, - o 30 60 200 Weight of Skeleton. The skeleton constitutes rather less than one-fifth of the body by weight. Uses of the Skeleton. i. It supports the softer parts of the body. 2 . Protects internal delicate organs. 3. Gives attachment to muscles, forming an essential part of the mechanism of movement. The Vertebral Column, averaging in length 28 inches, sup- ports at its upper extremity the skull, and in the region of the back the ribs. It rests below upon the hipbones, which are supported by the legs. Its Structure. It is composed of a number of similarly-shaped bones or vertebrae, piled up one above the other so as to form a column. A Yertebra is a very irregularly-shaped bone composed of a solid mass of bone in front, the Body or Centrum, and of an Arch of bone behind. Coming off from each side of the arch is a pro- jection of bone, or Transverse Process. From the middle of the back of the arch projects backwards, and more or less down- wards, another, often pointed, projection, or Spinous Process. The arch is thus divided into two parts on each side (i) a front portion between the body and transverse process called the Pedicel, and (2) a posterior much broader portion between the transverse and spinous processes, called the Lamina. On the upper and THE SKELETON. 2Q lower sides of the arch are two smooth, more or less inclined sur- faces, by means of which contiguous vertebrae are jointed together. These projections are the joint, or Articular Processes, of the ver- tebra. The surfaces of the upper pair face backwards ; those of the lower pair forwards. The upper surfaces fit on to the lower of the vertebra next above, and they are firmly connected so as to form a joint. Between the pedicels of each two contiguous vertebrae is a space or orifice, termed an interuertebral foramen, through which passes a spinal nerve. Peculiar Vertebrae. Some of the vertebrae show deviations from this type. The first vertebra, or Atlas, on which the skull rests, has no body, and only a very small spinous process. On the upper surface of its arch are two elongated, slightly concave, joint surfaces, on which rest two corresponding convex joint surfaces, one on either side of the foramen magnum of the skull. The second vertebra, or Axis, not only has a body of its own, but a long pro- jection odontoid process from the upper surface of it, which takes the place of the absent body of the atlas, being kept in position by a strong cord-like ligament (transverse ligament). The transverse processes of these two vertebrae, like all the other cervical ver- tebras, are pierced by a hole through which passes the vertebral artery. The four last vertebrae, consisting of little more than small solid masses of bone, are joined together in adult life to form one bone, called the Coccyx. The five vertebrae next above the coccyx are also joined to form one in the adult. This bone, the Sacrum, has a peculiar shape. Shaped like a V, with the point downwards, it is concave forwards and convex back- wards. The transverse processes, which are enormously developed, are joined to form a large lateral" mass of bone on either side, to which are connected the hipbones. The bodies of contiguous vertebrae are firmly joined by flattened discs of fibro-cartilage (intervertebral discs). The bodies and arches are also connected by ligaments. They are also jointed together by means of the articular processes. Regions of the Vertebral Column. The vertebrae are named according to their position : Cervical, or the 7 uppermost, Dorsal (12), Lumbar (5), Sacral (5), Coccygeal (4). Curves of the Vertebral Column. There are four curves, being from in front as follows: (i) Cervical, convex; (2) Dorsal, con- cave ; (3) Lumbar, convex ; (4) Sacral, concave. The cervical and lumbar curves are due to the intervertebral cartilages being 30 LECTURE V. thicker in front than behind, and in the case of the latter also to the body of the 5th lumbar vertebra being deeper in front. The dorsal and sacral curves arise from the bodies of the vertebrae in those regions being deeper behind than in front. These curva- tures increase the mobility of the column and make it less liable to injury. Sometimes a slight lateral curvature, convex towards the right side, is observed in the dorsal region. I. Bones directly attached to the Vertebral Column. These are : 1. The Occipital Bone of the skull resting upon the atlas. It is pierced by a large hole, the foramen magnum, forming the com- munication between the vertebral canal and the interior of the skull. 2. The Ribs, twelve on either side, are flattened, elongated, curved, slightly twisted bones, possessing a considerable amount of elasticity. The upper seven are attached directly by cartilage costal cartilages to the sternum, hence termed Sternal Ribs. Of the re- maining five Asternal Ribs, three are attached by cartilage to the costal cartilage next above, and thus indirectly to the sternum. The last two, not being so attached, are called Free or Floating Ribs. A rib ends in the Head jointed to the bodies of two contiguous vertebrae, except the first, which is jointed exclusively to the first dorsal vertebra : a narrower portion, the neck, lies between the head and the Tubercle by which the rib is articulated to the transverse process of the lower of the two vertebrae connected with the head. The Costal Cartilages are directly joined to the ribs. Those of the sternal ribs are (except the first) connected to the sternum by joints. 3. The Hipbones, or Ossa innominate, are firmly joined to the sacrum behind, and connected together in front by cartilage (Symphysis pubis}. The three banes together form the Pelvis. On the outer lower aspect of the hipbone is a deep cup-shaped depres- sion, the acetabulum. Into this fits the head of the femur forming the hip-joint. Until puberty the hipbone consists of three separate bones joined by cartilage. The connecting cartilage is visible as a Y-shaped mass in the acetabulum until the seventeenth or eighteenth year. The upper of these three constituent bones is called the Ilium, the lower posterior one the Ischium, and the front one the Pubis. In the lower part of the bone is a large oval hole, the Obturator foramen, closed by a fibrous membrane. The Pelvis is wider transversely, obliquely, and from front to back in the female than in the male. The pelvis is inclined to the horizontal 60-65. The pressure of the body on the sacrum is thus trans- mitted to the thigh-bones more directly than if it were horizontal. THE SKELETON. 31 II. Bones indirectly attached to the Vertebral Column. i. Bones of the Skull. Of the twenty-two skull bones eight are said to form the Cranium and fourteen the Face. a. The Cranium. Most posteriorly is the Occipital ; joined to it in front are the two Parietal, connected together along the middle line of the cranium, and in front to the Frontal, and below to the two Temporal. Each temporal bone is composed of three parts, viz., (a) Squamous in front and above, joined in front to the sphenoid bone, and by its zygomatic process to the malar bone, above to the parietal bone ; \b] Mastoid, behind joined to the parietal and occipital; (c] Petrous, a pyramidal-shaped mass in the middle and projecting internally. It is of importance, for it contains the internal ear. Extending downwards and forwards from this part is a long, tapering projection, the Styloid process. Two Sphenoid are most complex bones, each consisting of a central body and two pairs of lateral expansions, the great and small i&ings. The body is directly continuous behind with the body of the occipital bone. In front it is connected with the ethmoid and vomer. The great wings are joined to the frontal, parietal, and temporal. The Ethmoid, situated in front of the sphenoid and below the frontal, enters into the formation of the cranium, orbits, and nose. It is composed of a central vertical plate and two lateral masses, joined above by a horizontal sieve- like mass, the cribriform lamella, on which rest the olfactory lobes. b. The Face. The fourteen bones forming the face are two single, viz., the Inferior Maxillary and the Vomer, a small thin-plate bone placed vertically between the nostrils, and six pairs, viz., Superior Maxillary, Malar, Lachrymal, Nasal, and Inferior Tur- binate. The first three enter into the formation of the Orbit, together with the frontal and sphenoid. The anterior orifice of the nostrils is formed by the superior maxillary, nasal, and vertical plate of ethmoid. The inferior maxillary is jointed to the squamous portion of the temporal, its condyle being articulated to the glenoid fossa. Shape of the Skull. At birth the proportion which the face bears to the cranium is only about half that of the adult. Slight differences are usually observable between the male and female skull : the muscular prominences and the frontal sinuses are not so much developed in the latter, and the face is less in proportion to the cranium. The classification of the skulls of different races has been attempted by a comparison of the amount of projection 32 LECTURE V. of the jaws (prognathous and orthognathous), and of the length of the skull (br achy cephalic and doliocephalic]. The Hyoid Bone is a U-shaped bone situated at the back of the tongue. It is composed of a body and two pairs of horns or cornua, great and small. It is kept in position by the small horns being attached by ligaments to the styloid processes of the temporal bones. The body flattened from before back is concave posteriorly and situated in front of the epiglottis. A great number of muscles are attached to the hyoid bone, connecting it with the temporal and lower jawbones, to the scapula and sternum, to the tongue and larynx. ii. Bones of the Lower Limb a. The Thigh is formed by the Femur jointed at its upper extremity to the hipbone, and by its lower to the tibia. b. The Leg is formed by the Tibia and Fibula, the former (in- ternal) is much thicker and stronger than the latter. The fibula does not enter into the knee-joint, being jointed at its upper extremity to the tibia. The Patella, or kneepan, is situated in front of the knee-joint, between the femur and tibia : it may be regarded as a sesamoid bone developed in the tendon of the extensor quadriceps cruris muscle. The tibia and fibula are both articulated below to the astragalus. c. The Tarsus, or ankle, is composed of seven bones, viz., the Astragalus, behind and above to which are articulated the tibia and fibula ; it rests upon the heelbone, or Calcaneum, between which and the metatarsus is the Cuboid bone. The astragalus is jointed in front to the Scaphoid, and it in turn to the three Cuneiform bones outer, middle, and inner. d. The Metatarsus is composed of five similar shafted bones, jointed behind to the tarsus, in front to e. The Phalanges. These are fourteen in number, arranged in three rows, there being three phalanges for each toe except the great, for which there are only two. iii. Bones of the Upper Limb make up the shoulder girdle, the arm, the forearm, and hand. a. The Shoulder Girdle is composed of sternum, clavicle, and scapula. Sternum, or Breastbone, forming the front of the chest, is dagger- shaped, the lower extremity ending in a pointed piece of cartilage the xiphoid cartilage. The upper end is enlarged to form the manubrium, on each upper angle of which is a joint surface for articulation with the inner end of the clavicle. Clavicle, or Collar- THE SKELETON. 33 bone, is curved in shape somewhat like an italic f. Situated above the ribs, it passes outwards at the base of the neck, and is jointed at its outer extremity to the acromion process of the scapula. Scapula, or Shoulder-blade, is a flattened V-shaped bone lying at the back of the chest. On its posterior surface is a prominent ridge, or Spine, terminating in front in the acromion, to which the clavicle is jointed. At its upper outer angle is a shallow depres- sion, or glenoid fossa, into which fits the head of the humerus. b. The Arm is formed by the Humerus jointed above to the scapula, below to the radius (externally) and ulna (internally). c. The Forearm is formed by the Radius and Ulna. The Ulna large above, small below has a deep sigmoid cavity at its upper extremity, by which it articulates with the humerus, thus forming with that bone the elbow-joint. The Radius small above, large below is articulated by its head to a separate surface capitellum of the humerus, and also to the ulna, by its lesser sigmoid notch. The radius alone enters into the wrist-joint by its large lower extremity. The small end of the ulna is separated from the carpal surface by a piece of cartilage. Ex- ternally it is jointed to the radius. d. The Carpus. Eight bones arranged in two rows of four each, upper and lower, form the carpus. The bones are : Upper row Scaphoid (external), Semilunar, Cuneiform, Pisiform. Lower row Trapezium (external), Trapezoid, Os Magnum, Unciform. e. Metacarpus. Five shafted bones similar to those of the lower limb form this portion of the hand. f. Phalanges, fourteen in number, arranged in three rows, resemble those of the lower limb, but are longer. There are only two phalanges forming the thumb, as in the case of the big toe. LECTURE VI. THE MUSCLES. Y. Muscular Tissue. The physiological importance of this tissue is evident from the facts 1. That the muscles form fths of the body by weight. 2. By their contractions they produce the movements of the body, and of its parts. 3. A study of the chemical changes occurring in muscle as the result of its activity gives some knowledge of the nature of its nutrition, and so far throws light upon the chemico-vital changes of the body generally. General Arrangement of the Muscles. The muscles may be classified into two systems 1. Dermal, which is much better seen in some other verte- brate animals, and is represented in man by a few muscles only, viz., the Platysma my aides , a thin, broad muscle, passing up from the collar-bone and shoulder-blade to the lower jawbone, just below the skin, and the muscles attached to the hairs by which they are made erect in gooseskin. 2. Skeletal. There is an intimate connection between the development of the skeleton and the muscles attached to it, since both subserve the same use, viz., movement. When there is an increase or decrease of one there is a reciprocal increase or decrease of the other. The muscles are symmetrical, and, with few exceptions, are in pairs. Number of the Muscles. The muscles number about 240, and may be thus apportioned (i) Head and neck, 75 ; (2) vertebral column and trunk, 51 ; (3) upper limb, 58 ; (4) lower limb, 59. Coverings of Muscles. The muscles are covered and more or less enclosed by expansions of connective tissue, often called Fascia. When this covering is composed of the white fibres THE MUSCLES. 35 densely arranged so as to give it a shining appearance, it is spoken of as an Aponeurosis. Attachment of the Muscles to the Bones. There are two modes of attachment 1. In some cases the muscular substance is directly attached to the bone. 2. In most cases the muscle is attached to the bone through a tendon. A typical muscle may be said to have a tendinous extremity at either end, with a central fleshy mass called the belly. Rarely there is a central tendon dividing the fleshy mass into two parts or bellies, hence termed Digastric. The tendinous extremity which when a movement is produced by the contraction of the muscle is fixed is termed the origin ; the extremity which under similar circumstances is movable is called the insertion. Composition of Muscle. About 75 per cent, of muscle consists of water. The following gives the amount of the chief constituents in 1000 parts : a. Organic Matters,- -208 245 Proteids,. - 173-5 ' - I 97' 1 (Nitrogenous, - 2'o -j Non-nitrogenous,- 15-1 b. Inorganic Matters, 9 10 Phosphoric Acid, - 3 '4 4'8 Potash, - 3 3-9 Soda, Lime, Magnesia, - minute quantities. Sodium Chloride, Iron, Structure of Muscle. From the point of view of their microscopic structure the muscles are separable into three classes i. Skeletal Muscles, also termed Striped, Striated, and Volun- tary. These muscles are easily seen to be composed of coarse threads, or Fasciculi, which are separated by inward prolonga- tions of the investing fascia. The thin sheath of connective tissue thus surrounding a fasciculus is termed the perimysium. Each fasciculus is composed of a great number of elongated cylindrical cells (i to ij inches in length), with rounded ex- tremities, and an average diameter of ^J^th of an inch, known 36 LECTURE VI. as muscular fibres. Each fibre consists of a semi-fluid mass of living matter enclosed within a very fine, colourless, transparent, elastic sheath the sarcolemma and is curiously striated both longitudinally and transversely, especially the latter. Beneath the sarcolemma are nuclei. The transverse striae appear alter- nately light and dark under the microscope. It is a much- disputed question what is the cause of this curious and characteristic appearance, which has given to all muscles exhibiting it the name of Striated. Possibly it is merely an optical effect. These muscles are also often called " Volun- tary," from their being for the most part under the control of the will. 2. Unstriated Muscles, also called Plain and Involuntary. These muscles, which surround the bloodvessels, the alimentary, canal, &c., consist of much elongated fusiform cells contractile fibre-cells of various sizes. Within each cell is a much-elongated nucleus. The cells measure -^^ to y^ inch in length, and -Q-^QQ to 25 1 oo inch in breadth. 3. Muscle of the Heart. This is composed of oblong or square shaped cells exhibiting a faint transverse and longitudinal striation. Each cell has an oval nucleus. This kind of muscle, as well as the preceding, not being under the influence of the will, is some- times called " Involuntary ". Function of Muscle. Muscular tissue possesses in a very high degree that property observed in the Amoeba Contractility. Muscle is eminently contractile. When stimulated a muscle con- tracts, becoming shorter and broader ; on removal of the stimulus, it relaxes to its original dimensions. The contraction in the living body takes place as the result of a nervous impulse arriving at the muscle after passing along a nerve connected with it. There is good evidence to show, however, that Muscle is independently contractile- 1. The end of the sartorius muscle containing no nerve terminations contracts when stimulated. 2. After urari poisoning, which destroys the end structure by which the nerve is connected to the muscle, direct stimulation of the muscles causes them to contract. 3. Certain chemical substances do not cause contraction when applied to the nerves, but do so when directly applied to the muscles e.g., ammonia, carbolic acid. 4. Contraction may occur in the absence of nervous tissue a. The rhythmical contraction of the foetal heart. THE MUSCLES. 37 b. In vegetal protoplasm, by which movements of plants are produced (e.g., mimosa pudica). Graphical representation of a muscular contraction. The con- traction and relaxation of muscle can be graphically represented by a curve, when it is seen that a certain period elapses after the application of the stimulus, before the contraction begins (latent period]. The curve then rises to a maximum (contraction), and immediately after falls to the original level (relaxation}. The whole contraction occupies T Vh of a second, thus divided Latent period -j^-th, Ri se (contraction) T o-ths, Fall (relaxation) TJhj-ths. The instrument by which this is shown is termed a Myograph. It essentially consists of a drum or a plate covered with a coating of lampblack movable at a definite rate, against which the point of a fine lever can be brought. This lever has attached to it between the fulcrum and free point the lower extremity of a muscle. When the muscle contracts the lever is raised, when it relaxes the lever falls. The time is determined by making a tuning-fork in a similar manner record its vibra- tions. Tetanus. When a series of electric shocks, following one another very quickly, is passed into a muscle, it remains con- tracted so long as the stimuli continue to enter it. The myographic curve under these circumstances shows the first two phases, but not the last, the lever writing a straight line on a level with the summit of the curve. A muscle so con- tracted is said to be in a state of Tetanus. Tetanus occurs as the result of disease (lockjaw), and of poison (strychnin). The number of successive stimuli required varies from_i5 (frog) to 330 (insects) per second. Different kinds of Stimuli. In addition to the ordinary normal nervous stimulus, muscles may be excited to contract by (i) chemical, (2) thermal, (3) mechanical, (4) electrical stimuli. Changes occurring during contraction. Important changes take place in a muscle when it contracts a. Structural: [i.) It shortens, thickens, and hardens. (ii.) The light and dark striations change places. b. Physical : (iii.) There is an elevation of temperature. (iv.) The electrical conditions are changed, there being a negative variation of the natural muscle current, (v.) The extensibility of the muscle is increased. 38 LECTURE VI. c. Chemical : (vi.) Carbonic acid (CO..,) gas is evolved, (vii.) The muscle has an acid reaction due to the production of sarkolactic acid (C 3 H 6 O 3 ). The appearance of these bodies points to a chemical decomposition of the muscular substance, or at least of a part of it that part free from nitrogen. The components of the muscle soluble in water decrease, whilst those soluble in alcohol increase, in quantity. Rigor Mortis. Soon after death the muscles become stiff and rigid, a change termed Rigor Mortis. Many of the changes oc- curring during rigor mortis are strikingly similar to those con- nected with contraction e.g., (i) the muscle does actually contract ; (2) there is a rise of temperature ; (3) carbonic acid is evolved ; and (4) sarkolactic acid formed. The following differences are, however, apparent : (i) The extensibility is not increased; (2) the electric cur- rents altogether disappear; (3) The muscle becomes rigid and opaque. This rigidity is the cause of rigor mortis, and is due to the proteid constituent of the muscle, or Afyosm, coagulating. Fatigue and Exhaustion of Muscle. When from exertion the muscles are contracted beyond a certain limit, the products resulting from the contraction are too great to be immediately removed by the bloodstream, and by their accumulation make further contraction more difficult. If contraction be carried on further still (beyond what may be called the maximum) the accumulation is so great that the muscles cannot be thrown into contraction any longer, and are said to be exhausted. Re- covery occurs after a period of rest, during which the blood- stream both removes the accumulated products, and brings a supply of nutriment to make good what has been lost. During ordinary muscular contraction the blood supply is increased, and in consequence the amount of nutriment brought overbalances the muscular substance that has been lost, and hence the muscle increases in size. In thus increasing by use a muscle regarded as a machine differs from any machine of human contrivance. LECTURE VII. MOVEMENT NERVOUS TISSUE. Amoeboid Movement is seen in low forms of life, like Amoeba (animal) and Myxomycetes (plant). The white blood corpuscles also exhibit it. Ciliary Movement may produce locomotion of a complete organism, as in Infusoria (animal) and Protococcus (plant), or may exist in the stationary organs of the higher animals e.g., the epithelium lining the air tubes. Muscular Movement is produced either by involuntary or voluntary muscles. A. By Involuntary Muscles. These may be divided into three classes i. Those surrounding tubes, as the bloodvessels, alimentary canal, ureters, &c., which, by their contraction, diminish the calibre of the tube. ii. Those surrounding cavities, as the heart. By their con- traction they diminish the capacity of the cavity. iii. Those in other parts e.g., the muscles in the skin or arrector pili. B. By Voluntary Muscles. These, by their contraction, cause the bones to move over one another, thus producing the move- ments of limbs and other parts, and also movement of the whole body, or locomotion. The manner and extent of the movements of the bones are dependent upon the way in which they are con- nected together. Modes of Connection of the Bones. There may be said to be three a. Immovable connections, as in the bones of the skull by Sutures. b. Slightly movable connections, as by cartilage in the vertebral column and pelvis. c. Truly movable connections, as in all true joints. Structure of a Joint. A joint consists of the free ends of two 4O LECTURE VII. bones in contact, which can move over one another within an area circumscribed by the extent of the surrounding ligaments and muscles, and also by the shape of the bones. The surfaces of the bones entering into the joint are covered with gristle (articular cartilage}. Lining the inner surface of the ligaments, and extending a short distance on the surface of the bones, is a delicate smooth membrane (synovial membrane], which secretes a lubricating liquid (synovia). The extremities of the bones entering into the joint are kept in close apposition by the con- necting ligaments and muscles, by the oily covering, and by atmospheric pressure (hip-joint). Classification of Joints 1. Those having one axis of rotation. a. True Hinge Joints, e.g., the fingers and toes, in which there is movement in one plane only, there being either bending (flexion) or straightening (extension). b. Screw-hinge Joints, as the elbow and ankle-joint, in which the flexion and extension are complicated by a screw-like move- ment of one bone over the other (e.g., the ulna over the humerus). c. Pivot-joints, in which one bone rotates around another as the atlas round the axis, and the radius round the ulna in prona- tion and supination. 2. Those having two axes of rotation. d. Saddleshaped-joints, in which there is movement in two planes more or less at right angles to one another; e.g., the thumb-metacarpal bone, and the carpus (trapezium). 3. Those having several axes of rotation. e. Ball and Socket-joints, e.g., shoulder and hip-joints, in which there is movement in all planes. The varied movements of these joints are Flexion and Extension, Abduction and Adduction, Rotation around the long axis of the moving bone, Circumduc- tion. Circumduction is possible to a limited extent in the meta- carpo-phalangeal joints. 4. Rigid joints, in which slight movement is possible in all directions ; e.g., the carpal and tarsal joints. The Bones as Levers. The bones, moving in the joints under the action of the contracting muscles, act precisely as ordinary levers, and the different movements can be referred to the three systems of levers. The third kind of lever in which there is a gain in the rapidity at the expense of the force of its movement is the most common. Relation of Muscles to Joints. The arrangement of the muscles NERVOUS TISSUE. 4! around a joint depends upon the movements of which it is capable. Where there is movement in one plane only, the muscles are arranged on opposite sides in the plane of motion. Those causing bending are termed flexors ; those straightening extensors. If movement is possible in several planes, the muscles are on all sides of the joint. The Erect Posture, which so eminently distinguishes man from the lower animals, is maintained by the harmonious contraction of a great number of muscles. The centre of gravity of the whole body is at the promontory (the anterior projection at the top of the sacrum). A vertical line down from this point falls in front of the line joining the two ankle-joints. The consequent tendency of the body to fall forward is counteracted by the contraction of the muscles of the calf and other muscles of the leg. The centre of gravity of the head, trunk, and upper limbs is in front of the tenth dorsal vertebra. The vertical line down passes behind the line joining the hip-joints. The consequent tendency of the trunk to fall backwards is overcome by the attachment of ligaments and contraction of muscles (e.g., psoas and rectus femoris). The centre of gravity of the head is in front of the atlas, and hence the head tends to fall forwards, as is seen in sleep and death. Antagonistic to this tendency is the contraction of muscles passing from the neck to the back of the skull. The vertebral column being movable in the neck and loins, the contraction of strong muscles in these positions is necessary in order to fix it; e.g., extensor dorsi communis and quadratus lumborum. Sitting. In sitting, the body rests upon the tubera ischii of the hipbones. In the forward and backward movements of the body, possible in this posture, the trunk may be regarded as a lever moving on its fulcrum at the tubera ischii. Walking. In walking, each leg is alternately swung from back to front past the other, which then alone supports the body. In this, as in other pendulum motions, the extent of the swing depends on the length of the pendulum ; hence the length of the legs determine the natural rate of walking. The rate, also, de- pends on the length of time the two feet are on the ground together. As the rate quickens this time diminishes, until, in very quick walking, it becomes zero ; i.e., at the very moment one foot reaches the ground the other rises from it. Running. In running there is during each step a time when both feet are off the ground. By means of indiarubber bags (tambours), with which are con- 4 42 LECTURE VII. nected flexible tubes, fitted to the feet, Marey has been enabled to obtain graphic representations of the movements of the legs of men and animals in walking, running, galloping, &c. Also by taking a series of consecutive instantaneous photographs (Muy- bridge). YI. Nervous Tissue. This tissue is characterised by two distinct structures Structure 1. Nerve cells. These are usually irregular in shape, and passing off from them are processes, or poles, some of which are directly continuous with nerve fibres ; others join with similar processes from neighbouring cells, and by their union a network is often produced. 2. Nerve fibres, which are of two kinds medullated and non- medullated. i. A medullated nerve fibre consists of a central axis or axis- cylinder the essential part which is always in connection with a nerve-cell. This axis, which is fibrillated, is surrounded with a fatty layer, or medullary sheath, and the whole is enclosed within a very fine membrane the primitive sheath, or sheath of Schwann. The medullary sheath is divided at equal intervals into segments. The intervals thus regularly occurring are termed the nodes of Ranvier. Between each two consecutive nodes is a nucleus lying between the medullary and primitive sheaths. The axis cylinder and the primitive sheath are continuous throughout. These fibres differ much in size, the diameter varying from to T 2ihro th of an inch - ii. A non-medullated nerve fibre differs from the preceding in not possessing the medullary sheath. The non-medullated fibres are found chiefly in the nerves of the sympathetic nervous system, but also to some extent in the cerebro-spinal nerves, and the olfactory nerve is solely composed of them. Function. Nervous tissue possesses in a highly specialised manner the property of irritability. The irritability of nerve is sometimes termed excitability. Nerve-cells may be said to be endowed with the power of giving out, receiving, and storing up nervous impulses; whilst nerve fibres conduct these impulses either to or from the nerve-cells with which at their central ends they are connected. The excitability is diminished by cold, by lessened supply of blood, by being cut off from the centre with which it is con- nected. NERVOUS TISSUE. 43 A nervous impulse travels along a nerve at the rale of about 30 yards per second. White and Grey Matter. Nervous tissue containing nerve fibres only is white in colour, and is therefore called white nervous matter ; when nerve-cells are present the tissue has a grey colour, and it is hence known as grey nervous matter. Chemical Composition. 30 per cent. Water. It is neutral to test-paper. In the medulla of nerve fibres is Lecithin (con- taining Phosphorus). The axis cylinder contains proteids. LECTURE VIII. THE BLOOD AND LYMPH. YII. The Blood has been well described as the great internal medium by which the different parts of the body are brought con- tinually into connection with each other, and with the external world. From the nature of the uses the blood subserves in the animal economy, its composition must necessarily be continually altering as it circulates through the different parts of the body. Notwith- standing this continual variation, there are certain structural and chemical characteristics which are constant wherever the blood is found. General Characteristics. The average specific gravity of blood is 1055. The colour varies from bright scarlet (arterial) to a very dark red (venous). Venous blood is dichroic, for it is dark red by reflected, and green by transmitted, light. It is alkaline in reaction. The alkalinity diminishes after the blood is shed. The alkalinity is said to be diminished by great muscular exertion, probably due to the greater formation of acid in the muscles. It has a characteristic odour and salt taste. Structure of Blood. Under the microscope the blood is seen to consist of two distinct parts: (i) a transparent colourless liquid, the plasma, in which float enormous numbers of minute solid particles, (2) or corpuscles. These corpuscles are of two kinds, red and colourless or white. The Red Corpuscles, first observed in human blood by Leeuwenhceck in 1673, are circular, biconcave discs, ^^V^ tn mcn in diameter, and one-quarter this in thickness. This shape and size is characteristic of the red blood corpuscles of Mammalia generally, with the exception of the camel tribe, in which they are elliptical in shape, and in the musk deer and elephant, in which they are TYFU~^ tn mcn an( ^ ^To^h mcn m diameter respectively. In the rest of the vertebrata the corpuscles are oval in shape, with a central elevation on either side. They also possess a THE BLOOD AND LYMPH. 45 nucleus. They increase in size as you descend the vertebrate scale, being about ^wuu^ 1 m h m birds, or villi. Like the rest of the mucous membrane, they are covered with columnar epithelium resting on a basement membrane. Each villus contains in the middle of it a blindly ending lymphatic vessel, or Lacteal, surrounded by a close network of capillary blood- vessels. Muscular fibre-cells pass from the muscularis mucosag into the villi, ending in the basement membrane, and by their contraction shorten them. The villi have been calculated to be over four millions in number; they are most numerous in the duodenum and jejunum. Solitary Follicles are nodules of lymphoid tissue, scattered through the whole intestine, forming slight elevation of the mucous membrane and extending down into the connective tissue coat. In the ileum they are found in aggregated masses (Agminated follicles], forming patches from J inch to 2 or 3 inches in length and J inch wide (Peyer's patches). These are 20-30 in number, and are even more numerous in the young subject. These follicles are well supplied with blood, and are in most intimate connection with lymphatics. Blood is brought to the small intestine by the mesenteric artery. Its nerves are derived from the superior mesenteric plexus, which is formed by branches from the sympathetic (cceliac plexus, semi- lunar ganglion) and vagus nerve. Two well defined nervous plexuses are found in the wall of the intestine : an external in the muscular coat called the plexus of Auerbach ; an inner finer one in the submucous coat called the plexus of Meissner. The duodenum embraces the head of the pancreas, and into it, 3 to 4 inches from the pylorus, enter by a common orifice the com- mon bile and pancreatic ducts. The Large Intestine, 5 to 6 feet in length and ij to 2\ inches wide, extends from the ileum to the anus. It is divided into three parts, viz., Ccecum, Colon, and Rectum. The colon again is divided into an ascending transverse and descending portion and the Sigmoid flexure. Where the ileum joins the ccecum is a valve, the Ileo-ccecal valve. The walls are formed of coats similar to those of the small intestine. The outer peritoneal covering does not in all parts completely surround the tube, and in the colon it is developed into little projections enclosing fat called appendices epiploicce. The Muscular Coat is peculiar in the longitudinal layer, being much thickened in three parts; and, being here shorter, the 84 LECTURE XIII. intestine is thereby thrown into sacculations, which distinguish it from the smaller bowel. This sacculation causes inward projec- tions in the shape of ridges. The Mucous Membrane has no villi, but numerous tubular glands, shorter than those of small intestine. Solitary follicles are present, but no Peyer's patches. The Vermiform Appendix is a hollow, blindly-ending appen- dage to the coecum. It is peculiar to man, the higher apes, and the wombat. In structure it resembles the rest of the intestine, but the solitary follicles are more numerous. The Peritoneum is a serous membrane forming a closed sac in the male, but communicating with the Fallopian tubes in the female. The parietal layer lines the inner surface of the abdomen and pelvis and lower surface of the diaphragm ; the visceral layer covers more or less completely the organs in those cavities. LECTURE XIV. FOODS. The human body is often compared to a machine, e.g., a steam engine, and the food taken into the body is compared to the fuel put into the furnace of the steam engine. This comparison is correct to this extent, that in each case the food or fuel supplies material by the ultimate oxidation of which a supply of energy is forthcoming, by which the work of the machine is carried out. The human body, however, differs from any machine ever devised by man, inasmuch as it is able to make use of food not only as a source of energy, but also as a supply of material for the repair of those parts of the machine which have wasted away in the wear and tear of its activity. This remarkable property it owes to its being alive. It is the most obscure secret of life. It follows from this that the food must contain the same ele- ments as enter into the chemical constitution of the various parts of the body, or at least of those elements which are continually being lost from the body as the result of its vital activity. The essential point, therefore, in the definition of a food vs that it supplies material for the recuperation of the body ; in addition, it supplies energy in a potential form, which will be, other things equal, proportional to its chemical complexity. If any one element can be said to transcend others in its importance as a constituent of living organisms, it is Nitrogen. It characterises all those highly complex substances classed together under the term Proteid which form the basis of all protoplasmic or living structures. This element must therefore be present in the food, and from its im- portance the constituents of all foods have been divided into two great classes (i) Nitrogenous and (2) Non-nitrogenous. The animal body can make use of nitrogen as food only when com- bined with carbon, hydrogen, and oxygen, to form proteid. No simpler nitrogen compound can be made use of by animals as food. Food- stuffs, or the compounds which singly or, more generally, 86 LECTURE XIV. mixed together form the different foods, can be classified as follows : 1 . Nitrogenous, Proteid, or Albuminous, containing the elements carbon, hydrogen, oxygen, nitrogen, and sulphur. Examples of them are seen in Myosin of muscle, Casein of milk, Gluten of flour, Albumin of eggs. 2. Non-nitrogenous Bodies may be divided into a. Carbohydrates, consisting of carbon, hydrogen, and oxy- gen, in the proportion represented by the formula C^H 2?l O n . Starch, the different kinds of Sugars, and Cellulose are examples. b. Fats composed of carbon, hydrogen, and oxygen, the last element being in smaller proportion than in the carbo- hydrates. Butyrin of butter, Stearin of suet, and Palmitin are examples. Fats by the action of superheated steam or alkalies take up the elements of water and decompose into glycerine and a fatty acid of general formula QH^O, ; c., plexuses), and pelvis (Hypogastric and Pelvic plexuses), connected by nerves with the sympathetic trunk. Of these nerves the largest and most important are the Splanchnic, three on either side, termed the Great, Small, and Smallest. The Great splanchnic arises from the fifth to ninth thoracic ganglia, and passing through the crus of the diaphragm, ends in the semilunar ganglion, which forms part of the solar or epigastric plexus situated behind the stomach, on the aorta and pillar of the diaphragm. The Small splanchnic^ arising from the tenth and eleventh thoracic ganglia, ends in the cceliac plexus surrounding the cceliac axis. The Smallest splanchnic originates from the twelfth thoracic without joining one another, when of the medullated kind. Non- medullated fibres may branch and join. An ultimate fibre or, rather, axis cylinder may branch just prior to its final distribu- tion. This is well seen in the cornea. The fibres are collected into bundles, QT funicufy and these again collected together form the nerve. A nerve is enclosed by a connective-tissue sheath termed the neurilemma. Classification of NerYes. Nerves may be classified accord- ing to their origin as follows : Spinal Nerves. There are thirty-one pairs originating from the spinal cord. They are named according to the region of the vertebral column whence they issue, viz., 8 cervical, 12 dorsal, 5 lumbar, 5 sacral, i coccygeal. They are distributed chiefly to the muscles and integument. Cranial Nerves. There are twelve pairs arising from the brain. All these, with the exception of the first four, viz., olfac- tory, optic, third, and fourth, have their primary or deep origin in the medulla oblongata. They are all distributed to the head except the vagus and spinal accessory. Sympathetic Nerves, from the sympathetic trunk or ganglia. They consist largely of non-medullated fibres. They are connected with the spinal nerves through the communicating branches (rami communicantes], and thus receive medullated fibres from, and give non-medullated fibres to, the spinal nerves. They are distributed to the vascular system, the thoracic and abdominal viscera, and to glands. Origin and Termination of Nerves. Since a nerve-fibre originates from a nerve-cell in the (a) brain, (l>) spinal cord, (e) ganglia, nerves may be said to have their origin in nerve-cells, situated in a central organ. At their distal extremity nerve-fibres NERVES. ICQ end in motor end-plates (muscles), electrical organs, secretory cells, other nerve-cells, fine plexuses, or sensory end-organs. Functions of Nerves. The great function of nerves is to conduct nervous impulses to and from nerve-centres (especially the brain and spinal cord), and other parts of the body. At its distal extremity a nerve-fibre has one of two properties : 1. Communicative, transmitting the impulse, or its effect, to some structure, thereby causing some change or modification in it e.g., (a) muscular contraction, (b) secretion, (c) inhibition. 2. .Receptive, receiving impulses as the result of the action of stimuli. These stimuli may be of different kinds (a) electrical, (b} chemical, (c) mechanical, (d) thermal, (e) natural. Natural stimuli can act properly only through end- organs, or the mechanisms at the distal extremities of the sensory nerves ; e.g., the eye and ear. Efferent and Afferent Fibres. A nerve-fibre which conveys impulses from a centre to some part of the body is called an Efferent or Motor Fibre. A nerve-fibre conveying impulses to a centre from some part of the body is termed an Afferent or Sen- sory Fibre. These terms are not strictly synonymous ; for whilst all motor fibres are necessarily efferent, all efferent are not motor : e.g., those conveying impulses to a gland. In like manner, although all sensory fibres must be afferent, all afferent fibres do not convey impulses which give rise to a sensation, or, in other words, are not sensory. Examples of such afferent fibres, not sensory, are seen in those which convey impulses to the spinal cord, thereby causing a reflex action. A nerve composed solely of motor or efferent fibres is called a Motor Nerve ; e.g., third, fourth, sixth, facial, and spinal accessory cranial nerves. One composed only of sensory or afferent fibres is termed a Sensory Nerve ; e.g., olfactory, optic, auditory. A nerve containing both kinds of fibres is a Mixed Nerve. Such are the spinal nerves, and the fifth, vagus, and glosso-pharyngeal cranial nerves. The nature of a particular nerve is shown by observing the effects of stimulation (movement, or sensation, or both) of the cut surface of its distal and proximal ends after section. Anterior and Posterior Roots of Spinal Nerves. Shortly before the spinal nerves join the spinal cord their constituent motor and sensory fibres part company, and enter that structure separately as the Anterior and Posterior Roots. On the posterior root is an enlargement or Ganglion. The motor fibres pass by the anterior roots, the sensory fibres by the posterior roots. This 110 LECTURE XVII. important physiological fact was discovered by Sir Charles Bell (1821) and Majendie (1822). This fact can be readily demonstrated by experiment, for A. When the anterior root is divided 1. The muscles supplied by the nerve cannot be voluntarily contracted, but the parts to which the nerve is distri- buted are still sensible. 2. Stimulation of the distal portion causes the muscles supplied by the nerve to contract. Stimulation of the proximal portion gives rise to no sensations. B. When the posterior root is divided 1. The muscles supplied by the nerve can still be voluntarily contracted, but the sensibility of the structures to which the nerve is distributed disappears. 2. Stimulation of the proximal portion causes evidence of sensation. Stimulation of the distal portion causes no movement. An exception to the preceding statement is seen in what is called recurrent sensibility. This term is applied to the evidence of sensation sometimes observed when the distal stump of the anterior root is stimulated. It appears to be due to some of the fibres of the posterior root turning back into the anterior root soon after they have entered the trunk of the nerve. They soon bend back, and pass into the trunk again. Nutrition of Nerves. This is intimately associated with the connection of the fibres with nerve-cells at their central ends, for when a nerve is divided the distal portion soon degenerates, whilst the proximal portion much less readily does so. In the case of the spinal nerves, it is found that whilst the anterior root is dependent for its nutrition on the connection of its fibres with the spinal cord, the posterior root is quite different, for it depends on the ganglion on the posterior root. Section of the posterior root between the spinal cord and the ganglion causes degeneration of only that small part still connected to the spinal cord, whereas its section on the other side of the ganglion causes degeneration of all the sensory fibres of the trunk. This method of experiment affords a means of tracing the course of the motor and sensory fibres respectively, and, from its dis- coverer, is known as the Wallerian method. Rate of Conduction of Impulses. A nervous impulse travels along a nerve at the rate of 26 metres (frog), 35-40 metres (man), per second. This is shown in motor nerves by causing a NERVES. Ill muscle to contract by stimulating its nerve first close to the muscle, and then as far as possible from it. The difference in the length of time which elapses between the moment of stimulation and the moment of contraction in the two cases is the time taken by the impulse to travel over the length of nerve between the two points stimulated. In sensory nerves it can be shown in the human body by a person indicating with a movement of the hand when he feels a stimulus applied to the skin at a point distant from the brain (say the foot), and also at a point nearer (say the thigh) the brain. The difference in time between the application of the stimulus and the movement in the two cases indicates the time taken by the sensory impulse in passing along the length of nerve from the foot to the thigh, from which the rate can easily be determined. The personal equation, or the time taken by the mind to convert the sensation into a volition, is in both cases the same, as also is the time taken by an impulse in passing from the brain to the hand. Electrical Phenomena of Nerve Nerve Currents. In a divided nerve a current passes from the surface to the cut edge outside the nerve ; in other words, the cut surface is electrically negative to the sound. This electrical current undergoes a change when an impulse passes along it, viz., by becoming less, or it undergoes a negative variation. The phenomena are essentially similar to those observed in muscle at rest and in contraction (vide Lecture VII.). This electrical alteration is the only known change occurring in a nerve when it is transmitting an impulse. A current passed transversely through a nerve does not excite it. When a constant current passes along a nerve the natural nerve current in the same direction as this current is increased, that in the opposite direction is diminished. Electrotonus . When a constant electric current is made to flow along a portion of a nerve its irritability is increased in the neighbourhood of the negative, and decreased near the positive, pole. The nerve is not excited so long as the intensity of the current does not change. LECTURE XVIII. NERVE CENTRES THE SPINAL CORD. The Spinal Cord Shape. It is elongated and cylindrical. In the regions of the neck and loins it is somewhat larger, forming the cervical and lumbar enlargements. Extent and Size. From 15 to 18 inches in length, it extends from the medulla oblongata above to the first lumbar vertebra below. Its lower termination is conical in shape, the apex of the cone being opposite the first lumbar vertebra. It is continued below this as a slender filament the filum terminate. This terminal filament is surrounded by the sheath of the cord as far as the upper part of the sacrum ; then, passing through the dura mater, it is attached below to the coccyx. The spinal cord is rather broader from side to side than from front to back. It measures rather more than one-third of an inch in diameter, except in the cervical and lumbar enlargements, where it is about half- an-inch across. Membranes. There are three, continuous with those surround- ing the brain : i. The Dura mater, continuous with the membrane lining the skull, hangs freely in the vertebral canal, thus forming a loose investment of the cord. At its lower part there are some small fibrous connections with the vertebrae. It extends some distance beyond the apex of the cone to the upper part of the sacrum. After the roots of the spinal nerves have passed through the dura mater it is continued on the nerve becoming continuous with its sheath. Between the dura mater and the wall of the vertebral canal are connective tissue and fat, and a plexus of veins. ii. The Pia mater is a thin, delicate, fibrous, vascular membrane continuous with that investing the brain. It is, however, not so vascular, is thicker and more closely attached to the substance of the cord. It dips into the anterior THE SPINAL CORD. 113 and posterior fissures, especially the former. It is continued on the roots of the nerves becoming lost in their connective tissue sheaths. iii. The Arachnoid is a very fine, delicate membrane between the two preceding. Between it and the pia mater is a considerable space the subarachnoid space continuous with a similar space in the skull, and which is continuous with the ventricles of the brain by the foramen morgagni. In the subarachnoid space is a liquid the subarachnoid fluid. Fine bundles of fibres pass across this space, connecting the arachnoid with the pia mater. An imper- fect partition the ligamentum denticulatum divides the sub- arachnoid space into an anterior and posterior half. It passes from the pia mater between the anterior and posterior roots of the spinal nerves, and is attached by the denticulations to the dura mater ; below it becomes fused with the filum terminale. The ligamentum denticulatum thus fixes the spinal cord in a central position. Fissures. The spinal cord is divided almost completely into two symmetrical halves by two deep fissures, one in front, the other behind. They are known as the Anterior median and Posterior median fissures. Along each half of the cord, on a line with the posterior roots, is a groove. Columns. Each half of the cord is divisible by this groove, and the line of origin of the anterior roots into three columns : (i) Anterior, (2) Lateral, and (3) Posterior. Minute Structure. The spinal cord consists chiefly of nerve- fibres and nerve-cells having a definite and characteristic arrange- ment. Besides these essential elements, there is a certain amount of interstitial and supporting connective tissue. A transverse section of the spinal cord shows it to consist of two distinct parts 1. External White Matter, consisting chiefly of nerve-fibres. The two halves of the cord are connected by a certain amount of this matter, passing across the bottom of the anterior fissure, forming the White or Anterior Commissure. Surrounding and passing between the fibres is a fine network, or neuroglia. 2. Internal Grey Matter, containing large numbers of nerve- cells. It is somewhat crescentic in shape in each half. From the ends of the crescent the cornua or horns spring the roots of the spinal nerves. The two halves are connected by a thin strand of grey matter at the bottom of the posterior fissure. It is called the Grey or Posterior Commissure, and in its substance is a 114 LECTURE XVIII. fine canal the Central Canal of the cord. This canal is lined with a layer of columnar ciliated cells. Above it communicates with the fourth ventricle of the brain. Localisation of Nen>e-cells in the Grey Matter. The nerve- cells are especially aggregated in three longitudinal groups i. In the anterior cornu forming the Motor Ganglionic Column. ii. Clarke's or Posterior Vesicular Column, at the inner surface of the base of the posterior cornu. It is best marked in the dorsal region, but extends a short distance up into the cervical and down into the lumbar regions. iii. Column of the Intermedio-Lateral Tract is situated in the middle of the concavity between the two horns. It is present only in the dorsal region. Origin of the Spinal Nerves. Each spinal nerve springs from two roots, anterior and posterior. They pierce the dura mater separately, and the trunks of the nerves leave the vertebral canal by the intervertebral foramina. The fibres of the roots spring from the respective cornua of the grey matter. The exact course pur- sued by the fibres after entering the cornua is not thoroughly known. Some end directly in the cells of the anterior cornua ; others pass to the opposite cornua ; and others again find their way by the commissures to the other half of the cord. Functions of the Spinal Cord The two great functions of the spinal cord are (i) the con- duction of impulses to and from the brain and spinal nerves, and to and from different parts of the cord ; (2) a centre of reflex action. These two functions are correlated with its structure, for whilst the former is largely, if not wholly, the property of the white matter, the nerve-cells of the grey matter alone form the reflex centres. A. Conductor of Impulses The spinal cord conducts nervous impulses to and from the brain and spinal nerves. This fact, and the paths followed by the impulses are shown by the following experiments 1 . Transverse section of one-half of the cord is followed by a. Loss of Voluntary Movement, or Paralysis, on the same side in all parts supplied with nerves originating below the point of section. b. Loss of Sensation, or Anaesthesia, on the opposite side of all parts supplied with nerves from belowthe point of section. 2. Longitudinal median section causes THE SPINAL CORD. 115 Loss of Sensation on both sides of all parts supplied with nerves from that part of the cord cut, but no paralysis is produced on either side. These Experiments show that 1 . Motor Impulses pass down the cord on the same side as that from which they are emitted. 2. Sensory Impulses, as soon as they enter the cord, pass over to the other side, by which they pass up to the brain. There has been much difference of opinion regarding the exact paths in the white matter of the nervous impulses. Pro- bably the impulses pass up and down by the lateral columns a view supported by the fact that they form the larger portion of the white matter, and also by their area increasing from bottom to top to an extent corresponding to the united sectional area of the spinal nerves. If this view is correct, the anterior and posterior columns are probably conductors between different parts of the cord. The area of the grey matter varies according to the thickness of nerves given off. Hence it is much greater in the cervical and lumbar thickenings. This points to its intermediating directly between motor and sensory fibres entering it, or being segmental in its character. B. Reflex Action A reflex action is essentially the conversion of afferent impulses into efferent ones by nerve-cells independently of the will or con- sciousness. A reflex mechanism therefore consists of three parts (i) an afferent nerve, (2) an efferent .nerve, (3) a centre connecting i and 2. It is in virtue of the spinal cord containing many of these centres that it plays so important a part in reflex actions. A Nerve-centre, here as elsewhere, may be defined as a collection of nerve-cells endowed with the power of controlling or modifying some action or function of the body, either auto- matically or as the result of the arrival at it of afferent impulses. In the modus operandi of the reflex mechanism the resulting act is dependent on the strength of the afferent impulses, but more so on the nature of the changes taking place in the reflex centre. Reflex actions are very common in the human body, and are of a very useful and purposeful nature. The following examples are indicative of this : Winking, coughing, laughing, swallowing, vomit- ing, movements from irritation of the skin by an injurious object, contraction of pupil by light. Primary and Secondary Reflex Actions. Such acts as swallow- Il6 LECTURE XVIII. ing, coughing, sucking, c., are performed independently of volition from the earliest period of life, and are sometimes called Primary Reflex Actions, in contradistinction to those acts which for their performance in the first instance require an effort of will, or even repeated volitions, amounting in some cases to an education. Such actions, only after a time becoming reflex, are termed Secondary or Acquired Reflex Actions. Walking and other more complicated movements are examples. The evidence showing the spinal cord to be a seat of reflex action is of two kinds i. Experimental. It is well and easily demonstrated in a frog from which the brain has been removed. The animal is destitute of all voluntary movement, but a. If the flank be tickled, a contraction of the flank muscles of that side will take place. b. If one side be sharply pinched, the leg of the same side will be drawn up and swept over the spot pinched. c. A piece of blotting-paper moistened with strong acetic acid and placed on the flank will cause the leg of the same size to be raised and swept over it so as to remove the paper. d. (i.) If the animal be suspended by a hook and the tip of one of the toes be gently pinched, the leg is at once drawn up. (ii.) If one of the toes be dipped in dilute sulphuric acid, the same result after a time occurs. This experiment is explainable on the assumption that the centre stores up weak impulses, each oneof which is individually not strong enough to cause the discharge of an efferent impulse, until their combined influence is sufficient to cause the movement. This is called a Summation of stimuli. 2. Pathological. If by accident or disease the spinal cord is severed from the brain above, stimulation of afferent nerves, although producing no effect on the consciousness of the indi- vidual, causes distinct and very complex movements. Modification of Reflex Action. The importance of the centre as a part of the reflex mechanism is strikingly shown by the pro- found modification of certain reflex acts produced by nervous impulses affecting the condition of the centres concerned in their production. This modification may be in the direction of stop- ping or slowing the action (inhibition}, or of quickening or intensifying it (augmentation). Inhibition. The following illustrate this : THE SPINAL CORD. 117 a. When the Brain of a frog is removed reflex action takes place more readily than before. Hence it is assumed that the brain by sending down impulses to the centres of the spinal cord inhibits them. The greater ease with which reflex action occurs in sleep illustrates the same point. b. Crystals of sodium chloride placed in the Optic lobes of a frog retard reflex action. c. Strong stimuli applied to any sensory nerve may inhibit reflex action. The action is analogous to the Inhibition of the Heart-beat caused by stimulation of the vagus nerve (vide Lecture X.). Cold lengthens the time of a reflex action, no doubt, by diminishing the excitability of the centre. Potassium Bromide, Hydrate of Chloral, and Atropin act in an analogous manner. Augmentation. The possibility of this modification is inferred from the remarkable effect of Strychnin. This poison causes the excitability to be so increased that the slightest afferent impulse causes all the reflex centres in the cord to discharge efferent impulses, so that the body is thrown into convulsions. The action may be compared to that of the accelerator nerves on the beat of the heart (vide Lecture X.). Time taken for Reflex Action. The changes in the centre occupy '0555 to '0471 second. Ganglia. In addition to the sympathetic ganglia already referred to, there are four ganglia in the skull. These are con- nected with the sympathetic system, and are hence sometimes included in it. They are Ophthalmic or Ciliary Ganglion, situated in the orbit. It is very small, no larger than a pin's head. It has a motor root from the third nerve, a sensory root from the fifth (nasal branch of the ophthalmic division). It is also connected with Meckel's gan- glion and with the cavernous plexus, and sends ten to twelve short ciliary nerves to the iris and ciliary muscles. Spheno-palatine, or Meeker s Ganglion, the largest of the four, triangular in shape, of a reddish colour, is situated in the spheno- maxillary fossa, near the spheno-palatine foramen. Its motor root is from the facial nerve, its sensory from the fifth (spheno-palatine trunk). From it nerves pass to the periosteum of the orbit, to the gums, hard and soft palate, uvula, tonsils, mucous membrane of nostrils and part of pharynx, middle auditory meatus. Otic Ganglion, oval in shape, is situated just below the foramen ovale. Its motor root is from the facial, and its sensory from the Il8 LECTURE XVIII. fifth and ninth. It sends nerves to the mucous membrane of the tympanum and Eustachian tube, tensor tympani and palate, and to the cavernous plexus. Submaxillary Ganglion, situated on the submaxillary gland, is roundish and reddish-gray in colour. Its motor root is from the corda tympani branch of the facial, its sensory from the lingual branch of the fifth, nerve. It sends nerves to Wharton's duct, mucous membrane of the mouth, and to the submaxillary gland. Physiological Classification of Nerve Centres. Nerve Centres may be said to be of two kinds : (i) Reflex, (2) Automatic. The former are well seen in the spinal cord. It is doubtful whether the latter are present at all in that structure. It is doubtful whether either reflex or automatic centres are present in the ganglia of the sympathetic or in those just referred to. Considering the presence in these ganglia of nerve-cells and their connection with so many nerves, it would be expected, a priori, that they would act as centres. There is, however, very little conclusive experimental evidence on the point. The same ap- plies to the much smaller ganglia found in the heart, intestines, &c. The Submaxillary ganglion has been supposed to be a centre of re- flex action ; the movement of the Heart to be due to the automatic activity of the microscopic ganglia in its muscular substance ; the peristaltic action of the Intestine to microscopic ganglia in its walls ; the contraction of the arteries to nerve-cells in the vascular walls. In none of these cases, however, can the evidence be said to be absolutely conclusive, and the question as to how far these ganglia are nerve-centres reflex or automatic requires further elucidation. LECTURE XIX. NERVE-CENTRES THE BRAIN. The Brain. White and grey nervous matter arranged so as to form an organ of great complexity constitutes the brain, or en- cephalon. Roughly speaking, the brain may be said to consist of: i . The Cerebrum ', or large brain. 2.. The Cerebellum, or small brain. 3. The parts connecting these with the spinal cord. Of these the most important is the Medulla oblongata, or the small portion directly continuous with the spinal cord. General Arrangement of Parts. A general idea of the arrangement of the several parts of the brain can perhaps best be obtained by tracing the course of the nerve-fibres from the spinal cord below to the surface of the brain above. The fibres of the spinal cord pass up into the Medulla oblon- gata, which is its direct continuation. In their course, imme- diately above the medulla, the fibres are interrupted by others placed transversely. These cross fibres connecting the two halves of the Cerebellum are sufficiently numerous to cause a considerable projection just above the medulla. This projection is called the Pons Varolii. Above the pons varolii the fibres spread out somewhat, so as to form two large masses diverging slightly from one another. These are the Peduncles of the Brain, or Crura Cerebri. The crura cerebri lose themselves largely in two masses on either side containing a great deal of grey matter. These so-called basal ganglia are placed one in front of the other. The anterior are the Corpora striata, the posterior the Optic tkalami. Some of the fibres pass directly to the surface through the Internal capsule, situated between the optic thalamus and the outer nucleus of the corpus striatum. From the basal ganglia fibres radiate, forming the main in- ternal white matter of the Cerebral Hemispheres, and terminate in the convoluted layer of grey matter forming the Convolutions. I2O LECTURE XIX. The most important parts of the brain may be said, then, to be four, viz. : (i) Medulla oblongata, (2) Cerebellum, (3) Basal ganglia; (4) Convolutions of the Cerebrum. It will be most convenient to consider first the structure of these several parts, and then their functions. Structure of Parts- Medulla Oblongata is pyramidal in shape, with its broad end upwards. Its dimensions are : Length, i ~ inch ; breadth, i inch ; depth, | inch. In structure it roughly resembles the spinal cord, having anterior and posterior fissures, internal grey and external white matter. The anterior fissure extends up to the pons varolii. The posterior fissure becomes lost above by the back of the medulla opening out to form the lower half of the fourth ventricle of the brain a space continuous below with the central canal of the spinal cord. On the floor of the fourth ventricle the grey matter of the medulla is exposed to view. The roof of this part of the fourth ventricle is formed by a thin membrane consisting of pia mater lined with a layer of epithelium. On either side of the anterior fissure, just below the pons, are two slight projections of white substance called the anterior pyramids. These are of great importance, because in them a considerable number of the fibres coming up from the spinal cord cross to the other side before proceeding to the higher parts of the brain. This crossing of fibres is known as the Decussation of the Pyramids. From each side of the medulla a large bundle of fibres, the Restiform bodies, passes to the cerebellum constituting its inferior peduncles. Cerebellum (Hind or Little Brain]. Dimensions, 4 x 2\ x 2 inches. It consists of a body and three pairs of peduncles, or crura, by which it is connected with the rest of the cerebro-spinal axis. The Peduncles are (i) superior to the cerebrum, forming the sides of the upper half of the fourth ventricle ; (2) inferior to the medulla (restiform bodies) ; (3) middle to the pons varolii. All these pass into the cerebellum at its fore part. The Body is composed of two lateral Hemispheres joined to- gether by a central portion, the Vermiform process. Posteriorly the hemispheres are separated by a depression. The Surface is laminated or foliar, the lamince being nearly parallel. Structure. Externally there is a layer of grey matter; the white matter within has a curiously branched arrangement, hence THE BRAIN STRUCTURE. 121 the term arbor vittz applied to it. Within the white matter is a small nucleus of grey matter, from its irregular contour termed the Corpus dentatum. The grey matter of the surface con- sists of an outer layer containing small cells and fibres, and an inner or granule layer, largely composed of closely -packed, granule-like cells, ^nrth to ^-Vo^h inch in diameter. Between these two strata is a single layer of large, highly characteristic cells, the cells of Purkinje. They are pear-shaped, T/o^" tn to T(Tbir tn inch in diameter, and send processes into both the internal and external layers. In the latter the processes are large and much branched, the branches being connected with the small cells there present. Pons Yarolii forms a projection immediately above the medulla, due to its containing, in addition to the longitudinal fibres on their way to the upper parts of the brain, transverse fibres connecting the two hemispheres of the cerebellum. The transverse fibres are external. Internally there is a strand of grey matter continuous with that of the medulla. Its posterior surface forms the floor of the upper half of the fourth ventricle, the roof here being formed by the valve of Vieussens. Crura Cerebri. Each peduncle is divisible into two parts : (i) the Crust a, or anterior; (2) the Tegmentum, or posterior portion. Between the two is a mass of grey matter, the Sub- stantia nigra. The crusta is composed of fibres coming up from the medulla (pyramids), terminating largely in the internal capsule. The tegmentum is composed both of grey matter continuous below with that of the pons and medulla, and of fibres passing above into the optic thalamus. In it also are the fibres from the superior peduncle of the cerebellum. Between the diverging peduncles are the posterior perforated space and the corpora mamillaria. Corpora Quadrigemina. Behind the crura cerebri, sepa- rated from them by the aqueduct of 'Sylvius , are four hemispherical masses of grey and white matter, arranged in two pairs, anterior and posterior, called the corpora quadrigemina. They correspond to the optic lobes of the brain of lower vertebrates. The anterior pair is connected by fibres with the optic tract. Basal Ganglia Corpus striatum is situated in front and to the outer side of the optic thalamus. It is a large ovoid mass of grey matter streaked with white (hence the name). The greater part of it is embedded in the white substance of the hemisphere, whilst a portion of it comes to the surface in the lateral ventricle. There 9 122 LECTURE XIX. is thus an intra-ventricular (nucleus caudatus] and an extra-ventri- cular portion (nucleus lenticularis]. Between the latter and the optic thalamus is a mass of white matter called the Internal Cap- sule ; it is composed of fibres passing direct from the crus to the convolutions of the cerebrum. Thalamus opticus is oval in shape and rests on the peduncle of the cerebrum. The upper surface is white, free, and prominent : in front it is seen in the lateral ventricle. It forms the side of the third ventricle, the grey matter of the two thalami being connected across that cavity by the soft commissure of the third ventricle. Cerebral Hemispheres When looked down upon from above, the cerebrum or large brain covers all the rest of the encephalon. It consists of two symmetrical halves separated by the great longitudinal fissure. This fissure passes quite through to the base, both before and behind. In the middle it is interrupted by a large cross mass of white substance, or Corpus callosum, which connects the two hemispheres together. The great internal mass of the cerebrum consists of white matter ; the layer of grey matter on the surface is often called the Cortex. Surface. It is convoluted, being folded upon itself to form the Convolutions, or gyri, marked off from one another by depres- sions, or Sulci. There is thus a great increase of surface, and consequently of grey matter, without increase of size. Into the sulci passes the Pia mater, a fine vascular membrane closely investing the surface of the brain, and continuous with the mem- brane of the same name on the spinal cord (vide Lecture XVIII.). Lobes. The surface is again marked off into five lobes by deeper depressions, or fissures. The lobes are: (i) Frontal, or anterior; (2) Parietal, or middle; (3) Occipital, or posterior; (4) Tempora-sphenoidal, or lower; (5) Central, or island of Reil. Fissures. There are three primary or interlobar fissures : 1. Fissure of Sylvius, between the temporal and frontal lobes. 2. Fissure of Rolando, between the frontal and parietal lobes. 3. Parieto-occipitai ', between the two lobes indicated by the name. Structure. The internal white matter consists of great num- bers of fibres diverging in a fan-like fashion, hence termed the Corona radiata. Also of commissural fibres connecting different parts of the cortex of the same hemisphere, and fibres passing to the other hemisphere by the corpus callosum. In the external grey matter, ^th to Jth inch in thickness, several strata can be dis- THE BRAIN STRUCTURE. 123 tinguished of alternate lighter and darker colour, due to the different nature and arrangement of the constituent nerve-cells. The most characteristic cells are those forming a thick stratum in the middle part. They are pyramidal in shape, with a large nucleus. They measure ygWth mcn across at the base. The cells of the cortex differ in size in different parts, an interesting fact in connection with the question of the functions of the cortex (vide Lecture XX.). For example, in the region of the fissure of Rolando (motor centres) the cells are large ; in the occipital lobe (sensory region) they are smaller. The cortex is very abundantly supplied with bloodvessels. Membranes of the Brain Dura mater is a strong, thick connective-tissue membrane, closely attached to the inner surface of the skull. It sends three projections or partitions inwards into the cavity of the skull, viz., (l) the Falx cerebri^ vertically between the two cerebral hemi- spheres, extending nearly to the corpus callosum ; (2) the Tento- rium cerebelli, horizontally between the cerebrum and cerebellum ; (3) the Falx cerebelli, vertically between the hemispheres of the cerebellum. The dura mater consists of two distinct layers, which, becoming separated in certain definite positions, leave spaces termed Sinuses, containing blood. There is one along the upper border and another along the lower border of the falx cerebri (superior and inferior longitudinal sinuses). Along the line of attachment of the tentorium cerebelli to the occipital bone is the lateral sinus, and of the falx cerebelli the posterior occipital sinus. Internally the dura mater is lined with a layer of epi- thelioid cells. Pia mater is a delicate, highly vascular membrane, closely investing the surface of the brain. It dips into all the sulci. It also passes into the interior of the brain (at what is called the transverse fissure) to the lateral ventricles, over the third ventricle, forming the velum interpositum. It forms also the choroid plexuses. Similarly below it forms the roof and choroid plexuses of the fourth ventricle. Arachnoid is for the most part in contact with the pia mater, but it does not pass down into the sulci. The space between the dura mater and arachnoid is called subdural ; that between the arachnoid and pia mater the sub- arachnoid. The latter communicates with the ventricles of the brain. It contains a liquid, the cerebro-spinal fluid. Ventricles of the Brain 124 LECTURE XIX. The Fourth ventricle, bounded in front by the medulla oblon- gata and pons varolii, and behind by the pia mater and valve of Vieussens, ends above in a small canal, the aqueduct of Sylvius, situated between the corpora quadrigemina behind and the crura cerebri in front. At its other extremity this canal opens into a cavity, the Third ventricle. The sides of the third ventricle are formed by the optic thalami, the floor by the infundibulum, a funnel-shaped structure, the roof by the velum interpositum and pineal gland. The third ventricle communicates in front by a Y-shaped aperture, the foramen of Monro, with two large cavities, one in each half of the cerebrum. These are the Lateral ventricles. Each lateral ventricle, besides a central part, or body, situated beneath the corpus callosum, has an anterior cornu descending into the frontal lobe, a descending cornu, and a posterior cornu in the occipital lobe. The Fifth ventricle is a small cavity situated in the septum lucidum, or delicate double partition between the two lateral ventricles, extending from the corpus callosum above to the body of the fornix below. The Fornix is a longitudinal mass of fibres connecting the ante- rior and posterior parts of the brain. It is a longitudinal, as the corpus callosum is a transverse, commissure. Its body rests upon the velum interpositum, and proceeding from it in front are two ante- rior pillars which arch downwards, and \.\^Q posterior pillars behind, passing down into the descending cornua of the lateral ventricles. Development An acquaintance with the development of the brain makes it easier to understand and remember the somewhat complicated arrangement of its parts. The brain is developed from what are known as the cerebral vesicles in the embryo. They are three in number : anterior, middle, and posterior. These vesicles are transformed into the fore, mid, and hind brain respectively, and the remains of their cavities form the ventricles of the brain. Anterior Vesicle. From this are developed the different parts of the fere brain. Two hollow prolongations are formed in front, from which are developed the cerebral hemispheres. A secondary budding gives rise to the olfactory lobes. The outer and under walls of these prolongations thicken to form the Corpora striata. Their cavities form the lateral ventricles. The body of the original vesicle forms the third ventricle and structures surround- ing it, viz., from THE BRAIN STRUCTURE. 125 Floor, the Infundibulum. Roof, the Pineal Body. Sides, thicken greatly to form the Optic thalami. Middle Yesicle is modified to form the Mid brain. Roof forms the Corpora quadrigemina. Floor forms the Crura cerebri. The Cavity is transformed into the aqueduct of Sylvius. Posterior Yesicle develops into the hind brain, consisting of the Medulla oblongata and Pons varolii in front, and the Cere- bellum behind. Its cavity becomes t\\e fourth ventricle. Comparative Anatomy In the lowest-known vertebrate (Amphioxus) there is no difference observable between the spinal cord and brain. The Optic Lodes (mid brain) are highly developed in Fishes and Amphibia, being as large as, or even larger than, the cerebral hemispheres. In Reptiles and Birds, although still considerable structures, they are by no means so prominent as the cerebrum, This is specially so in Birds. In Mammals this portion of the mid brain is reduced to very small dimensions. Four bilaterally- placed small rounded elevations, the Corpora Quadrigemina, repre- sent it. The Cerebellum is represented in the lowest Fishes and Am- phibia by a narrow band. In Reptiles it is a little larger, and in Birds is of considerable size and laminated externally ; it is the central part only (vermiform process), and hence there is no pons varolii. It is, however, quite uncovered by the cerebrum. In Mammals it becomes more complicated as we rise in the series. The Cerebral Hemispheres increase in size as we rise in the vertebrate scale. In Mammals the changes are chiefly connected with the increased development and complexity of the cerebral convolutions. As they increase in size the olfactory lobes diminish. At the same time the cerebrum extends backwards until the cere- bellum is completely covered by it. The surface, at first plain, becomes convoluted, and (with reservations) the convolutions may be said to increase in number, depth, and complexity as the intelligence of the animal increases. The increase in the number of the convolutions indicates an increase in the quantity of grey matter. Male and Female Human Brain. The comparison of the weights of the brain and body of a great number of adults of both sexes, and the study of these relations at different stages of life, go far to support the view that the difference in weight between 126 LECTURE XIX. the male (48*2 oz.) and the female (43*09 oz.) is simply an expres- sion of the difference in weight of the bodies. Cranial Nerves. There are twelve on either side from before back, as follows : 1. Olfactory (nerve of smell). 2. Optic (nerve of sight). The roots of the two nerves join soon after leaving the skull, forming the optic chiasma. 3. Oculo-motor y to all muscles of eye-ball except external rectus and superior oblique. To circular muscular fibres of iris and to ciliary muscle. 4. Trochlearis, to superior oblique muscle. 5. Trigeminus, a great mixed nerve arising, like a spinal nerve, by two roots a motor and a sensory. On the latter is a ganglion (Gasserian ganglion). It divides into three great branches: (i) Ophthalmic; (2) Superior maxillary ; (3) Inferior maxillary. 6. Abducens, to external rectus muscle. 7. Facial, to muscles of the face ; is hence nerve of expression. 8. Auditory (nerve of hearing). 9. Glossopharyngeal, a mixed nerve to tongue and pharynx (nerve of taste). 10. Pneumogastric, or Vagus, a mixed nerve; efferent fibres to pharynx, larynx, oesophagus, stomach, intestines, heart (inhibitory) ; afferent fibres to air-tubes, pharynx, gullet, stomach, vasomotor and respiratory centres. i T. Spinal Accessory, to stern o-mastoid and trapezius muscles. It receives sensory fibres from cervical nerves. Part of it joins the vagus, giving it efferent fibres. 12. Hypoglossal, to muscles of tongue and most of muscles connected with hyoid bone. It is connected with the fifth and vagus, three upper cervical, and sympathetic nerves. LECTURE XX. NERVE-CENTRES FUNCTIONS OF THE BRAIN. Functions of the Brain. The functions of the brain are very varied and of the highest importance; for besides being the seat of volition and the psychical faculties, the most important functions of the body are dependent upon and are controlled by nervous impulses descending from this portion of the cerebro- spinal axis. The proofs that the brain is the organ of mind have been well summarised by Bain as follows : " i. The physical pain of excessive mental excitement is localised in the head. 2. Injury or disease of the brain affects the mental powers. 3. The pro- ducts of nervous waste are more abundant after mental excite- ment. 4. There is a general connection between the size of brain and mental energy. 5. By specific experiments on the brain and nerves, it is shown that they are indispensable to mental functions." Medulla Oblongata. This small pardon of the brain is of importance in three ways : 1. As a conductor of impulses. All impulses passing up and down between the spinal cord and the higher parts of the brain must pass through the medulla. Many of the descending motor impulses cross by the decussation of the pyramids. Hence it follows that injury to one side of the brain causes paralysis on the opposite side of the body. 2. All the Cranial Nerves ; except the olfactory, optic, third and fourth, have their ultimate or deep origin in the grey matter of the medulla oblongata. Of these the Vagus, or Pneumogastric , is the most important, for it is most intimately associated with the three important functions of circulation, respiration, and digestion. 3. It is the seat of very important nerve-centres. Amongst these are the centres presiding over and controlling the functions of circulation and respiration (respiratory and vasomotor centres], Injury to a certain small part of the medulla between the vaso- motor centre and the calamus scriptorius causes respiration to 128 LECTURE XX. cease. The heart's beat is usually at the same time stopped, due to the vagus nerve being connected with the medulla close to this point (cardio-inhibitory centre]. Death in consequence instan- taneously ensues on this spot being injured. For this reason it was termed the nceud vital by Flourens. The vasomotor centre extends for about 3 mm. on the floor of the fourth ventricle, from about 5 mm. above the calamus scriptorius to within 2 mm. of the corpora quadrigemina. Besides the foregoing centres there are others connected with Swallowing, Digestion, and Secretion. Cerebellum Physiological experiment (as by injury, removal, and electri- cal stimulation) and the facts of comparative anatomy and disease point to the cerebellum as being a centre for the co- ordination of movement necessary for the maintenance of equilibrium. a. Injury. Injuries cause the movements to be disorderly, and a greater effect is produced when they are lateral or unsym- metrical. Lateral injury gives rise to lateral movements. Injury to the anterior or posterior median portions causes the animal to fall forwards or backwards respectively. b. Removal. The centre is deep-seated, for the greater the amount removed the more lasting the effect. c. Electrical Stimulation. Ferrier has shown that electrical stimulation of the surface causes movements of the eyes, and sometimes of the head and limbs. He considers that his results " are such as to bring out the relation of the ocular movements to those necessary for the adjustment of equilibrium generally ". They also support the view that the centre in the cerebellum is affected by visual impressions. Physiological experiment further goes to show that the centre is influenced by afferent impulses from the internal ear (semicircular canals], irritation of these structures causing similar results to direct irritation of the surface of the cerebellum. Lastly, the centre is affected by tactile and muscular impressions. d. Comparative Anatomy. A comparison of different animals points to the conclusion that the cerebellum is developed in pro- portion to the variety and complexity of the movements of which the animal is capable. e. Disease. Disease of the cerebellum has been observed to be associated with difficulty in co-ordinating the movements necessary for the maintenance of equilibrium. Corpora Quadrigemina THE BRAIN FUNCTIONS. I2Q In the animal scale the development of the optic lobes varies directly as the spinal cord, and inversely as the cerebral hemi- spheres. Experiments by injury and electrical stimulation have failed to thoroughly demonstrate the functions of the corpora quadrigemina. There seems little doubt, however (as their con- nection with the optic tracts would indicate), that they are in some subordinate manner connected with vision, or at least with visual impulses. These visual impulses are thereby associated with the production of certain movements, e.g., those of the iris and of accommodation. Basal Ganglia It has been supposed that the corpus striatum is a motor centre for the emission of motor or efferent impulses, and the optic thalamus a sensory centre for the reception and elaboration of sensory or afferent impulses. In accordance with this theory these ganglia have been regarded as the centres in the cerebrum presiding over those manifestations of activity which take place independently of consciousness and volition; in other words, form a great reflex centre. Such actions are termed senson- motor, in contradistinction to those taking place consciously when the convolutions are called into request, and known as ideo-motor. The evidence, both experimental and pathological, bearing upon this theory is inconclusive. This evidence, however, to- gether with their anatomical relations, points to the corpus striatum being more particularly associated with motor and the optic thala- mus with sensory impulses. The localisation of distinct sensory centres in the optic thalamus (Luys) is not generally accepted. Possibly the optic thalamus is a centre of sub-conscious sensation, and the corpus striatum a motor centre corresponding to the motor region of the cortex, in which the different individual parts are less specialised. Internal Capsule. Section of the anterior two-thirds causes paralysis of motion, of the posterior one-third loss of sensation on the opposite side. Cerebral Hemispheres These are considered the seat of those faculties termed mental. Physiology and pathology agree in pointing to conscious- ness being inseparable from the activity of the cerebral hemi- spheres. The definite determination of the functions of this important part of the brain has been attempted in several ways : 130 LECTURE XX. I. Observing the Effect of Complete Removal. Generally it may be said that this is followed by absence of sensation, ideation, and volition. The animal is reduced to a mere machine, its activity being called forth only as the result of external stimulation. a. In a Frog. Its movements are just like those of a normal frog, but an external stimulus is required for their production. It maintains its normal position, moves when touched, croaks when its flanks are stroked, swims when thrown into water, and is affected by different degrees of illumination. From this it is evident that there is in some part of the brain, other than the cerebral hemispheres, a central nervous mechanism by which these complex movements can be performed, only some afferent stimulus is required to set this mechanism going. And we may assume as probable and plausible that in the normal animal voluntary movements are produced by this mechanism receiving impulses from the cerebral hemispheres above, and being thus set in action. b. In a Bird. An absence of volition is observed, an ex- ternal stimulus being required for the production of movement. c. In a Mammal. Whilst volition is absent, movements are caused by tactile, visual, and auditory stimuli. It moves when touched, avoids objects casting a shadow, and follows a strong light with its head, and is affected by sounds. These facts are evidence, if at all, of sensation in only a very restricted sense of that term, for although stimulation of the sensory organs may result in movements, they are probably more akin to reflex actions than anything else. That the sensory impulses do not give rise to perception and ideas is evident from the fact that the animal is unaffected by food or any pleasant or terrible object. The result of this mode of experimentation is negative, and applies to the cerebral hemispheres as a whole. The question may be asked, Does the cerebrum act as a whole, or have certain parts certain definite functions ? Is there, in a phrase, a division of labour in the cerebral hemispheres ? Owing to the great influence of Flourens the view was long held that the cerebral hemispheres did act as a whole, and that removal of any part simply caused enfeeblement of all the functions. Scepticism of the justness of Flourens' conclusion has gradually increased, until at present it is very largely rejected. Facts which have tended to this end are (i) The very general association of aphasia with a definite part of the cerebral surface. (2) The association of limited paralysis THE BRAIN FUNCTIONS. 13! with limited injury. (3) The connection between epileptiform convulsions and the irritation of certain convolutions. (4) The effects of electrical stimulation of the cortex. II. Electrical Stimulation of the grey matter of the convolu- tions. The experiments of Hitzig and Fritsch and of Ferrier have shown that there is a distinct connection between electrical stimulation of the cortex and movements. Stimulation of certain definite areas of the cortex by an electric current is followed by definite movements involving the co-ordination of several distinct muscular contractions, and are similar to those produced as the result of volition. Ferrier, as the result of his numerous and elaborate experi- ments, has mapped out a considerable portion of the cerebral surface into series of well-defined areas, electrical stimulation of each of which gives rise to a definite movement. These areas he terms motor centres. They are chiefly situated around the fissure of Rolando, and the portion of the surface embracing them is known as the motor region. These motor centres have a definite and interesting relation to one another, as regards the movements with which they are associated, viz., that the lowest centres are those for the movements of the mouth, tongue, and face ; next above these come the centres for the upper limbs, and at the top those for the lower limbs ; in the marginal convolution within the great longitudinal fissure are centres for the muscles of the trunk. Notwithstanding Ferrier's remarks that " the reactions pro- duced by electrical stimulation of the cortex are definite and predictable, and vary with the position of the electrodes areas close together give different results, certain areas no result," his theory of localisation of function in the convolu- tions, as far as motor centres are concerned, must be taken only in a restricted sense. From I., a. b. c., it is seen that the most complex and elaborate movements may take place in the absence of the cortex, provided there be suitable external stimuli. It is, therefore, plain that the cortex cannot contain motor centres in the sense of being essential to the production of the particular movements with which electrical stimulation shows them to be associated. We may assume, however, that they are the centres called into activity when those movements are pro- duced voluntarily, and to them the term psychomotor (which Ferrier has suggested) would be very appropriate. If the conclusion that these areas are motor centres is correct, 132 LECTURE XX. we should expect that removal of them would cause paralysis of the movement resulting from their electrical stimulation. III. Removal of definite parts shows that such is the case. When an area corresponding to a motor centre is destroyed, paralysis of the movements caused by its electrical stimulation ensues. Removal of the whole motor region causes general paralysis. This applies to animals, especially the monkey. Of the human body Ferrier says : " There is not on record a single unequivocal case of destruction involving the motor area unac- companied by paralysis of greater or less extent or limitation ; while, on the other hand, there is not another region of the brain which has not been the subject of destructive lesion times without mention, without any motor paralysis whatever ". In some animals the paralysis, after a time, largely, if not al- together, disappears. The recovery is not due to the vicarious action of the same centre of the other hemisphere, for on its removal the paralysis does not return. This question of recovery is the chief obstacle in the way of the acceptance of the theory of motor centres. It has been held by some to be fatal to the theory, but Ferrier goes a long way towards removing the objection when he says : " The more the movements are dependent on conscious discrimination and volitional impulse, the more marked and enduring is the paralysis resulting from lesion of the centres . . . the more independent movements are most affected. The varied and delicate movements of the hand are most of all impaired, and are the last to be re-established." In monkeys, he says : " There is not recovery of movements dependent on intelligent volition ". Hence the appropriateness of the term psycho-motor as applied to these centres. Sensory Centres. Removal or destruction of other areas causes loss of sensation. These areas have consequently been termed Sensory Centres. Destruction of the angular gyrus and occipital lobe was followed by loss of sight. This area is hence called the visual centre. On similar grounds the auditory centre is localised in the superior temporo-sphenoidal convolution. Greater difficulty has been experienced in localising the centres for the remaining special senses. In the case of smell and taste this partly arises from the difficulty of obtaining certain evidence of their absence. Ferrier localises the olfactory centre in the hippocampal lobule, and this is supported by its relation to the olfactory nerves, and by the fact that electrical stimulation of this part causes torsion of the lip and nostril on the same side. The THE BRAIN FUNCTIONS. 133 gustatory centre he places at the lower extremity of the temporo- sphenoidal convolution, an area electrical stimulation of which is followed by movements of the lips, tongue, and cheeks. The tactile centre, lastly, is located in the falciform lobe, but no differentiation of this into centres for touch for the different parts of the surface of the body has been made out. IV. Disease- Localisation of function in the cortex is strongly supported by the disease known as Aphasia, which has for a long time been known to be associated with disease of the posterior portion of the third frontal convolution, sometimes called Broca's convolu- tion. Injury to the cortex from disease or from a fall has often been observed to be followed by convulsions or paralysis of movements corresponding to those resulting from electrical stimulation of the same part. In other words, the convulsions or paralysis arise from stimulation or destruction of the appropriate centres. The theory of localisation has thus enabled the seat of injury of the cortex to be diagnosed, which in some cases has been followed by successful surgical operation. LECTURE XXL SENS A T ION TO U C H. Definition of Sensation. Physiologically, sensation may be said to be the modification of consciousness following on physical changes taking place in the brain as the result of the arrival at it of sensory impulses. Furthermore, under normal conditions, these sensory impulses are always the result of the stimulation of a definite terminal organ, in which the sensory nerve ends at its distal extremity. These terminal organs are the organs of special sense. Sensiferous Apparatus. The sensiferous apparatus or mechanism necessary for the production of normal sensations con- sists, therefore, of three parts : (i) Brain, (2) End-organ, (3) a Sen- sory nerve connecting (i) and (2). The modus operandi of the ap- paratus is that an external agent acting upon the end-organ stimulates it in such a manner that sensory impulses pass from it up the nerve to the brain ; on arriving there, they cause physical changes within it, producing those modifications of consciousness we call light, hearing, smell, taste, and touch. The brain is the essential part of the apparatus, and, under certain circumstances, sensations may be produced in it independently of the nerve and end-organ (subjective sensations]. The end-organ and nerve are subsidiary parts of the apparatus especially adapted to enable a particular external agent to produce certain definite physical changes in the brain. Pain is the strong, unpleasant sensation resulting from the intense stimulation of a sensory nerve. Sensations of pain are not referred to an external object as ordinary sensations are. Classification of Sensations. The foregoing description does not include all sensations not those, e.g., which are of a vague and general character, and are not caused by the stimula- tion of any external agent. Sensations may hence be arranged in three classes SMELL, TASTE, TOUCH. 135 1. Special sensations, produced by the stimulation of some organ of special sense. 2. General sensations, in which the sensation is not referred to any particular locality, nor is there any consciousness of the particular agent producing the sensation. Fatigue, exhaustion, faintness, dejection, are examples. 3. Internal sensations are those which result from the stimu- lation of certain internal organs of the body to which they are more or less referred. Thirst and hunger are referred more or less distinctly to the digestive organs, though they arise from a general want all over the body. They may be said to be inter- mediate between special and general sensations. Suffocation, and, less distinctly, the exhilaration from inhaling an invigorating atmosphere, are referred to the respiratory organs. Repletion, indigestion, and nausea are referred to the organs of digestion. Law of Eccentric Sensation is applied to special sensations being referred to the sense-organs at the terminations of the nerves, i.e., to the point where the external agent acts, notwithstand- ing that the sensation really occurs in the brain on the arrival there of the sensory impulses. This is strikingly illustrated by the effect of stimulation of the nerve stump of an amputated limb. To explain this, it seems necessary to assume that the imaginative faculty is entirely formed by experience. Limen. A stimulus acting upon the end-organ must reach a certain intensity before it can be felt, or, in other words, give rise to a sensation. This minimum intensity is termed the limen. It is constant in the same individual. This intensity of stimulus must be increased a certain proportional amount before a differ- ence in its intensity can be appreciated, or, in other words, a different sensation can be produced. The necessary increase, or sensible increment, is proportional to the previous stimulus. The ratio of the sensible increment to the previous stimulus differs in different cases e.g., for light, it is i to 100 ; for sound only, i to .3 ; for touch, i to 30. TOUCH. The End-organ is situated on the surface of the body, especially in the skin, for the structure of which vide Lecture III. The sensory nerves to the surface of the body have been observed to terminate in the following structures : Tactile corpuscles, situated in the papillae of the skin. They are oval bodies, ^J^th inch long and o-jj^th mc ^ b roa d- One or 136 LECTURE XXI. more nerve-fibres enter each corpuscle. The corpuscle appears transversely marked, due partly to the coiling around it of the fibre on its way to the top of the corpuscle, and partly to an external covering of connective tissue with oblong nuclei placed transversely to the axis. End-btilbs are spheroidal bodies, g^th inch in diameter, found in the lips. They have a simple nucleated sheath continuous with the sheath of the fibre, and in the middle a soft granular core. Padnian bodies are large terminal structures at the extremity of sensory nerves in the fingers, toes, joints, &c. They are visible to the naked eye, being y^th inch long and ^th broad. Each body has a stalk in which a nerve-fibre passes to the centre or core of the corpuscle, which is surrounded by 40 to 50 membranous capsules, the inner being more closely placed than the outer. Nerve plexuses. This mode of termination is well seen in the cornea, where the axis cylinders of the nerve-fibre divide and sub- divide to form a very fine network or plexus. Tactile Sensations may be arranged under the following heads : Pressure. Organs adapted for touching are endowed with mobility. When two or more sensations follow one another, as the result of stimuli applied to the same spot, at a sufficiently short interval, they are fused into one. A pressure sensation quickly reaches its maximum, and im- mediately after decreases. The intensity of a sensation is affected by the pressure on contiguous parts e.g., a finger dipped in mer- cury. The additional increment of pressure necessary to be felt, or,, in other words, produce a new sensation, is for the fingers ^th the previous pressure. It appears to be more for other parts. Some parts of the skin are less sensitive to pressure than others. The greatest sensitiveness is on the forehead, temples,, and back of hand and forearm ; next the fingers ; then the nose, chin, and abdomen. Sensiferous Apparatus for Touch in its simplest form may be said to consist of (a) brain nerve-cell, (b} nerve-fibre, (c) tactile corpuscle. Its modus operandi will be that pressure in the skin stimulates the tactile corpuscle, causing an impulse to pass up the fibre to the nerve-cell in the brain. The impulse on arriving at the cell causes certain physical changes in the cell, of the nature of which we are entirely ignorant. All we know is that those changes. SMELL, TASTE, TOUCH. 137 are followed by the modification of consciousness we call touch. Whether the former is caused by the latter is probably at bottom a barren question. As far as we know the one never exists with- out the other, a connection which all analogy would point to being best explained as one of cause and effect. If we imagine that under certain circumstances the physical changes (say, molecular redistributions) produced in the cell by the nervous impulse on arriving at it, are rendered permanent or fixed, we have a physical basis of memory. The after effect is very distinct, especially with heavy weights and after long application. Temperature Although it is difficult or impossible to determine absolute temperature, a difference of J C. can be perceived through the finger. The more sudden the change in temperature of the skin, the more intense the sensation produced. Those portions of the skin most sensitive to variation of pres- sure are not identical with those most sensitive to variation of temperature. This, together with the fact that nerve-trunks are not affected by heat, so as to give rise to the sensation of heat, has suggested that in the skin there are special end-organs for tempera- ture alone, apart from pressure, and that they are connected with separate parts of the central organ by separate nerve-fibres. In other words, that there is a special sense of temperature. Pathological cases have also been brought forward in support of this view. Locality When a portion of the skin is touched we are able with more or less accuracy to say where the spot touched is. We are able to localise pressure sensations. This power of localisation is not the same on all parts of the skin. This can be shown by experi- mentally determining how far apart two points applied to the skin must be to be felt as two i.e., give rise to two distinct sensations. The distances in the several parts of the body are as follows : Tip of tongue, ri j finger tip, 2-2 ; centre of palm, 8; tip of nose, 6*6; cheek, 15*4; back of hand, 29; sternum, 44; back, 66 mm. If at these places the two points are nearer together than the above, they are fused into one ; they give rise to only one sensation. The experiments can be carried out with a pair of compasses, though a more elaborate apparatus has been devised for the purpose. The two sensations are more easily produced the lighter the pressure ; on the other hand, they are distinguished with greater 10 138 LECTURE XXI. difficulty if there are pressure sensations being produced in neigh- bouring parts simultaneously. Sensory circles are imaginary figures on the skin, within the circumference of any one of which two points give rise to only one sensation. They are smaller the more sensitive the part. They cannot be explained by assuming that one nerve-fibre supplies each circle. A more probable explanation is that there are a number of isolated fibres distributed to each sensory circle, and that a certain number of spaces supplied by these isolated fibres must intervene between the two points of irritation in order that a double sensation may be produced. The sensory circles vary in size at different times. Practice diminishes their size. The circles of the blind are smaller than those of other people. From this it is evident that the property of the skin of measuring the distance between two points is really a property of the brain. The sensory circles are not situated really in the skin, but in the brain. The improvement of the sense of touch by practice may therefore be explained by the sensory areas in the brain becoming more defined and restricted. Muscular Sense In calculating the weight of a body there are two sensations involved (i) sense of presssure, (2) the muscular sense. It is by aid of the latter especially that we are able to distinguish the difference in weights of bodies. A difference of one-fortieth can thus be distinguished. We are always conscious when we put forth muscular exertion. It is a questiq>n whether this conscious- ness results from changes taking place in the first instance centrally in the brain, or peripherally in some end-organ ; in other words whether there is or is not a true muscular sense. In favour of the former view may be adduced the following facts : 1. Anatomical. Afferent nerves have been shown to end in muscles in fine plexuses. 2. Physiological. (a) Muscular sense is experienced when muscles are thrown into contraction by electrical stimulation. (fr) After section of the posterior roots of the spinal nerves movements have all the appearance of lacking the guidance of the muscular sense. 3. Pathological, (a) The keen pain experienced in cramp. () Loss of tactile sensibility while muscular sense still remains. (f) Locomotor ataxy, or impairment of power of co-ordinating the movements of the limbs, may occur without diminution of cu- taneous sensibility. LECTURE XXII. SMELL TASTE VOICE. The End-organ of Smell is the simplest of all the end- organs of the special senses. It consists of the mucous mem- brane lining the upper part of the nasal cavity. By the inward projection of two curved bones (turbinate bones], the nasal cavity is divided into three passages. The olfactory nerve is distri- buted only to the mucous membrane lining the two upper of these passages. Here the fibres of the olfactory nerve are believed to end in peculiar spindle-shaped cells (olfactory cells] situated between the ordinary columnar epithelium of the mucous membrane. These cells therefore constitute the real end-organ of smell. The olfactory nerve-fibres arise from the olfactory lobes, from which they pass to the nostrils by piercing the cribriform plate of the ethmoid bone. In order that they may excite the end-organ, the odorous substances must be in the form of gas or vapour, or at least be conveyed in a gaseous medium. The epithelium of the mucous membrane lining the lower (non-olfactory) portion of the nostril is ciliated. Sensiferous Apparatus for Smell in its simplest form con- sists of (a) nerve-cell of brain, (fr) nerve-fibre of olfactory nerve, (c) olfactory cell in nasal mucous membrane. Its modus operandi an odorous particle stimulating (c), causes an impulse to pass up (V) to (a\ producing in it physical changes followed by sensa- tion of smell. Sensations of Smell. The delicacy of the sense of smell far surpasses that of the other senses. It has been calculated that u ou^Vo'o'o'th of a grain of musk can be perceived. Sensations of smell can only be roughly divided into pleasant and unpleasant. Most substances of an unpleasant odour are injurious to the animal economy. Sensations of smell may be produced subjectively. Mental changes are more tenaciously associated with smell than with any other sense. 140 LECTURE XXII. From the greater acuteness of the sense of smell in some animals there seems reason to believe that the larger the olfactory surface the more intense the sensations of smell. The mucous membrane of the nose may as the result of excitation give rise to sensations other than those of smell ; e.g., ammonia excites the terminations of the fifth nerve distributed to the nasal mucous membrane. TASTE. The End-organ of Taste is situated in the tongue, a mus- cular organ covered with mucous membrane, which is looser and smoother on the under surface and at the sides. The mucous membrane has a thick stratified epithelium. Along the middle line on the upper surface is an irregular slight furrow, or Raphe, terminating about half-an-inch from the base in a depression the foramen ccecum of Morgagni. On the upper surface are numerous PapillSB which gradually become smaller towards the edges, and are altogether absent from the under surface. The papillae are of three kinds, viz., Circumvallate, Fungiform, Filiform or Conical. i. Circumvallate Papilla. These are the largest, and, seven to twelve in number, are arranged somewhat in a V-shaped manner, the arms meeting towards the foramen caecum. They derive their name from the fact that each papilla is surrounded by a wall, or vallum, separated from it by a depression, or fossa. The epithelium is thick and stratified, and the substance of the papilla consists of dense connective tissue like that of the dermis. Along the sides of these papillae, embedded in the thick epithelium, are oval or flask-shaped bodies called taste-buds. The base of each taste-bud rests upon the substance of the papilla, while the apex, appearing as an orifice when seen from above, emerges between the ordinary epithelium cells. A Taste-bud is composed of two parts, viz. a. Cortical, or outer part, made up of long, flattened cells, with pointed ends, in contact by their edges. They extend from base to apex, and are called the cover-cells. b. Central part, composed of spindle-shaped cells (rod-cells). The upper process of each rod-cell extends to the apex, and a fine styliform extremity projects at the orifice ; the lower passes down to the substance of the papilla, where it is said to be con- nected with a plexus of fine nerve fibrils. In these rod-cells, then, we have the real end-organ for taste, and they are hence often VOICE, SPEECH. 141 called the gustatory cells. Taste-buds are also found on the fungiform papillae, on the soft palate, uvula and epiglottis. 2. Fungiform Papilla. These, as their name suggests, are narrowed at their point of attachment. They are most numerous at the apex and edges. They are red, being very vascular. 3. Filiform Papilla are the most numerous and smallest. They are minute conical, tapering, or cylindrical eminences. Nerve of Taste. The base of the tongue is supplied by the glossopharyngeal nerve, which is connected at its terminations with the taste-buds. The back of the tongue is undoubtedly a seat of taste, and hence the glossopharyngeal is regarded as especially the gustatory nerve. The front of the tongue is supplied by the lingual branch of the fifth nerve, and it is regarded as the nerve of taste for that portion of the tongue. Sensiferous Apparatus for Taste may be said in its simplest form to consist of (a) nerve-cell of brain, (b) nerve-fibre of glossopharyngeal nerve, (cj rod-cell of taste-bud. Its modus operandi will be that a sapid substance stimulating (c) causes an impulse to pass up (b\ which on arriving at (a) causes in such changes as result in the modification of consciousness called taste. Taste Sensations may be classified as bitter, sweet, salt, and acid. Those sensations which cannot be referred to one of these probably result from the combination of taste-sensations with those of smell and touch. The union of taste and smell gives rise to what is called fat flavour of a substance. In order that the end-organ may be stimulated it is essential that the stimulating substance be in solution. The larger the surface acted upon, the more intense the sensation!" A tempera- ture of 35 to 40 C. is that most favourable to the production of taste-sensations. Sensations of taste can be produced by stimuli other than sapid substances e.g., mechanical stimuli and the passage of an electrical current, when there is a sensation of acidity at the positive and of alkalinity at the negative pole. The sense of taste is not so delicate as that of smell : ^^ths of a grain of sulphuric acid can be detected. SOUND, VOICE, SPEECH. As a preliminary to the study of the next sense, viz., Hearing, it is desirable to know something of the nature of Sound, its pro- duction and transmission, and especially of that kind of sound called Voice. 142 LECTURE XXII. Sound Production of Sound. Sound is produced by the vibration of bodies, e.g., Air (organ pipe), Strings (violin), Membranes (drum, vocal chords). Propagation of Sound. Sound is transmitted through any elastic medium as waves or undulations. When the medium is the atmosphere these sound-waves consist of alternate condensa- tions and expansions of the air. In an unenclosed medium, a series of spherical waves alternately condensed and rarified is produced around each centre of disturbance. Sound thus pro- pagated travels at a velocity of 1 1 oo feet per second. Intensity of Sound varies inversely as the square of the distance of the sonorous body from the ear. Classification of Sounds. Sounds are either non-musical (noises) or musical. The latter are termed tones. Tones are produced when the vibrations are periodical i.e., occur and recur at regular intervals. Tones are either simple or compound. Compound tones are produced by the union of vibrations of different rates of frequency. Characteristics of Tones. A musical note or tone has three qualities 1. Intensity ', dependent on the amplitude or extent of the vibrations. 2. Pitch, dependent on the number of vibrations occurring in a given time. 3. Quality i or Timbre, due to the number, intensity, and relative preponderance of the harmonics or overtones which accompany the primary tones. Harmonics. When any given note is sounded, not that tone only is produced, but a series of tones, each being of less intensity than the one preceding. These are called the harmonics of the primary tone; e.g., if C, the primary tone, be represented by unity, then i, 2, 3, 4, 5, 6, 7, &c., represents the whole series. The harmonies of the primary C are its octave, then the fifth to that octave, second octave, the third, fifth to the second octave, c. Voice Voice is produced by the vibration of the vocal chords. These are two elastic membranes situated in the larynx, the box-like expansion at the top of the trachea or windpipe. The Larynx is composed of four cartilages, joined together by membranes and ligaments and connected by muscles. The cartilages are VOICE, SPEECH. 143 (1) The Thyroid, a large V-shaped cartilage, the open part being at the back. The point of the V in front forms what is felt in the neck as " Adam's apple ". The posterior edges end above and below in projections termed the superior and inferior cornua. The superior cornua are connected by ligaments to the great cornua of the hyoid bone ; the upper edge of the thyroid is connected by a ligament to the body of the hyoid. (2) The Cricoid, situated below the thyroid, is in shape like a signet ring, the thicker portion being behind. Posteriorly it is partly overlapped by the sides of the thyroid, to which its upper edge is attached at the sides and in front by ligaments. Its lower edge is similarly connected to the first cartilage of the trachea. On the outer surface on each side posteriorly is a smooth depres- sion where the inferior cornua of the thyroid is jointed. On the upper border of the deeper posterior part there is on either side of the middle line an articular surface to which are jointed (3) and (4) The two Arytenoid cartilages. These are pyramidal in shape, the base of the pyramid entering into the aforesaid joint. Each arytenoid has three lateral surfaces posterior, external, and internal, and at the base three angles anterior, external, and posterior. To the anterior angles are attached the posterior ends of the vocal chords, which are attached close together in front at the re-entering angle of the thyroid. The arytenoids are connected together by muscle, and their angles to the cricoid and thyroid also by muscles. By the contraction of these muscles the aryte- noids can be moved on the cricoid, with the result of approaching or separating the anterior angles, and thereby the posterior ends of the vocal chords. Muscles of the Larnyx. Crico-thyroid, from external surface of thyroid forwards and downwards to the front part of lateral external surface of cricoid. There are two views regarding its action: (i) that it pulls down the thyroid, thereby rendering tense the vocal chords; (2) that it pulls up the front of the cricoid, thereby depressing its posterior part, and thus putting the vocal chords on the stretch. Thyro-arytenoids, from anterior angle of arytenoid to angle of thyroid lying immediately external to the vocal chords. Their action is supposed to be to raise up the thyroid after its depression by the preceding muscle and thereby slacken the chords. It may also act directly upon the vocal chord, rendering tense limited portions of it. Lateral Crico- arytenoids, from external angle of arytenoid downwards and for- wards to the external surface of the cricoid. By their contraction 144 LECTURE XXII. they move forwards the external angles, thereby causing the anterior angles of the arytenoids to come nearer together, and in consequence the vocal chords to become more parallel. Posterior Crico-arytenoids, passing from the external angles down and back to the posterior surface of the cricoid, by their contraction pro- duce exactly the opposite result to the last muscle, viz., a separation of the vocal chords. Arytenoids pass from the posterior surface of one arytenoid to that of the other, and by their con- traction are said to cause the arytenoids to come nearer together. Whether their action will cause the anterior angles, and hence the vocal chords, to come nearer together, or just the reverse, seems doubtful, probably the latter. Glottis. The opening from the oesophagus into the larynx is called the glottis. At the opening, attached to the back of the tongue, at the top of the larynx, is a spoon-shaped piece of car- tilage, or Epiglottis. During swallowing the epiglottis is bent down so as to cover over the glottis. Vocal Chords are composed of yellow elastic connective tissue, covered over by a closely-attached mucous membrane. Their free edges, which by their vibration produce voice, are sharp and straight, and are directed upwards. The surfaces below 7 face inwards towards one another. Just above the vocal chords, separated'from them by a space, the ventricle, are two marked folds of the mucous membrane, forming \.hefa/se vocal chords, for they take no active part in the production of voice. Conditions of Voice Production. When at rest the vocal chords are separated by a considerable space, which becomes wider the stronger the inspiratory act. In order that they may produce voice, two conditions are necessary, viz. : 1. That they shall be made parallel. 2. That they shall be rendered tense. These conditions are brought about by the contraction of the preceding and perhaps other muscles. " The action of these several muscles is excessively complicated, and anyone who should be able fully and exactly to describe it would dispel the mystery which still surrounds the whole subject of voice produc- tion " (Morell Mackenzie). The action of the chief muscles is probably as follows : Parallelism Lateral Crico-arytenoids. Separation Posterior Crico-arytenoids : Arytenoids. Tension Crico-thyroids. Relaxation Thyro-arytenoids. VOICE, SPEECH. 145 The blast of air necessary to throw the chords into vibration is supplied by the lungs, and hence normal vocalisation takes place only during expiration. The vocal organ is essentially a reed instrument, the lungs being the bellows, the trachea the pipe, the larynx the reed-box, the vocal chords the reeds, and the pharynx, nose, and mouth the resonator. Characters of Voice. As of musical sounds generally, so of the voice. It varies in 1. Loudness, dependent on the force with which air is expelled from the lungs, or on the amplitude of vibration of the chords. 2. Pitch, dependent on the length and tension of the chords, for on this depends the number of vibrations in a given time. 3. Quality, or Timbre. This depends on the number and character of the over-tones accompanying the fundamental tone. The chief factor in determining the timbre of a voice is the size and shape, &c., of the resonance chambers, amongst which must be counted the cavity of the chest, the pharynx, the mouth, and the nasal cavities. Speech Speech is the voice modified by the palate, tongue, lips, and teeth, these structures being therefore sometimes called the organs of speech. Voice may exist without speech, and speech may exist without voice (i.e., voice produced by the vocal chords), as in whispering. Speech sounds are distinguished as vowels and consonants. Vowel sounds are due to particular over-tones accompanying the fundamental tone produced by the vocal chords. The different over-tones of the different vowels are caused by the resonance chamber (pharynx and mouth) taking different shapes. The essential alteration appears to be in the length of the reso- nance tube (mouth). In sounding, for example, the vowels e, a, ah, o, oo, in the order named, a gradual lengthening of the tube of the mouth will be observed. The production of vowel sounds is therefore essentially a question of quality or timbre. In fact, by combining a fundamental tone with the appropriate over-tones, Helmholtz has been able to artificially produce the vowel sounds by synthesis ; and Kcenig, by his ingenious manometrical flame apparatus, shows in a graphical manner how there is a particular form of flame-picture for each vowel, and how it varies with the pitch. By a combination of this method with a series of reso- nators, he is also able to analyse a vowel sound into its funda- mental tone and overtones. 146 LECTURE XXII. Consonants are not produced by the vocal chords, but by the outrush of air being interrupted or modified in passing through the throat and mouth. Consonants are classified in several different ways: (i) Liquids and mutes; (2) according to the mechanism of formation into explosives, aspirates, vibratives, and resonants ; (3) according to the place where the interruption of the outrush of air takes place into a. Labials when at the lips ; e.g., B, P. b. Dentals at the teeth ; e.g., T, D. c. Gutturals at the throat ; e.g., K, G. Defective Speech. Stammering is due to the abnormal action of the diaphragm, thereby interfering with the appropriate respi- ratory movements on which the production of the voice is dependent. Stuttering is due to inability to normally manipu- late the larynx so as to produce the appropriate sounds. It is accordingly much more difficult to overcome than stammering. Aphasia is due to disease of the brain (vide Lecture XX.). LECTURE XXIII. HEARING. To understand this sense we have to consider first the Ear, or end-organ, and then the way in which it is acted upon by sound- waves so as to produce the sensation. I. The Ear. The human ear is composed of three parts an outer, middle, and inner ear. External Ear. This consists of two parts : 1. The Auricle, or the feature usually known as the ear. It is an irregular mass of cartilage covered with skin, exhibiting elevations and depressions the purpose of which is probably to reflect the sound-vibrations into the other part of the external ear. In the human ear this is probably a very subsidiary func- tion of the auricle compared with the same structure in some of the lower animals. 2. The Auditory Canal, or External Auditory Meatus. This tube, about i \ inch long, is partly cartilaginous and partly osseous. It is slightly narrower in the middle than at either end. It is closed internally by a membrane, the membrana tympani. At its outer part are a number of modified sweat-glands, the ceruminous glands, which secrete a waxy substance. Middle Ear. The Tympanum, or Drum, is a cavity in the petrous portion of the temporal bone of the skull. It is an irregularly-shaped cavity, separated from the outer ear by the Membrana tympani, a thin fibrous membrane, almost circular in shape, attached at its circumference to the bone. It is a little over \ inch in diameter. The fibres are arranged radially on the outside, circularly on the inside. Externally it is covered by a thin layer of skin, internally by a mucous membrane with squamous epithelium. All the rest of the tympanum is lined with ciliated epithelium. The membrane is directed downwards and inwards. It is not flat, but bulged in towards the tympanum, where the handle of the malleus is attached to it. It is thrown into vibration by sound-waves. " The membrani tympani has the 148 LECTURE XXIII. remarkable property of answering equally well to tones of any pitch within the range of tones perceptible to our ear : to tones of about 60 to 4000 vibrations per second, which is impossible with an ordinary stretched membrane." The Tympanum is \ inch high, and 2 lines broad. Above, it is separated from the brain by a thin piece of bone, and below similarly from the jugular vein ; posteriorly it is continuous with the mastoid cells ; in front it communicates with the pharynx by the Eustachian tube, a canal about ij inch long, said to be normally closed, but is momentarily opened during the act of swallowing. In the inner wall of the tympanum are two minute openings communicating with the internal ear. One of these is oval in shape, and is called the fenestra ovalis ; the other, round, is the fenestra rotunda. They are both closed by membrane. To the membrane of the former is attached the bottom of the stapes, or stirrup-bone. Ossicles of the Ear. There are three minute bones in the Tympanum, viz. : The Malleus, or hammer-bone, is fixed by its handle-process to the inner surface of the membrana tympani; its head is jointed to The Incus, or anvil-bone, which in addition to a body has two processes one short by which it is fixed to the wall of the tympanum, another longer which at its extremity is joined to The Stapes, or stirrup-bone. This is firmly joined to the membrane, closing up the fenestra ovalis. These bones are so connected as to move as one lever, and as a result any vibration of the membrana tympani is communicated through them to the membrane of the fenestra ovalis. Internal Ear, or Labyrinth. This is the essential part of the ear, for in it terminates the auditory nerve. It consists of a series of channels hollowed out in the substance of the petrous portion of the temporal bone. It is divisible into three parts, viz. 1. The Vestibule, or central part. Into this opens the fenestra ovalis. 2. The Semicircular Canals, three in number, placed pos- teriorly to the vestibule with which they communicate. 3. The Cochlea, in front of the vestibule, also opening into it. Within these bony spaces, often spoken of together as the bony or Osseous Labyrinth, are membranous structures of similar shape, forming the Membranous Labyrinth. HEARING. 149 Between the osseous and membranous labyrinths is a liquid, or Perilymph. Within the membranous labyrinth is another liquid, or Endolymph. The Vestibule. The membranous vestibule consists of two fibrous sacs, called the Utricle and Saccule. Into the former, which is much the larger, open the membranous semicircular canals. The latter is continuous with the central canal of the cochlea. The utricle and saccule communicate with one another by a narrow canal. The Semicircular Canals are designated according to their position superior, posterior, and external. The superior and pos- terior are vertical, and meet one another at an angle, the former lying transversely, the latter antero-posteriorly. The external canal is horizontal and communicates with the vestibule by two distinct openings ; whereas the superior and posterior canals have each only one independent communication with the vestibule, the other being common to both. At one end of each canal, just where it joins the vestibule, there is an enlargement called the Ampulla. The Membranous Semicircular Canals have a similar shape. They are only about one quarter as wide as the osseous canals, and are attached at one side to the bony wall. Their connections with the utricle correspond to those of the bony canals with the vestibule. Terminations of Auditory Nerve. The auditory nerve divides into two branches, distributed respectively one to the cochlea, the other to the vestibule and semicircular canals. The Vestibular portion divides into five branches, viz., one to each ampulla, to the saccule, and to the utricle. On the internal surface of each membranous structure, corresponding to the place of termination of the branch of the nerve, is a cushion- like elevation covered by a columnar epithelium, surmounted by fine cilium-like processes, which project stiffly upwards. These so-called auditory hairs do not, however, project freely into the endolymph, for they are covered by a gelatinous mass containing numerous minute calcareous bodies called otoliths. This auditory epithelium is the real end-organ, for the columnar cells are con- nected at their base with the final terminations of the auditory nerve. The Cochlea is a dome-shaped organ about J inch in height, and the same in breadth at the bottom. It is composed of a conical tube 1 1 inch long and y^th inch wide at its beginning, where widest, coiled upon itself 2\ times. There is a central bony axis, 150 LECTURE XXIII. the modulus, or column of the cochlea. This tube, forming the osseous cochlea, is divided into two compartments by a partition, partly bony, projecting out from the column, forming the spiral lamina of the cochlea^ and partly membranous, connecting the spiral lamina to the outer wall, forming an important structure, the Basilar membrane. The two parts into which the cochlea is thus divided are termed respectively the Scala tympani (the lower), opening by the fenestra rotunda into the tympanum, and the Scala vestibuli (the upper), because it communicates at the bottom with the vestibule. These two compartments communicate with one another at the apex of the cochlea, and they contain perilymph. A small triangular part of the scala vestibuli is cut off by a fine membrane (membrane of Reissner), passing upwards and outwards to be attached to the bony wall. The portion thus divided off is termed the duct or Central Canal of the Cochlea. Of its three walls, the upper and lower are membranous, and the outer bony. It is the membranous cochlea. At the bottom it communicates with the saccule by a fine canal (canalis reuniens\ at the top it ends blindly. It contains endolymph continuous with that in the saccule. The inner surface of the central canal is lined with epithelium ; and this on the basilar membrane undergoes modification to form the remarkable Organ of Corti. The central part of this complicated structure is formed by the Rods of Corti. These are inclined towards one another in pairs, meeting at their heads, which project up into the central canal of the cochlea. In this way they form a tunnel, extending along the whole length of the basilar membrane, which is filled with en- dolymph. The inner rods are more numerous than the outer in the proportion of 6000 to 4500. Internal and external to the rods are rows of columnar cells surmounted by stiff hairlets. The inner hair-cells form a single row ; the outer hair-cells are arranged in four rows. The hairs on the latter project through rings formed by fiddle-shaped structures connected with one another at their ends, the most internal being connected with the tops of the rods. A reticular lamina is thus formed, covering this part of the organ of corti like a wire net. On either side the cells forming the epi- thelium gradually diminish in size until they resemble the cubical cells lining the canal generally. The whole organ is covered by the tectorial membrane^ which projects from the spiral lamina. Termination of the Auditory Nerve. The branch of the HEARING. 151 auditory nerve to the cochlea passes up the central column and sends fibres through the spiral lamina to the organ of corti. In what part of that complicated structure the fibres actually ter- minate is a matter of some uncertainty. They have been described as ending in the hair-cells, which from analogy are certainly the part of the structure we should expect to be the end-organ. Sensiferous Apparatus for Hearing. In its simplest form the sensiferous apparatus for hearing may be said to be (a) nerve - cell in brain, (b) nerve-fibre of auditory nerve, (c) hair-cell of utricle, ampulla, or cochlea. Its modus operandi that sound agitating (c) causes an impulse to pass up (), which on arriving at (a) produces in it such physical changes as to bring about the modification of consciousness called hearing. We have now to consider how the agitation or excitation of the hair-cell or end- organ is caused by sound-waves. II. How End-organ excited by Sound. Those vibrations of the air from a sounding body, called sound-waves, passing down the auditory canal, fall on the membrane tympani and set it vibrating. The vibration of this membrane (connected to the handle of the malleus) is communicated to the ossicles of the ear, which, acting as one lever, rotate round an axis, passing through the heads of the incus and malleus. Their movement throws into vibration the membrane closing the fenestra ovalis. Since the handle of the malleus is half as long again as the stapedial process of the incus, it follows that the movement of the stapes will be only two-thirds as great as that of the onalleus. But, on the other hand, although there is a loss in the extent, there will be a corresponding gain in the force of the movement communicated to the membrane closing the fenestra ovalis. The vibration of this membrane agitates the perilymph of the labyrinth. This, causing a disturbance of the endolymph, will agitate the auditory hairs projecting from cells connected with the terminal fibres of the auditory nerve. Sound may be conveyed to the perilymph through the bones of the head. Such sound is much less perfectly referred to the external world than that which acts through the membrana tympani. Muscles of the Tympanum. The vibration of the membrana tympani is controlled and modified by the tensor tympani muscle. This muscle, arising in the Eustachian tube, is inserted into the malleus near the top of the handle. By the strong contraction of 152 LECTURE XXIII. this muscle, the membrana tympani may be rendered so tense as not to respond to sonorous vibrations. In this way very loud sounds would be prevented from communicating their vibrations to the inner ear, and thereby injuring the delicate end-organ. Again, by varying the degree of its contraction, the tension of the membrane may be varied so as to alter its fundamental tone, thus enabling it to take up more readily the vibrations produced by sounds of different pitch and intensity. There is another muscle, the stapedius, inserted into the head of the stapes. By its contraction it tends to fix the stapes, and thereby prevents too strong vibra- tions from being communicated to the perilymph. The lowest sound recognisable as musical is produced by about 32, and the highest by about 32,000, vibrations per second. Functions of different parts of Internal Ear. There is very little certain knowledge concerning the functions of the several parts of the internal ear. From the striking differences in structure of the three parts, Vestibule, Semicircular Canals, and Cochlea, they doubtless have different functions. Vestibule. From its simplicity, and from the fact that it is the first to appear in the animal scale, the vestibule probably gives rise to the sensation of sound as such. Semicircular Canals. From their position and arrangement these have been supposed to enable us to learn the direction of sounds. Experiment goes to show that they are intimately as- sociated with the co-ordination of movements and maintenance of equilibrium (vide Lecture XX.). Cochlea. From its remarkable complexity, the cochlea must have an important function. What that function is, however, it is impossible to say with certainty. The remarkable Organ of Corti has been supposed to be the organ through which we are enabled to appreciate the pitch and quality of musical sounds. This is suggested by the number and arrangement of the rods of Corti, for they vary regularly from the bottom to the top of the cochlea in length and in the span of their arch. There are, however, the following objections to the view that the rods of Corti constitute the organ by which we analyse musical sounds : 1. The nerve-fibrils end in the hair-cells, not in the rods. 2. The variation in length of the rods is not sufficient. 3. They are absent in birds. In view of these objections, the basilar membrane, considered as a series of tense parallel strings, may be the organ required. HEARING. 153 It increases in width gradually from the bottom to the top of the cochlea. Judgments of distance and of direction of sounds are by no means easy or precise. They appear to be more certain and exact for noises than for musical sounds. Comparative Anatomy. There is considerable doubt whether auditory organs exist in the lower Invertebrata. In Ccelenterata and Vermes a small capsule containing minute hard particles (otoliths) has been regarded as a rudimentary ear. From the sounds produced by many Insects^ they are probably endowed with the sense of hearing. In Crustacea there appears to be an undoubted organ of hearing which may be compared to the utricle of the human ear, for it consists of a sac lined with hairs and containing otoliths. The vestibule is apparently the only part of the ear represented in the auditory organ of Invertebrata. In Vertebrata the vestibule and semicircular canals are present in all classes from fishes upwards. The cochlea in Fishes is merely a small projection ; in Birds it is short, conical, and straight, except for a bending at the extremity forming the lagena. In Mammals it is essentially the same as in man. ii LECTURE XXIV. THE EYE LIGHT. To understand the sense of sight, three things are necessary : 1. That form of energy called light. 2. The structure of the eye. 3. The way in which light acts upon the eye, so as to cause vision. I. Light is held to be due to the vibratory disturbance of an exceedingly tenuous medium pervading all space the luminiferous ether. A luminous body gives a rapid undulatory motion to the molecules of this ether, which motion is propagated as spherical waves in all directions. The smallest conceivable portion of light is called a ray. It is represented by a straight line. A. pencil oi light is a collection of rays from the same source : it may be parallel, divergent, or convergent. Refraction. When a ray of light passes obliquely from one medium to another (e.g., air to water, or air to glass), it is bent or refracted. Refraction takes place when light passes through a lens. The straight line which passes through the two centres of a biconvex lens is called the principal axis. All rays of light parallel to the principal axis, falling on a biconvex lens, are so refracted as to be brought to one point behind it, called the principal focus. All the rays of light proceeding from any lumin- ous point (beyond a certain distance from the lens) are refracted so as to be brought to a focus after passing through it. Images. Any object, being virtually a series of luminous points, when placed in front of a biconvex lens, beyond the prin- cipal focus of the lens, has a luminous representation of it formed behind the lens. This luminous representation is termed an image. The image in this case is reversed. Camera. A camera is a box within which by means of a lens images of external objects are formed clear and distinct upon a screen. In a photographic camera this screen is movable, so that images of objects at different distances may be formed upon it LIGHT. 155 at different times. The eye is a camera, images of external objects are formed by a biconvex lens (the crystalline) on a screen (the retina). Colour. Sunlight can be decomposed by a prism into a number of colours, comprising all the simple colours, and which, when combined, produce all the colours which occur in nature. The sum of the colours thus obtained by decomposing sunlight is called the solar spectrum. That the colours of the spectrum are simple may be shown by cutting off all the colours except one by means of a screen. When this remaining colour is allowed to pass through a prism, it emerges unbroken. By re-combining the different colours of the spectrum by allowing them to fall on a reversed prism white light can be reproduced. Colour of Bodies. The colour of a body depends on that portion of the spectrum which it reflects or transmits. The two extremes are the case in which a body reflects or transmits all colours in the proportion in which they exist in the spectrum, when it is white, and that in which it transmits or reflects none, when it is black. Fundamental Colours. This term has been applied to red, green, and violet , for by combinations of these three all possible colours can be formed. Quality of Colour depends on (i) the part or parts of the spectrum reaching the eye, (2) the intensity of this light, (3) the amount of white light mixed with the coloured light. Mixed Colours are the impressions of colour resulting from the coincident action of two or more colours on the same part of the retina. This impression is single, and cannot be resolved into its constituents. This is a physiological combination, and is quite different from the effect produced by the mixture of differently coloured pigments. Complementary Colours. If a portion of the spectrum be cut off, the remaining portions give a definite colour ; if, on the other hand, the latter be cut off, the former give another de- finite colour. These two resulting colours, when combined, give white, and they are termed, in consequence, complementary colours. The following are examples of complementary colours : Red and Greenish-blue ; Orange and Light-blue ; Green and Purple. Properties of the Spectrum Luminous. The rays of the spectrum consist of light vibra- tions of different wave lengths which diminish from the red to the 156 LECTURE XXIV. violet. Since the different kinds of light travel with equal velocity, violet makes more vibrations in the same length of time than red light; the former about 667 billions, the latter 456 billions per second. All the light rays of the spectrum differ solely in the wave lengths of their vibrations, and possess no other special mark of distinction. Colorific. The spectrum has no defined limit at either end, but passes gradually into black, the transition being more gradual at the violet than at the red end. The greatest heating effect is produced by the ultra-red rays. Chemical. The violet rays are more powerful, chemically, than the rest of the spectrum. This action extends beyond the visible portion of the spectrum. To these invisible rays the term actinic is applied. II. The Eye- Comparative Anatomy. The pigment spots of some Actinia (sea anemones, &c.), have been supposed to be rudimentary eyes. In its simplest form an eye is little more than a lens fixed on the end of a nerve. Such an eye is found in Insects. In these animals the eye consists of one or more six-sided lenses placed at the end of a nerve. In the Cuttle-fish (Cephalopoda), an eye of great com- plexity is found. In the eye of this invertebrate animal all the main parts of the human eye are represented. Structures surrounding the Eye The Eyelids consist of a mass of dense connective tissue (tarsus], covered externally by skin, and internally by the con- junctiva. Opening on the edge of the lids are a number of sebaceous glands, the so-called Meibonian glands, imbedded in the substance of the eyelid. The extent of separation of the eyelids determines what is usually called the " size of the eye ". Conjunctiva is the delicate continuous mucous lining of the inner surface of the eyelids, and the anterior surface of the eyeball : over the cornea it consists simply of epithelium. Lachrymal apparatus for the secretion and passing off of the liquid by which the exposed surface of the eyeball is kept moist consists of The lachrymal gland, of the racemose variety, about the size of a small almond, situated at the upper and outer part of the orbit, a little behind the anterior margin. Its upper surface is attached to the periosteum by fibrous bands. From it pass 12 to 14 ducts which open on the surface of the conjunctiva. LIGHT. 157 The lachrymal canals are two small tubes beginning at two orifices the puncta lachrymalia at the inner angle of the eyelids. They open internally into The lachrymal sac, situated at the side of the nose imbedded in a deep groove in the superior maxillary bone. It opens below into The nasal duct, about a half-inch long, in a groove of the superior maxillary bone. It descends to the fore part of the lower meatus of the nose. It is lined by a ciliated epithelium, as is that of the lower portion of the nostril. The Eyeball, globular in form, measures about one inch in all diameters except the antero-lateral, which is a little less. It is kept in position in the orbit by the optic nerve behind, the eyelids in front, a mass of fat, and six muscles by which its varied move- ments are produced. Chambers. Its interior is divided into parts, or chambers, by a dark opaque screen (iris), in the centre of which is a round aperture (pupil), and the crystalline lens, situated just behind the pupil. The posterior chamber, forming the greater part of the eyeball, is filled with a gelatinous fluid the vitreous humour ; the anterior chamber, much smaller, is filled with a similar fluid the aqueous humour. Coats of the Eye (a) The eyeball is enclosed by a strong, thick membrane, the posterior, fths of which is white and opaque. This is the Sclerotic. The anterior, Jth slightly more convex than the rest, is transparent, and is known as the Cornea. Together they form the corneo-sclerotic, or outer coat. (b) Lining the internal surface of the sclerotic is the second, or middle coat, the Choroid. It consists of a dense network of fine bloodvessels, amongst which are numerous branched cells, con- taining pigment granules ; hence the choroid has a black colour. In front it is continuous with the iris at a line corresponding with that of the junction of the sclerotic with the cornea. Just before joining the iris, the substance of the choroid is thickened, so as to form a fringe of projections the ciliary processes of the choroid. The Iris consists of a framework of fibres and muscular tissue, with pigment-cells scattered through it. The amount of pigment present determines the " colour of the eye ". Posteriorly, it is covered with pigment-cells uvea continuous with the hexagonal pigment-cells of the retina. There are two sets of muscular fibres ; one arranged circularly sphincter which, by their contraction, 158 LECTURE XXIV. diminish the size of the pupil ; the other arranged radially dilator causing by their contraction the pupil to enlarge. By the action of these muscular fibres, the amount of light entering the eye is diminished or increased : this action is wholly reflex. (c) Lying on the inside of the choroid is the third or inner coat of the eye, the Retina. It is the essential part, the end-organ. The retina extends only about two-thirds as far forwards as the choroid. It ends in an irregular border, or ora serrata. Beyond this the retina is represented merely by a single stratum of elon- gated columnar cells, with the pigmentary layer external to them (pars ciliaris). The middle point of the retina is slightly tinged with colour, and is hence known as the yellow spot, or macula lutea. The centre of the yellow spot is slightly depressed, forming the fovea centralis, the part of the retina most sensitive to light. The retina may be regarded as an expansion of the optic nerve which pierces the sclerotic coat at the back of the eyeball a little to the nasal side of the middle point. Crystalline Lens. This is situated immediately behind the pupil, and is invested by a very fine membrane continuous with the hyaloid membrane, surrounding the vitreous humour. The portion of the membrane passing from the edge of the lens to the hyaloid is called the zonula of Zinn, or suspensory ligament of the lens. It keeps the lens in position, and in a slightly stretched condition. The crystalline lens is biconvex, the posterior con- vexity being slightly greater than the anterior. It is firmer and more strongly refractive in its central part. It is \ inch in diameter, and \ inch thick. It is made up of concentric lamina, composed of long riband-shaped microscopic fibres, ^Vo" inch broad. These adhere together by their edges, which are often serrated. Ciliary Muscle, or Tensor Choroidece. This muscle arises by a thin tendon from the fore part of the sclerotic, close to the cornea. Its fibres spread out, and are directed backwards to be inserted into the choroid opposite the ciliary processes, and partly further back. When this muscle contracts, it draws the choroid forward, and thereby renders less tense the suspensory ligament of the lens. The crystalline lens then, in virtue of its elasticity, becomes more convex (vide Accommodation Lecture XXV.). The Retina This is the essential part of the eye, for in it terminate the fibres of the optic nerve. It is the end-organ for vision. Al- though very thin and delicate, being little thicker than a piece of LIGHT. 159 thick paper, it has a very complex structure. No less than eight layers can be distinguished in it. These are from within out 1 . Layer of nerve-fibres, being the terminal fibres of the optic nerve. 2. Ganglionic layer, composed of nerve-cells. 3. Inner molecular layer, probably of the nature of a fine network. 4. Inner nuclear layer, containing numerous large nuclei. 5. Outer molecular layer, a thin stratum, apparently similar in nature to (3). 6. Outer nuclear layer, containing large nuclei connected with the constituents of 7. The Bacillary layer, or layer of rods and cones. 8. A layer of hexagonal pigment- cells. In addition to this proper substance of the retina, there is a supporting tissue. Internally, this forms a fine limiting membrane membrana limitans interna. Immediately internal to the bacillary layer, it forms another limiting membrane the membrana limitans externa. In the centre of the optic nerve is a small artery. It branches out, forming a network of bloodvessels in the innermost layers of the retina. Rods and Cones. These are the most important part of the retina, for they are the part sensitive to light. They form, in fact, the real end-organ. The rods and cones are continuous through the intermediate layers with the nerve-fibres of the inner- most layers. In the yellow spot there are only cones, which are here much longer and narrower, especially in the fovea. All the other layers are very much thinned at the fovea, and the layer of nerve-fibres is absent. That the cones are the real end-organ for vision is supported by the following facts : 1 . The blind spot (where the optic nerve enters) has no rods and cones, but only nerve-fibres. This spot is not excited by light. 2. The centre of \hz yellow spot, or fovea, the part most sen- sitive to light, has this layer highly developed at the expense of the others. Here there are only cones, no rods. 3. The experiment of Purkinjds figures. 4. The distance at which two points can be distinguished, i.e., give rise to two sensations, corresponds very nearly with the diameter of a cone. 160 LECTURE XXIV. Sensiferous Apparatus for Sight. In its simplest form this may, therefore, be said to consist of (a) nerve-cell in brain, (b) fibre of optic nerve, (c) a cone of the retina. Its modus oper- andi, that waves of light having traversed the eyeball and other layers of the retina on reaching the bacillary layer excite the cone, and, in consequence, an impulse passes up the fibre of the optic nerve to the nerve-cell, in which it produces such physical changes as are followed by the modification of consciousness called sight. Optical centre of the eye lies in the crystalline lens, not, how- ever, exactly in its centre, but between that point and the posterior surface of the lens. The optic axis is the straight line joining the object observed and the retina which passes through the centre of the crystalline lens. Optic angle is the angle formed by the two optic axes from the same object. Visual angle is the angle under which an object is seen. If the visual angle, formed by straight lines from two objects to the eye be less than 60 seconds, they appear as one. LECTURE XXV. VISION. III. How Light acts upon the Eye Formation of the Image Light, from an object passing through the transparent cornea, by which it is refracted, the aqueous humour and the pupil, falls upon the anterior surface of the crystalline lens. Refracted by the lens, an image of the object is formed behind it within the eye. In a normal (emmetroptc) eye the image (unless the object be nearer than five or six inches) is always formed on the retina and on the yellow spot. This is rendered possible by the power possessed by the eye of Accommodation. Inverted Image. The image upon the retina is reversed, yet we see every object in the field of vision upright. Much discus- sion and mystery have been thrown around this fact. "The true scientific answer is found in what is called the ' Law of Visible direction '." This law may be thus stated : " When the rays from any radiant strike the retina, the impression is referred back along the ray line into space, and therefore to its proper place " (Le Conte). This is only in keeping with the way in which other sensory impressions are referred to some outside object in a definite direction. Accommodation is affected by an alteration of the convexity of the anterior surface of the crystalline lens. This alteration is brought about by the contraction of the tensor choroideae muscle (vide Lecture XXIV.). When contracted, this muscle draws for- ward the choroid, thereby rendering less tense the suspensory liga- ment, and thus gives free play to the elasticity of the lens. That accommodation is produced in this way is proved by observing the three reflected images of a luminous object seen in an eye when accommodated for long and short distances. These images are formed by the anterior surface of the cornea, the anterior convex and posterior concave surfaces of the crystalline lens. During accommodation only the middle image undergoes a l62 LECTURE XXV. change. In accommodation for far objects it becomes longer, for near objects shorter, showing that the anterior surface of the lens is more convex in the latter than in the former case. That the tensor choroidese is the muscle of accommodation can be shown experimentally by stimulation of the ciliary nerves going to the muscle, and by the action of atropin, which paralyses it. Near limit of accommodation. The nearest point at which an object can be seen distinctly is called the near limit of accommoda- tion. For the normal eye it is 5 to 6 inches, for the myopic eye 2 to 3 inches, for the hypermetropic eye at some distance. The near limit is easily determined by Schemer's experiment. The Pupil. In accommodation for near objects the pupil contracts ; in accommodation for far ones it dilates, thus appro- priately altering the amount of light entering the eye. Long and Short Sight. If the crystalline lens or cornea be more than normally convex, or if the antero-posterior diameter of the eyeball be too great, the image is formed in front of the retina. This defect is called short-sightedness, or Myopia. It is corrected by concave spectacles. If, on the other hand, the sur- faces of the cornea and lens are too flat, or the antero-posterior diameter of the eyeball too small, the image is formed behind the retina. This is long-sightedness, or Hypermetropia. It is cor- rected by convex spectacles. Normal vision is termed Em- metropia. Binocular Vision. Under ordinary circumstances both eyes are used, and there must therefore be two images of any object observed. The two images give rise to a single perception be- cause they fall on corresponding points of the two retinas. If this is prevented, as by squinting, we see double. Horopter. The horopter is an imaginary figure in space com- prising all those points projected on to corresponding points of the retinas. Judgment of solidity depends on binocular vision. It is shown by the stereoscope to be due to the blending of two slightly dis- similar images on the two retinas. Judgment of distance is also connected with binocular vision, for it is partly due to the sensation of muscular effort necessary to bring the axes of the two eyes to the proper convergence. The sensation of effort of accommodation, no doubt, also assists. Movements of the Eyeball. These are produced by six muscles, all, with one exception (inferior oblique), having their origin in the osseous wall of the orbit near the point where the VISION. 163 optic nerve enters it. Four muscles pass directly to the eyeball. They are the straight or Recti muscles, denominated from their position external, internal, superior, and inferior. The two others are attached to the eye in an oblique direction, and hence termed Oblique. One above is the superior, and is peculiar in its anterior tendon working through a pulley-like loop, so that the direction of its action is at an angle to the direction of the main part of the muscle ; the other below, the inferior. As regards their action, they are divisible into two groups, (i) External and internal recti rotating the eyeball round its vertical axis. (2) Superior and inferior recti and the two oblique acting together (rectus superior with obliquus inferior, and rectus inferior with obliquus superior) rotate the eyeball round its horizontal axis. The straight line connecting the apex of the cornea with the fovea centralis is called the visual axis. All rotations of the eyeball take place round axes axes of rotation which meet the visual axis at right angles about 17 mm. behind its middle point (Listing). Observation of the Interior of the Eye. Part of the light reaching the retina is reflected. The reflected light returns along nearly the same paths as those by which it arrived. If by a mechanical arrangement this reflected light be made to pass into the eye of an observer, he is able to see the interior of the ob- served eye. An instrument invented by Helmholtz, called the Ophthalmoscope, is such an arrangement. The brilliancy of the eyes of some animals is due to the reflection of light by a carpet of glittering fibres, called the tapetum, lying behind the retina. After Effect. The retina continues in an excited condition for some time after the stimulus ceases. There is an after effect. In other words, the sensation lasts longer than the stimulus. After-images result from this. When they are continuations of the sensation, they are termed positive after-images. A practical application of this is seen in the toy called the zoetrope. The positive after-images are followed by images just the reverse, i.e., what was dark is light, what was light is dark ; or if coloured, by an image having its complementary colour. These succeeding images are termed negative after-images, and are explained as the result of exhaustion. Colour Sensations. The theory of colour vision generally accepted, named after its originator and chief supporter the Young-Helmholtz Theory, supposes that there are three primary colour sensations red, green, and violet and that all other colour sensations are due to a combination of these. Further, that every 164 LECTURE XXV. visual unit of the retina (every cone) contains three terminal ele- ments, one of which is most excited by red rays, another by green, and a third by violet rays. All three are more or less excited by all the rays. These three terminal elements are assumed to be connected with three sets of fibres. Combinations of these fundamental colour sensations may be made by the colour top. If one or more of the primary colour sensations are absent, colour blindness results. The edge of the retina is often colour blind for red rays. Herings Theory. This theory is opposed by Hering, who has put forward an ingenious view, that colour sensations are due to two antagonistic processes in the visual substance, one of a con- structive nature, or assimilation (black, green, blue), the other of a destructive character, or disassimilation (white, red, yellow). The visual substance of the retina, according to this view, is com- posed of three constituents, viz., a white-black substance, decom- posed by white light, reconstructed in darkness ; a green-red sub- stance, decomposed by red, reconstructed by green rays; and a blue-yellow substance, decomposed by yellow, reconstructed by blue rays. This theory affords a good explanation of comple- mentary coloured after-images, and also of colour blindness. Retinal Purple. The retina is coloured by a violet-red colour- ing matter the retinal or visual purple which is bleached by light. Its connection with vision is unknown, for it is found only in the rods, not in the cones, and, therefore, not in the fovea, where vision is most distinct. Optical Illusions. Examples of these are seen in (a) Zollner's figures, (ft) Irradiation, (c] Parallel horizontal and verti- cal lines in the form of a square giving rise to illusion as to length and depth, (d) Colour illusions, or the simultaneous contrasts of Helmholtz. Imperfections of the Eye 1. Colour blindness, before alluded to. 2. Astigmatism, due to the curvature of the cornea being less in the vertical than the horizontal direction. In consequence, the horizontal and vertical portions of the image are not at exactly the same level. 3. Aberration, spherical 'and chromatic. The former due to the different degree of refraction of the rays falling upon the circum- ferential and more central portions of the cornea and lens ; the latter to dispersion, caused by the refractive media. Normally, these defects are discoverable only by making experiments on the eye. VISION. 165 4. Entoptic phenomena, or shadows of objects floating in the humours of the eye, thrown upon the retina. They usually have the form of round particles or bands, forming the so-called musccz volitantes. Summary A luminous body throws the luminiferous ether around it into a concentric series of waves or undulations. These ether waves only -olnroth i ncn l n travel towards the eye with a velocity of 186,000 miles per second. They make up the rays of light which travel in straight lines, and on reaching the transparent cornea pass through it. After traversing the aqueous humour, they pass through the pupil, and immediately fall upon the crystalline lens. By this lens they are so refracted that an image of the object is formed upon the retina, especially that part of it called the yellow spot. The light waves have the peculiar power of exciting that characteristic structure in the retina known as the layer of rods and cones, especially, if not exclusively, the cones. These bodies are connected through the other layers of the retina with the most internal layer of nerve-fibres forming the termina- tion of the optic nerve. The excitation of the cones is thus trans- mitted to the optic nerve, up which it passes as a nervous impulse to the brain. On arriving at that organ, it so affects it as to pro- duce that state of consciousness known as vision. APPENDIX. METRIC SYSTEM OF WEIGHTS AND MEASURES. This system is almost universally adopted in scientific calcu- lations. It has the advantages (i) of being a decimal system, and (2) its units being constants. The unit of Length is the Metre. A metre is 39*37 inches in length. It is very nearly roooVo^o P art of a quadrant of the earth's circumference, i.e. t the distance from the pole to the equator. The dimensions and multiples of a metre are as follows : To metre = i decimetre. i~Jo metre = i centimetre. ilfo~o metre = t millimetre. 10 metres = i decametre. 100 metres = i hectometre. 1000 metres = i kilometre. The unit of Volume is the Litre = i cubic decimetre. Its divisions and multiples are precisely similar to those of the metre, decilitre, centilitre, millilitre, decalitre, hectolitre, kilolitre. A litre = 61-027 052 cubic inches (rather less than a quart). The unit of Weight is the Gramme. A gramme is the weight of i cubic centimetre of pure distilled water at a temperature of 4 Centigrade. A gramme = 15*432349 grains. A kilogramme weighs 2*2046 Ibs. Divisions and Multiples of the gramme are : f5 gramme = i decigramme. 100 gramme = i centigramme. :ToVo gramme = i milligramme. 10 grammes = i decagramme. 100 grammes = i hectogramme. 1000 grammes = i kilogramme. APPENDIX. 167 The Thermometer usually employed in scientific experiment is the Centigrade. The Centigrade Thermometer differs from that in general use in this country (Fahrenheit '*) in the freezing point being zero or o instead of 32, and boiling point being 100 instead of 212. Putting C. for Centigrade, F. for Fahrenheit, oC. corresponds to 32F. IOOC. 2I2F. Therefore, 100 divisions of the C. corresponds to 180 of the F. Or, 5 9 ^r, i ,, ,, ,, 1-5 ,, With these data, it is very easy to convert a Centigrade into a Fahrenheit reading, or vice versa. THE ABERDEEN UNIVERSITY PRESS.