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 Croskey, John Welsh, I858- 
 
 Anatomy and physiology of the eye and iti 
 appendages . 
 
UNIVERSITY OF CALIFORNIA 
 SAN FRANCISCO LIBRARY 
 
Anatomy and Physiology 
 
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 OF THE 
 
 YE AND ITS Appendages 
 
 BY 
 
 JOHN WELSH CROSKEY, M. D. 
 
 OPHTHALMIC SURGEON 
 
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 PHILADELPHIA GENERAL HOSPITAL 
 Philadelphia, Pa. 
 
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 Anatomy and Physiology 
 
 OF THE 
 
 Eye and its Appendages 
 
 BY 
 
 JOHN WELSH [CROSKEY, M. D. 
 
 OPHTHALMIC SURGEON 
 
 TO THE 
 
 PHILADELPHIA GENERAL HOSPITAL 
 PhUadelphia, Pa. 
 
 
 PUBLISHED BY 
 
 SMITH-BDWAUDS CO. 
 
 129 NORTH laTH STREET 
 
 PHILADELPHIA 
 
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PREFACE 
 
 This little pamphlet is the outgrowth of the instruction given 
 to students and nurses at the Philadelphia General Hospital during 
 the past ten years. The author believes that if the anatomy of the 
 eye is thoroughly understood, it renders subsequent study less diffi- 
 cult. 
 
 Every effort has been made to simplify the text so that the 
 subject matter can be readily understood. 
 
 The author's thanks are due to his son-in-lavsr, Mr. Lew^is 
 Elberon Bailey, for the tw^o most excellent illustrations. 
 
DEDICATION 
 
 Affectionately dedicated 
 
 to 
 
 my preceptor ^ the late Henry Earnest Goodman^ M. Z>., 
 
 to whom I owe much of my success. 
 
Anatomy and Physiology of the Eye 
 AND ITS Appendages 
 
 JOHN WELSH CROSKEY, M. D. 
 
 OPHTHALMIC SURGEON 
 
 TO THE 
 
 PHILADELPHIA GENERAL HOSPITAL 
 
 PHILADELPHIA, PA. 
 
 ANATOMY, 
 
 The eye is the organ of vision. 
 
 Vision is the faculty or sense of sight. 
 
 The Orbits are the cavities which contain the eyeball, and are four. 
 sided, pyramidal, boney cavities, with the bases in front and the apices be- 
 hind. They are formed of seven bones: superior maxillary, malar, frontal, 
 palate, sphenoid, ethmoid and lachrymal. 
 
 The orbital plate of the superior maxillary, a portion of the malar, the 
 orbital process of the palate bone form the floor of the orbit. The orbital 
 plate of the frontal and a small portion of the sphenoid form the roof. The 
 ethmoid, lachrymal, sphenoid and the nasal process of the superior maxillary 
 form the inner wall, while the outer wall is formed by the sphenoid and 
 malar. 
 
 The Floor of the Orbit is inclined, being higher on the nasal side 
 than on the temporal side. It is thin and smooth and separates the orbital 
 cavity from the antrum of Highmore. 
 
 The Roof of the Orbit, except at its margins and especially the 
 anterior margin, is very thin, is easily penetrated and quite unable to resist 
 pressure from either side. On its opposite sides are the frontal sinus anter- 
 iorly, and the cranial cavity posteriorly. 
 
 The anterior margin of the roof is thick and overhangs the orbital 
 cavity. The inner wall is extremely thin and upon its opposite side, in 
 front, is the frontal sinus; below and behind is the nasal cavity. The outer 
 wall is very thick at the rim, thin immediately behind the rim, then thick 
 again on the sphenoidal portion. On the opposite side of the frontal half 
 of this wall is the temporal fossa. At the posterior portion, the wall sepa- 
 rates the orbit from the cranial cavity and at its extreme portion, both 
 cavities are united through the sphenoidal fissure. 
 
The Foramina of the Orbit are nine in number: 
 
 1. The Optic Foramen, for the passage of the optic nerve and ophthal- 
 mic artery. 
 
 2. The Sphenoidal Foramen (fissure Foramen Lacerum Anterius), 
 for the passage of the third, the fourth, the ophthalmic division of the fifth 
 and sixth cranial nerves, some filaments from the cavernous plexus of the 
 Bjmapathetic, the orbital branch of the middle miningeal artery, a recurrent 
 branch from the lachrymal artery to the dura mater, and the ophthalmic 
 vein. 
 
 8. The Spheno Maxillary Foramen (fissure), for the passage of 
 the superior maxillary nerve and its branches, the infraorbital artery and 
 ascending branches from Meckel's ganglion. 
 
 4. The Lachrymai, Foramen (groove), is occupied by the lachrymal 
 or nasal duct. 
 
 5. The Supraorbital Foramen (sometimes a groove), for the passage 
 of the supraorbital artery, vein and nerve. 
 
 6. The Infraorbital Foramen (groove), for the passage of the in- 
 fraorbital nerve and artery. 
 
 7. The Malar Foramen (sometimes two), for the passages of the 
 facial nerve and vessels. 
 
 8 and 9. The Anterior and Posterior Ethmoidal Foramina, for 
 the passage of the anterior Ethmodial artery and the nasal nerve. 
 
 The orbit is lined by periosteum which, after entering the orbit through 
 the optic foramen, is divided into two parts : one part lining the orbit and the 
 other part continuing in a fibrous sheath known as Tenon's capsule, which 
 invests the posterior parts of the eyeball. 
 
 As the periosteum enters the orbit through the optic foramen it forms 
 a tendinous ring, which ring gives origin to the ocular muscles. 
 
 The Orbital Periosteum is covered with a layer of connective tissue 
 and fat, upon which the eye rests. 
 
 In the roof of the orbit are two depressions: the fovea trochlaris at 
 the inner angle, and the fossa lachrymalis at the outer anterior angle. 
 
 THE EYE LIDS. 
 
 The Eye Lids are the curtains, movable at will, that cover the eyeballs 
 in front. They are the appendages of the eye and are composed of integu- 
 ment, connective tissue, orbicularis, palpebrarum muscle, tarsal cartilage. 
 Meibomian glands, and conjunctiva. 
 
 The opening formed by the edges of the lids is called the palpebral 
 fissure. This fissure at the outer side is called the outer canthus,, at the inner 
 side the inner canthus. The outer canthus is more acute than the inner, and 
 the lids here lie in close contact with the globe ; but the inner canthus is pro- 
 longed for a short distance inwards, towards the nose, where the two lids are 
 separated by a triangular space called the lachrymal lake or locus lachrymalis^ 
 
 6 
 
The lids contain a semilunar framework of condensed connective tissue 
 which is called the tarsal cartilage. This gives the lids their shape and firm- 
 ness. In the upper lid it is oval and is thickest at its anterior edge. In the 
 lower lid it is thinner and narrower and is of nearly uniform breadth 
 throughout. The lids are held firmly in position by fibrous tissue internally 
 to the tendo oculi, externally to the malar bone, and above and below to 
 the margins of the orbit by the palpebral ligament. The upper lid contains 
 about 30 large sebaceous glands called Meibomian glands, and the lower 
 lid contains about 20. The ducts from these glands open on the free borders 
 of the posterior lip of each lid. Their function is to secrete a sebum which 
 lubricates the edges of the lids, preventing them from sticking together and 
 also preventing the overflow of the lachrymal secretions. 
 
 The Eyelashes (Cilia) are attached to the free edges of the eyelids. 
 They are short, thick, curved hairs, arranged in a double or triple row at the 
 margin of the lids ; those of the upper lid are more numerous and longer than 
 the lower, and are curved upwards; those of the lower lid are curved down- 
 wards, by which means they do not interlace on closing the lids. 
 
 The Conjunctiva coats the posterior surface of each lid and also the 
 anterior surface of the eyeball. It is divided into three portions: the part 
 covering the posterior surface of each lid, which is known as the conjunctiva 
 tarsi; that part which coats the anterior surface of the eyeball called the 
 conjunctiva bulbi and the connection between these two parts which is called 
 Conjunctiva Fornix. The conjunctiva tarsi is best seen by everting the lid. 
 It has a smooth surface and is intimately and immovably adherent to the lids. 
 
 The Region of Transition (or fornix) is very readily brought to view 
 in the lower lid by drawing the lid down while the eye looks up. In the 
 upper lid, on the contrary, the fold of transition is hard to see, unless we 
 make a double eversion of the lid. The region of transition is the loosest 
 part of the conjunctiva, being so abundant that it lies in horizontal folds. 
 This arrangement insures the eye its free power of movement. If the 
 conjunctiva were to pass directly from the lid to the eye, as is sometimes 
 observed in consequence of disease of the conjunctiva, every movement of 
 the eyeball would be transmitted to the lids ; and if one of the lids were held 
 still with the finger, the eyeball would be hampered by it in its movements. 
 The conjunctiva is present in such quantity at the fornix that the eye 
 is able to move in complete independence of the lids, the folds in the region 
 of transition being smoothed out or crumpled together, as the movement may 
 require. Appearing through the lower fold of transition are the extensive sub- 
 jacent plexus of veins and also the white glistening fascia. Its lax char- 
 acter and also its abundant blood supply render the fold of transition par- 
 ticularly liable to great swelling in inflammations of the conjunctiva. 
 
 The microscope shows that the palpebral conjunctiva is covered with 
 a laminated cylindrical epithelium. The mucous membrane proper is of 
 adenoid character — that is, even in the healthy state it contains an abundant 
 quantity of round cells (lymph corpuscles), which notably increase in num- 
 ber with every inflammation of the conjunctiva. Of glands it possesses 
 acinous mucous glands, which are found along the convex border of the 
 tarsus. Analogous glands are present in the fornix conjunctiva. 
 
"The conjunctiva of the upper lid obtains its blood supply from two 
 arterial arches, the arcus tarseus superior and the arcus tarseus inferior. 
 These lie upon the anterior surface of the tarsus, near its upper and its 
 lower edges. To reach the conjunctiva, the branches of the arcus tarseus 
 inferior perforate the tarsus through its entire thickness from before back- 
 ward, two or three m.m above the free edge of the lid. The line along 
 which the vessels come out from the tarsus is marked by a shallow furrow 
 (sulcus subtarsalis) on the conjunctival surface of the lid. On the lower 
 lid there is only one arterial arch. 
 
 The Conjunctiva Bulbi covers the anterior surface of the eyeball. 
 It has no aperture corresponding to the cornea, but continues, even if with 
 altered character, over the latter. This continuity of the conjunctiva makes 
 it plain to us why morbid processes of the latter do not stop at the margin 
 of the cornea but are continued upon its surface, as we see very 
 clearly in trachoma and in conjunctivitis eczematosa. The two divisions 
 of the conjunctiva bulbi are distinguished as the conjunctiva scleraa and 
 conjuctiva cornea. 
 
 The Conjunctiva Cornea is perfectly transparent, and is so intimately 
 adherent to the cornea proper that it must be regarded as the uppermost 
 layer of the latter, and is better treated in connection with the cornea itself. 
 
 The Conjunctiva Sclerae covers the anterior segment of the sclera 
 in the form of a thin pellicle. It is connected with the sclera by lax con- 
 nective tissue (the episcleral tissue) so loosely that it can readily be moved 
 about from side to side upon the sclera. It is only at the periphery of the 
 cornea, where it ends in a sharp edge, the limbus (Limbus-hem) conjunctiva, 
 that the conjunctiva sclerae is intimately adherent to its substratum. It is 
 very thin and elastic and lets the white sclera be seen through it plainly, thus 
 forming the **white of the eye." 
 
 In old people there is a spot at the inner and the outer margins of 
 the cornea which contrasts by its yellow color with this whiteness. This 
 has the shape of a triangle with its base at the corneal margin, and projects 
 a little above the rest of the conjunctiva. It is called the interpalpebral 
 spot or the Pinguecula, and is caused by an alteration, that has taken place 
 in the tissues of that part of the conjunctiva which, being included in the 
 interpalpebral fissure, is constantly exposed to atmospheric influences. 
 
 The conjunctiva sclerae is covered with laminated pavement epithe- 
 lium and contains no glands. At the inner angle of the eye it forms a 
 crescentic duplication, the semilunar fold (plica semilunaris) ^ which repre- 
 sents an abortive remnant of the palpebra tertia of animals. To the inside 
 of the semilunar fold is a small, reddish, nipple-like prominence, the car- 
 uncle, which occupies the bottom of the horseshoe-shaped excavation at the 
 angle of the eye. This is shown to be histologically a small island made of 
 skin, containing sebaceous and sweat glands and having its surface covered 
 with delicate, light-colored hairs. 
 
 The conjunctiva of the eyeball receives its blood vessels chiefly from 
 the vessels of the fold of transition — the posterior conjunctival vessels. 
 Furthermore, the anterior ciliary vessels take part in supplying the con- 
 junctiva with blood. These vessels come from the four recti muscles and 
 run under the conjunctiva (through which they are visible, shining with a 
 
 8 
 
bluish luster) until near the edge of the cornea, where they suddenly dis- 
 appear, since they pass through the sclera into the interior of the eye. But, 
 before this happens, they give off branches which end in vascular loops, 
 in the limbus conjunctiva directly at the margin of the cornea (marginal 
 network of the cornea). This latter is of great importance for the cornea, 
 which is chiefly dependent upon it for nutrition. Other branches of 
 the ciliary vessels (anterior conjunctival vessels) run backward in the 
 •conjunctiva toward the posterior conjunctival vessels and anastomose with 
 them. 
 
 We have, therefore, in the conjunctiva two vascular systems — ^that of 
 the posterior conjunctival vessels and that of the anterior ciliary vessels. 
 According as the one or the other system is over distended with blood, 
 the conjunctiva has a different aspect, which we designate respectively as 
 conjunctival and as ciliary injection. 
 
 The Lachrymal Apparatus. 
 
 The Lachrymal Apparatus consists of the lachrymal gland and the 
 lachrymal passages. 
 
 The Lachrymal Gland (glandula lacrimalis) is an acinous gland which 
 consists of two divisions. The larger of these, known as the superior 
 lachrymal gland, lies in the upper external angle of the orbit in a depres- 
 sion in the bony wall of the latter, the fossa glandulae lacrimalis. The 
 excretory ducts of the superior lachrymal gland pass downward and empty 
 into the external half of the superior fornix conjunctiva. 
 
 The second division of the lachrymal gland — the inferior lachrymal 
 gland — is much smaller, and consists only of one or two lobules, for which 
 reason it is also known as the accessory lachrymal gland. Its lobules lie 
 along the excretory ducts of the superior gland directly beneath the mucous 
 membrane of the fornix. If the upper lid is everted and at the same time 
 the eye is made to look downward, we often see the conjunctiva of the 
 fornix in the vicinity of the outer angle of the lid pushed forward by a 
 soft mass which is in fact the inferior lachrymal gland. Krause's glands 
 form a sort of continuation of the lobules of the inferior lachrymal gland 
 over the fornix as far as its inner end. Their structure is that of the 
 lachrymal gland, so that they may be regarded as the ultimate scattered 
 outlying portions of the latter. 
 
 The Lachrymal Passages begin with the puncta lacrimalia. These 
 lie on the free border of the upper and lower lid (upper and lower punctum) . 
 In its downward course the lachrymal duct deviates a little backward 
 and outward from the vertical. Hence the two lachrymal channels diverge 
 as they go down, the lachrymal sacs being less far apart than are the lower 
 orifices of the lachrymal ducts. We can represent the course of the lachry- 
 mal channel on the living subject by placing a straight sound in such a way 
 as to lie at its upper part upon the middle of the internal palpebral liga- 
 ment, and below upon the furrow forming the boundary line between the 
 cheek and the alae of the nose. The position of this sound gives precisely the 
 direction of the lachrjmial duct. If we place a sound in this way on each 
 
 9 
 
side of the nose, we see how the sounds diverge as they go down, and we can 
 readily convince ourselves that the degree of divergence differs in different 
 individuals. The divergence, in fact, depends upon the breadth of the root 
 of the nose on the one hand, and upon the breadth of the inferior nasal 
 orifice on the other. These facts are of importance with regard to the 
 operation of sounding the lachrymal duct, in the performance of which the 
 sound must be pushed along in the direction of the duct. 
 
 The mucous membrane of the lachrymal sac and that of the lachrymal 
 duct form one continuous whole. There is, therefore, no sharp dividing 
 line between these two structures. They are mainly distinguished by the 
 fact that the lachrymal sac lies against the bone {the lachrymal bone) at 
 one side only, and everywhere else is free, while the lachrymal duct is 
 inclosed on all sides by boney walls. It follows from this that, in engorge- 
 ment of the lachrymal channels with fluid, it is only the lachrymal sac 
 which is distended so as to appear as a visible swelling at the inner angle 
 of the eye. The lachrymal duct cannot be distended; on the contrary, it 
 is the favorite seat of constrictions, which again do not occur in the lachry- 
 mal sac. The formation of these constrictions is facilitated by the fact that 
 a dense plexus of wide veins, analogous to the venous plexuses lie beneath 
 and near the inner extremity of the lid at the spot where the tarsus ter- 
 minates. They are situated upon small elevations, the lachrymal papillas 
 {papillae lacHmales) , and form the orifices of the canaliculi lacrimales. 
 These latter, starting from the puncta, run at first vertically for a short 
 distance — i. e., in the upper lid run upward and in the lower lid down- 
 ward; then they bend at a right angle and become directed toward the 
 lachrymal sac. In so doing they first pass behind the caruncle, and con- 
 verging more and more, at length reach the lachrymal sac. Into this they 
 empty, either separately or after having united to form a short common 
 trunk. 
 
 The Lachrymal Sac {sa^cus lacrimalis) lies in the inner angle of the 
 eye in the cleft {fossa sacci lacrimalis) which the lachrymal bone forms for 
 its reception. The lachrymal bone bounds the lachrymal sac on the inside, 
 while to the front and outside it is inclosed by the two branches of the liga- 
 mentum palpebrale mediale. This relation of the lachrymal sac to the in- 
 ternal palpebral ligament enables us to determine the position of the former 
 — a matter which is of importance when operations are concerned. If by 
 drawing the lids outward we put them on the stretch and so cause the pal- 
 pebral ligament to project, it can be seen that the lachrymal sac lies behind 
 the latter, and in such a way as to rise just above it by its summit or fundus. 
 
 At the spot where the cleft of the lachrymal bone merges into the 
 bony canal the lachrymal sac passes into the nasal or Lachrymal Duct 
 {ductus hbcrimalis) . The point where this transition occurs constitutes the 
 narrowest part of the whole lachrymal channel, and is, therefore, particu- 
 larly liable to the formation of pathological contractions {strictures) . From 
 this point the lachrymal duct passes downward and empties into the nasal 
 fossa below the inferior turbinated body beneath the mucous membrane of 
 the turbinated bodies and is interposed between the mucous membrane of the 
 lachrymal duct and the bony wall. The swelling of these veins is alone suf- 
 ficient to contract or to close entirely the lumen of the duct. 
 
 10 
 
The lachrymal passages are always filled with a small quantity of 
 lachrjnnal fluid. If air is found in them, it is to be regarded as a patho- 
 logical condition. 
 
 The Lagjirymal Secretion contains only a few solid constituents, 
 the main part of which is sodium chloride {hence "salty" tears). In the 
 normal state the lachrymal gland secretes scarcely any more liquid than 
 is lost by evaporation from the surface of the eyeball, so that but very small 
 quantities of fluid are discharged into the nose. It is only when the secre- 
 tion is increased, either in consequence of physical stimulation or of irrita- 
 tion of the eye, that any considerable quantity of tears is discharged into 
 the nose, where its presence is manifested by repeated blowing of the nose. 
 
 The moistening of the eyeball is not due to the lachrymal glands alone. 
 The secretion of the conjunctiva itself, and also of its mucous glands, par- 
 ticipate in the performance of this act. Hence it follows that even after 
 removal or degeneration of the lachrymal gland the eye does not become dry. 
 
 In the Conduction of Tears into the nose there are two factors to be 
 considered : the entrance of the tears into the lachrymal sac ; and their trans- 
 mission from the latter to the nose. 
 
 (a) The conveyance of tears through the puncta into the lachrymal 
 sac is effected by the act of winking. This takes place in such a way that 
 the palpebral fissure is closed by a movement beginning at the outer and 
 extending to the inner angle of the eye. Since the tears cannot flow off over 
 the border of the lid owing to the way in which the latter is lubricated 
 with fatty matter, they accumulate in the horseshoe-shaped notch in the 
 inner angle of the eye, and form the lacus lacrimalis into which the punctum 
 dips. If there is a perfect and water tight closure of the lids, the pressure 
 exerted by the latter at length forces the tears into the puncta. The pas- 
 sage of the tears into the lachrymal sac is facilitated by the passive dilata- 
 tion of the latter that occurs as the lid is closing; for the fibers of the 
 palpebral portion of the orbicularis arise in part from the internal palpebral 
 ligament, and hence, in contracting as they do during the closure of the lids, 
 draw the ligament away from the lachrymal bone. The anterior wall of 
 the lachrymal sac being connected with the palpebral ligament, is drawn up 
 at the same tim.e with it, so that the lachrymal sac is dilated and the con- 
 tents of the canaliculi are, so to speak, sucked into the sac. 
 
 (b) The conveyance of the tears from the lachrymal sac into the nose 
 is due partly to the constant entrance of fresh charges of tears from the 
 canaliculi and partly to the weight of the fluid; but the chief part in the 
 process is performed by the elasticity of the lachrymal sac. In virtue of this 
 elasticity the sac when distended by the tears tends to contract again and 
 thus expel the tears. Hence, in those pathological cases in which the lachry- 
 mal sac has lost its elasticity (atony of the sac) we observe that the con- 
 duction of tears downward is arrested, even though the nasal duct is 
 completely pervious. 
 
 The mucous membrane of the canaliculi is lined with laminated pave- 
 ment epithelium; that of the lachrymal sac and nasal duct with a single 
 layer of cylindrical epithelium. Acinous mucous glands are frequently 
 found in the latter. The mucous membrane at different spots projects in 
 the form of folds into the lumen of the lachrymal passages, a phenomenon 
 which has been described as a formation of valves. 
 
 11 
 
The largest of these folds is Hasner's valve, at the lower orifice of the 
 nasal duct. This, however, is not a true valve, any more than are the others 
 — that is, not a valve which could close up the lumen of the lachrymal 
 channel. On the contrary, it is simply a fold produced by the great obliquity 
 with which the nasal duct passes through the mucous membrane of the nasal 
 fossa. Like the other folds of mucous membrane in the lachrymal passages, 
 it is not of constant occurrence. 
 
 Duplication of the puncta and canaliculi, and also their absence, have 
 been recorded as congenital anomalies. 
 
 Different theories have been put forth to account for the transmis- 
 sion of tears into the nose. It is certain that perfect closure of the lids 
 forms an indispensable condition for the conduction of tears. If this closure 
 is interfered with — e. g., by paralysis of the orbicularis, narrowing of the 
 lids, notching of the border of the lids, etc. — epiphora at once makes its 
 appearance. The passage of tears into the lachrymal sac takes place even 
 when the rest of the way to the nose is cut off by the obliteration of the 
 nasal duct. Hence it follows that the tears are not sucked into the lachry- 
 mal sac through the rarefaction of the air in the nose during inspiration. 
 
 THE EYEBALL. 
 
 The Eyeball is situated in the anterior part of the orbit, to its outer 
 side and rests upon a cushion of cellular tissue and fat. It is protected 
 in front by the eyelids. In form it is a sphere of about 25 mm. in diameter 
 having the segment of a smaller sphere engrafted upon its anterior surface, 
 which slightly increases its anterior posterior diameter. The axes of the 
 two balls are parallel with each other, but do not correspond with the axis 
 of the orbits, which are directed outward. The eyeball does not reach its 
 full size until about 20 years of age. Its weight is about 95 grains. The 
 eyeball is composed of tunics or coats and of refracting media called hu- 
 mours. The tunics are 3 in number. Named from without inward they 
 are first sclerotica and cornea; second choroid, iris and ciliary processes; 
 third retina and zonula cilaris. 
 
 The First Tunic. The Sclerotic and Cornea are the outside coats 
 of the ball. Four-fifths of the ball is enveloped by the sclerotic ; the remain- 
 ing one-fifth by the cornea. 
 
 The Sclerotic, from a Greek word meaning hard, is a tough white 
 fibrous tunic or coat, and serves to give shape to the eye and protect its more 
 delicate interior. The Sclerotic is thickest at its posterior portion and 
 gradually becomes thinner as it approaches the cornea, where it again 
 thickens *at the sclero corneal junction. Entirely within the sclera at the 
 sclero corneal junction is the circular venous sinus (canal of Schlemm), 
 which is the outlet, whereby the aqueous humor finds its way into the cir- 
 culation. 
 
 The sclero corneal junction is beveled and receives the cornea in the 
 same way as the crystal of a watch is received by the groove in its case. 
 
 The Cornea {comu, a horn) is the transparent part of the external 
 coat of the eyeball and projects beyond the curving surface of the sclerotic 
 and is, therefore, the segment of a smaller sphere. In form it is slightly 
 
 12 
 
elliptical, the vertical diameter measuring 11 mm., while the horizontal 
 diameter measures 12 mm. The curvature of the posterior surface is some- 
 what greater than the anterior surface, the difference in the curvature 
 resulting in an increase in the thickness of the edges, the edges measuring 
 about 1 mm. A vertical section of the cornea shows it to consist of five 
 layers : 
 
 1. The Anterior Epithelium. 
 
 2. Bowman's Membrane (Anterior elastic Lamina). 
 
 3. The Stroma (Substantia propria). 
 
 4. Descement's Membrane (Posterior elastic Lamina). 
 
 5. The Posterior Epithelium. 
 
 1. The Anterior Epithelium is a continuation of the conjunctiva 
 and consists of pavement epithelium made up of cylindrical round and flat 
 cells. It is thickest at the periphery and thinnest at the center of the cornea. 
 
 2. Bowman's Membrane is a delicate, thin, homogeneous membrane 
 firmly attached to the lamellae of the stroma. In pathological conditions 
 and after death, the anterior epithelium separates very rapidly from Bow- 
 man's membrane. 
 
 3. The Stroma or true cornea forming 95% of the whole, is com- 
 posed of a ground substance and of cells. The ground substance is com- 
 posed of fine fibrillas of connective tissue, united together by a cement sub- 
 stance into flat bundles arranged in nearly regular lamellas. Between these 
 lamella are open spaces which are filled with lymph and are called lymph 
 spaces; these are connected with one another by small canals. Through 
 these spaces and canals circulate the Ijnnph upon which the cornea is de- 
 pendent for its nourishment. Circulating also in these canals and spaces 
 are a few white blood corpuscles. In the normal eye they are very few in 
 number, but as soon as any pathological condition occurs these white blood 
 corpuscles escape from the net work of blood vessels at the margin of the 
 cornea and increase very rapidly and play a very important part in any 
 inflammation of the cornea. 
 
 4. Descement's Membrane is a thin homogeneous hyaloid membrane 
 which serves to give the cornea its correct curvature. It is very resistant 
 to all pathological changes and when the entire cornea is broken down into 
 pus, this membrane will remain unimpaired for days. 
 
 5. The Posterior Epithelium is a single layer of flattened cells lining 
 the posterior surface of Descement's membrane. 
 
 The Corneal Nerves arise partly from the terminal branches of the 
 long ciliary and partly from the nerves of the bulbar conjunctiva. They 
 are abundantly distributed to the upper-most layers of the stroma directly 
 beneath Bov/man's membrane. Owing to the large nerve supply lesions of 
 the cornea are particularly painful. 
 
 The Second Tunic is composed of the Choroid, Ciliary Body and Iris. 
 It lines the inner side of the sclerotic, is perforated posteriorly by the optic 
 nerve, and in front has a circular opening called the pupil. 
 
 The Choroid is a vascular membrane of a rich chocolate brown color 
 upon its internal surface and a deep black within. It lies between the 
 
 13 
 
sclerotic and retina and extends from the optic nerve to the ciliary body. 
 It is pierced posteriorly for the passage of the optic nerve and is connected 
 anteriorly with the iris, ciliary processes and with the line of junction of 
 the cornea anu sclerotic, by a dense white structure, the ciliary ligament, 
 which surrounds the circumference of the iris like a ring. The choroid 
 membrane is composed of three layers: external, middle and internal. The 
 external layer is composed largely of veins and has been named Venae Vor- 
 tecosoe. The middle is composed largely of minute arteries. The internal 
 layer (or membrana pigments) as its name implies is composed of pigment 
 cells. The presence of these cells give the black appearance to the choroid. 
 
 The Ciliary Body is composed of the Ciliary Muscle and the Ciliary 
 Process and is that portion lying between the choroid and iris. It is con- 
 tinuous with both, and is about ^ of an inch in width. 
 
 The Ciliary Muscle is composed of circular and radiating fibres. The 
 circular fibers occupy that portion of the ciliary body near its junction with 
 the iris, while the radiating fibers are confined to the outer portion of the 
 ciliary body arising tendinously from the sclero-corneal junction, the poster- 
 ior elastic lamina, and from the ligamentum pectinatum iridis, and are 
 directed backward and inward where they terminate in the choroid. 
 
 The Ciliary Process is formed by folds (processes) of the choroid 
 at its interior margin. The folds or processes form a circle behind the lens 
 and cover the inner surface of the ciliary body. They are deepest and 
 thickest at their forepart and gradually taper into the choroid. Within the 
 folds of the ciliary processes are received the corresponding folds of the 
 suspensory ligament, which is also called Zonula Zinnii. 
 
 The Iris is a disk-shaped membrane of about one-half inch in diameter 
 (12 m) , perforated in the center, which perforation is called the pupil. By 
 its peripheral or ciliary border it is continuous with the ciliary body and is 
 held in position by the pectonate ligament and also by the anterior capsule of 
 the crystaline lens upon which the pupillary border rests. 
 
 The anterior surface of the iris is lined with a continuation of the 
 posterior epithelium of the cornea, and if examined closely is found to be 
 composed of elevations and depressions, which are caused by the blood vessels 
 lying in the stroma of the iris and running from the ciliary to the pupillary 
 margin. Near the pupillary margin these vessels interlace with a ring of 
 circular ridges. This divides the iris into two zones: the ciliary zone and 
 the pupillary zone. 
 
 The posterior surface of the iris is covered by the posterior lining 
 membrane and the retinal pigment layer, which layer gives the different 
 colors to the iris. The color of the irides in the same individual are, as a rule 
 similar, but there are instances of congenital differences in color in perfectly 
 normal irides. 
 
 The iris, by its ability to open and close, makes the pupil larger or 
 smaller and regulates the amount of light which enters the eye. Ordinarily 
 the pupil is about one-eighth to one-fifth of an inch (3 to 5 mm.) in dia- 
 meter. It should be round and promptly react to light stimulation. Under 
 the same illumination both pupils should be uniform in size. If one eye 
 should be shaded the pupil of the shaded eye should act in harmony with 
 the one exposed to light. This is termed the consenual action of the pupils. 
 
 14 
 
If the iris be thin and the rays of light pass through its substance, they 
 are immediately absorbed by the uvea, and if that layer be insufficient, they 
 are taken up by the black pigment of the ciliary process. In Albinos, 
 where there is an absence of the black pigment, the rays of light traverse or 
 pass through the iris, and even the sclerotic and so overwhelm the eye with 
 light, that the sight is very much diminished, except in the dimness of even- 
 ing. In the manufacture of optical instruments, care is taken to color their 
 interior black with the same object, the absorption of all extraneous rays of 
 light. 
 
 The Third Tunic, or inner tunis of the eye, is the Retina, a thin, 
 delicate membrane formed from the fibers of the optic nerve. In the living 
 eye it is perfectly transparent and of a purplish red color. The retina is 
 composed of two kinds of tissue, the nervous tissue and the supporting tissue. 
 The function of the latter is to maintain and support the extremely delicate 
 nervous tissue. The retina is attached at the entrance of the optic nerve 
 and at the ora serrata. It is not attached to the choroid, but simply lies on it. 
 
 An examination of the normal eye with the ophthalmoscope reveals the 
 optic disk, the retinal vessels and the macula lutea, which is the thinnest 
 part of the retina and the part that has the most delicate perception; this 
 is also called the fovea centralis, yellow spot. When we wish to get a precise 
 perception of an object we so adjust our eye that the image shall fall upon 
 this point. We fix the object or bring it within our central vision. 
 
 In contradistinction to central vision, we have peripheral or indirect 
 vision, which is vision with all the balance of the retina. The farther from 
 the fovea centralis the image is produced upon the retina, the less distinct 
 is the perception of its shape. The images falling upon the periphery of 
 the retina give us warning signals, which make us cast our eye directly upon 
 the objects which excite the image. Peripheral vision is, therefore, very im- 
 portant in walking. 
 
 The microscope shows the retina to be composed of ten different layers. 
 They are as follows: 
 
 1. Memhrana limitans interna. 
 
 2. Fibrous layer. 
 
 3. Vesicular layer. 
 
 4. Inner molecular layer. 
 
 5. Inner nuclear layer. 
 
 6. Outer molecular layer. 
 
 7. Outer nuclear layer. 
 
 8. External limiting membrane. 
 
 9. Jacob's membrane or rods and cones. 
 10. Pigmentary layer. 
 
 Jacob's membrane or rods and cones is the most important part of the 
 retina, as it is connected by nerve-fibrils with the layer of nerve fibers which 
 convey the visual impressions through the optic nerve to the brain. The optic 
 nerve collects its fibers from the retina. It pierces the choroid and sclera 
 and comes out a little to the nasal side of the posterior pole of the eye. This 
 portion of the nerve is called the intra ocular, while the part extending 
 from the back of the ball to the optic foramen is called the orbital, and that 
 part extending from the foramen to the optic chiasm is called the intra 
 
 15 
 
cranial, so that we have the optic nerve divided into three portions: THE 
 
 INTRA OCULAR, THE ORBITAL and THE INTRA CRANIA. It is about one-sixth 
 
 of an inch in diameter (-4 mm.) and is supplied by blood vessels from the 
 ophthalmic artery. The optic disk is that portion of the optic nerve included 
 within the margins of the choroidal foramen. It is usually round in shape 
 and is pierced in the center by the central retinal artery and vein. Both 
 the artery and vein bifurcate, one branch of each passing vertically upward 
 and the other downward to the retina. By the aid of the ophthalmoscope 
 the vein can easily be recognized from the artery by its size and color. 
 
 The Humors of the Eye Ball. The Aqueous Humor is a clear, 
 colorless, transparent liquid filling the anterior and posterior chambers of the 
 ball. It is faintly alkaline and has a sp. gr. of about 1005. It is secreted by 
 the ciliary processes and the posterior surface of the iris. If the anterior 
 and posterior chamber should be evacuated by accident or by an operation 
 the aqueous humor is rapidly reproduced. 
 
 The Vitreous Humor is a transparent, colorless, gelatinous mass which 
 fills the vitreous cavity. It is surrounded by a transparent capsule called 
 the hyaloid membrane. The anterior portion of the vitreous humor has a 
 deep depression called the hyaloid fossa. It is within this fossa that the 
 posterior portion of the crystalline lens rests. Beginning at the papilla of 
 the optic nerve and extending to the posterior surface of the crystalline lens 
 is a canal (canalis hyaloideus). During foetal life the hyaloid artery runs 
 into this canal. In the fully developed eye it probably serves as a lymph 
 channel. 
 
 The principal function of the vitreous humor is to hold the retina in 
 place. 
 
 The Crystalline Lens lies within the circle formed by the ciliary 
 processes, and is kept in position by the suspensory ligament or zonula 
 ciHaris. In this position it divides the eye into a smaller anterior and a 
 larger posterior section. It is a transparent and colorless structure of len- 
 ticular shape, the anterior surface of which is less, the posterior surface 
 more curved. It causes the rays of light to focus, in the normal eye, upon 
 the retina. 
 
 The function of the lens is to enable us to see distinctly all objects within 
 the range of vision, no matter at what distance. This is spoken of as the 
 power of accommodation. We can easily convince ourselves that the normal 
 eye possesses such a power, when we go to a baseball game and sit back of the 
 wire screen. We cannot see the screen and the player at one and the same 
 time. Another proof is found in the veils worn by women. This power of 
 accommodation is brought about by an alteration of form of the lens, its an- 
 terior surface becoming more convex and approaching the cornea. This al- 
 teration in form is accomplished by the ciliary muscle. When we arrive at 
 a certain age, which is about 45, if the eye was normal at birth, we require 
 spectacles for close work. This is due to the fact that at this time in life 
 the ciliary muscle loses some of its elasticity, and as a consequence has not 
 this ability to maintain the effort required in accommodating. When this 
 occurs we designate it as presbyopia, which means old sight. 
 
 The Ocular Muscles of the Eye are divided into the external extrinsic, 
 and the internal intrinsic. 
 
 16 
 
The External (extrinsic) Muscles control the movement of the eye 
 ball and are the four recti (straight) muscles and the two oblique muscles. 
 All six of these muscles are named from their relative positions: superior, 
 inferior, external and internal. 
 
 The Four Recti Muscles take their origin from around the optic 
 foramen. They come forward and are attached, as their names indicate, by 
 means of short tendons which spread out in the form of a fan and become 
 fused with the sclera. 
 
 The Two Oblique Muscles, the superior and the inferior, have a 
 more complicated course. 
 
 The Superior Oblique arises from the margin of the optic foramen and 
 runs forward upon the upper and inner wall of the orbit, to the upper and 
 inner margin of the orbit. It here passes through a firm fibrous loop, known 
 as the trochlea. From here it bends back at an acute angle and passes beneath 
 the superior rectus to the eyeball. Here it spreads out and is inserted into 
 the upper half of the ball about in the vertical meridian and behind the 
 equator. 
 
 The Inferior Oblique arises from the orbital plate of the superior 
 maxillary, and from here it runs upward and outward and is inserted about 
 in the horizontal meridian and also behind the equator of the ball. 
 
 The Nerve Supply of the External Muscles is derived from the 
 sixth cranial or abducens, the third cranial or motor oculi, and the fourth 
 cranial or patheticus. The sixth cranial supplies the external rectus. The 
 third cranial supplies the internal rectus, the superior rectus the inferior 
 rectus and the inferior oblique. The fourth cranial supplying the superior 
 oblique. 
 
 The Internal Muscles of the eye are the sphincter pupillsB and the 
 ciliary muscle. 
 
 The eye balls and their appendages receive their blood supply from 
 the branches of the ophthalmic artery. 
 
 The Supra-orbital Artery supplies the superior rectus and levator 
 palpebraB. 
 
 The Superior Palperbral Artery supplies the upper lid. 
 The Inferior Palpebral Artery supplies the lower lid. 
 
 The Lachrymal Artery supplies the lachrymal glands and the con- 
 junctiva. 
 
 The Nasal Artery supplies the lachrymal sac. 
 
 The Superior Muscular Artery contributes to the levator palpebrae, 
 superior rectus and superior oblique muscles. 
 
 The Inferior Muscular Artery contributes to the external rectus, 
 inferior rectus and inferior oblique muscles. 
 
 The Infra-orbital Artery supplies the inferior rectus and inferior 
 oblique muscles ( a branch of the internf\l maxillary artery) . 
 
 The Short Ciliary Arteries, about twelve in number, enter the 
 sclerotic around the optic nerve entrance and supply the choroid and ciliary 
 processes. 
 
 17 
 
The Long Ciliary Arteries (two in number) enter the sclerotic on each 
 side of the optic nerve, coming forward between the sclerotic and choroid 
 to the ciliary ligament, where they divide into two branches, forming the 
 circulus iridis, major and minor. 
 
 The Blood Supply of the external muscles is derived from the 
 ophthalmic artery. 
 
 The veins of the muscles empty into the ophthalmic and facial veins. 
 
 If we disregard the conjunctiva, there are no lymphatic vessels in the 
 eye. They are replaced by lymph channels and lymph spaces. These are 
 divided into two portions, the anterior lymphatic system and posterior 
 lymphatic system. The lymph of the anterior system is collected into the 
 anterior and posterior chambers of the ball, which chambers communicate by 
 means of the pupil. The lymph is secreted from the iris and ciliary pro- 
 cesses flowing from the posterior chamber through the pupil into the anterior 
 chamber. From here it filters through the sieve work of the ligamentum 
 pectinatum into the subjacent canal of Schlemm, and from here gets directly 
 into the anterior ciliary veins. 
 
 By far the greatest amount of lymph leaves the eye through the an- 
 terior system, and this, therefore, is more important than the posterior 
 system. 
 
 The posterior lymphatic system occupies the hyaloid canal, perichoroidal 
 space and tenon's space. The outflow of lymph from these spaces passes 
 into the lymph passages, which spread out along the optic nerve. The pas- 
 sages are found between the sheaths of the optic nerve (the intervaginal 
 space and around the sheaths called the supravaginal). 
 
 18 
 
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