IRLF ~r& 27 EXCHANGE BIOLOGY LIBRARY G BWl'WWIVd 'A 'N ' UNIVERSITY OF PENNSYLVANIA A STUDY OF THE KNEE JERK BY EDWIN B, TWTTMYER A THESIS PRESENTED TO THE FACULTY OP THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY PHILADELPHIA THE JOHN C. WINSTON COMPANY 1902 UNIVERSITY OF PENNSYLVANIA A STUDY OF THE KNEE JERK BY EDWIN B. TWITMYER .11 A THESIS PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY PHILADELPHIA THE JOHN C. WINSTON COMPANY 1902 &TB7.2- BIOLOGY LIBRARY G CONTENTS PAGE I. THE NORMAL KNEE JERK 5 II. VARIATIONS IN THE OSCILLATIONS FOLLOWING THE KNEE JERK 20 III. THE KNEE JERK WITHOUT BLOWS ON THE TENDONS. . . 24 ^4 4322 I. The knee jerk is immediately caused by the sudden and vigorous contraction of the quadriceps muscle of the thigh. The quadriceps is the great extensor muscle of the leg, forming a large fleshy mass, which covers the front and sides of the femur. Sherrington 1 has demonstrated by experimentation upon animals that two of its four divisions, the vastus internus, covering the inner surface of the femur, and the crureus, covering the front of the femur, are chiefly involved in the knee jerk mechanism. These two portions are so intimately blended anatomically as to form a single muscle. So considered it is related by its superficial surface with the psoas and iliacus, the rectus, satorius, pectineus, adductors, and fascia lata ; by its deep surface with the femur, sub-crureus, and synovial membrane of the knee joint. The tendons of the different portions of the quadriceps unite at the lower portion of the thigh, so as to form a single strong tendon which invests the patella throughout its whole surface except underneath where a free articular surface is presented, and con- tinues downward to be inserted in the rough bony prominence of the tibia. From the lower border of the patella to the insertion in the tuberosity of the tibia it is known as the ligamentum patellae. The patella may therefore be properly regarded as a sesamoid bone, developed in the tendon of the quadriceps and the ligamen- tum patellae. Immediately below the lower edge of the patella one-half inch to an inch the tendon passes over a slightly hollowed space in the parts beneath. The nervous structures involved in the knee jerk mechanism have also been determined with exactness. Sherrington 1 has located the position of both the afferent and efferent conduction paths. He found the afferent path to lie in the posterior root of the fifth lumbar of the Rhesus monkey, which corresponds to the fourth of man. This posterior root receives afferent fibers from the obturator and anterior crural nerves, and from the external and internal popliteal nerves./ By the method of severance and other modes of destruction of the conductivity of the nerves 1 Journal of Physiology, Vol. 13, p. 666. (5) entering the root, e. g., cooling to the freezing point, etc., Sherring- ton found that of the fibers entering the root from these sources, those on which the jerk depends are not from any except the anterior crural nerve, and in the anterior crural nerve only those fibers which issue from the vastus internus and crureus muscles. The peripheral terminations of the sensory conduction path are therefore found in the muscles whose contraction causes the movement. Further it is probable that some of these fibers also terminate in the tendon of the muscle. 1 By the same method the efferent conduction path was found in the anterior roots of the fifth and fourth lumbar nerves of the Rhesus (fourth and third of man) and was traceable along the anterior crural nerve into those of the muscular branches of that trunk which supply the vastus internus and crureus divisions of the quadriceps, i. e., in exactly the same muscles in which the afferent condition path has its peripheral termination. Pathological evidence, offered by cases presenting degeneration of posterior nerve roots and certain portions of the posterior columns of the cord in which the knee jerk is observed to be entirely absent, makes it clear that the spinal center involved in the knee jerk mechanism is situated somewhere between the second and fifth lumbar segments inclusively. Mills 2 agrees with Edinger, Starr and others in placing it in the second or third lumbar segments or probably both. A blow on the patella' tendon (a) stimulates the peripheral sensory fibers terminating freely in the superficial tissue, (6) stimulates the peripheral sensory nerve fibers terminating in the tendon, (c] depresses the tendon thereby drawing it taut and exerting through its attachments a longitudinal stress upon the fibers of the quadriceps, thus offering a stimulus to the peripheral sensory nerve fibers terminating in the muscle itself, (d) by 1 As to the manner in which sensory nerve fibers terminate in the tendons, Philipp Stohr, in Lehrbuch der Histologie, 1901, says: "Die Sehnenspindeln sind meist spindelformige Auftreibungen von Sehnenbundeln, die von einer gut entwickelten bindegewebigen Hulle umgeben werden. Das eine Ende der Spindel geht in Sehnenbundel uber, das andere setzt sich in Muskelfasern fort. Die an die Mitte herantretenden Nervenfasern theilen sich wiederholt, verlieren ihr Mark und gehen in ein reich entwickeltes Astwerk uber mit oft keulenformig angeschwollenen Enden. 2 Journal of Nervous and Mental Disease, 1899, p. 142. depressing the tendon serves as a mechanical stimulus to the quadriceps. With the given anatomical structures comprising the knee jerk mechanism and the given results of a blow on the ligamentum patellae, three explanations of the physiological cause of the knee jerk are possible. 1. The knee jerk is a reflex. Stimulation of the sensory fibers terminating in the superficial tissue by contact or by an electrical current does not produce the movement, consequently if the jerk is a reflex, the peripheral excitation has its origin in the tendon or muscle (or both). In either case the course of the afferent impulse is the same. ''From the tendon of the patella, the excitation is carried to the spinal cord by crural sensory fibers, and enters by the dorsal root into a certain portion of the dorsal column, and thence passes to the dorsal horn; next it takes its course through the intermediate gray matter until it reaches cells of the ventral horn; and thence the motor excitation goes through the motor roots and crural nerve to the anterior muscles of the thigh. 1 " Waller 2 objects to this theory on the ground that the time elapsing between the blow and the response is too short for the trans- mission of an excitation over a reflex arc. By exact measurement he found the time between the percussion of the tendon and the contraction to be between .03 and .04 second and between the direct percussion of the muscle and the contraction .03 second. Exner's reflex, the winking of the eyelid, has a latent time of .05 seconds. Aside from this one instance a knowledge of normal reflex times is still wanting. 2. The knee jerk is a muscular contraction due to the direct mechanical stimulation of the muscle. On this theory the func- tion of the nervous structures involved, i. e., an afferent and efferent conduction path and a spinal segment, any interruption of which results in a total loss of the jerk, is to maintain by means of a constant reflex influence, the tonicity of the muscle. With the tone present the muscle reacts directly to the mechanical stimulus. Opposed to this theory is the fact that the knee jerk may be present when muscle tone appears to be wanting and may be absent in the case of men who apparently have a normal 1 Mills, Journal of Nervous and Mental Disease, 1899, p. 142. 2 Journal of Physiology, Vol. 11, p. 384. :.;{ -%i ; ,v. % ": 8 amount of tone. 1 Further the theory does not satisfactorily explain the facts of reinforcement, e. g., the increase in the extent of the kick when the hands are clinched just before the tendon is struck. Lombard 2 made a study to determine the changes in the tone and irritability of the quadriceps and found that neither increased under reinforcing conditions. 3. The knee jerk is first due to the mechanical stimulation of the muscle and second to the reflex excitation, i. e., the muscle contracting in response to the mechanical stimulus represents the beginning of the kick, while the reinforcement or continuance of the movement is the result of the reflex impulse. Lombard 3 reports the results of one case which showed an irregularity "which one might expect if when the original contraction of the muscle had reached its highest point or when the muscle had even begun to relax, a second impulse had reached it, and caused it to contract still further." Although this result lends support to the theory, the non-observance of this irregularity in all knee jerk records is not evidence to the contrary. The reflex influence may gradually become operative quite before the movement due to the mechanical stimulation had reached its maximum height, in which case the irregularity would not appear in the record. Improved methods of recording the excursion of the leg may reveal different rates of movement during different divisions of the excursion and thereby furnish more conclusive evidence on this point. Some evidence for this theory is presented in Part III of this study. The following study comprises (1) an attempt to determine as nearly as possible the extent of the unaugmented or normal knee jerk for normal subjects and (2) a consideration of the modifica- tions it is observed to undergo during an extended period of experimentation. When the patella tendons are struck at exactly the same place with blows of constant force and at regular inter- vals, no two of the resulting knee jerks are of the same extent. This variation is usually referred to some accidental stimulus acting upon the subject, e. g., a loud or distracting sound or to an idea to which the subject directs his attention, an emotional state of greater pr less intensity, any one of which conditions may 1 See Diagram, p. 17. 1 Journal of Physiology, Vol. 10, p. 122. 4 American Journal of Medical Science, Vol. 93, p. 88. produce a variation in the extent of the jerk, or in a general way to a change in the activity of the central nervous system. 1 From the very nature of the case such sources of variation are not wholly avoidable. However secure the subject may be kept from accidental sensory stimuli, the stream of consciousness is never altogether within control of the experimenter and the organic processes of digestion, circulation, etc., are constantly producing some slight or more profound modification in the equilibrium of the nervous system. The results of a study made by Noyes 2 on the unaugmented knee jerk in sleep in a case of terminal dementia suggest a more definite explanation for the variations occurring in kicks following in close succession when the conditions remain precisely the same. Noyes obtained the knee jerk curve and the Traube-Hering curve for the same period and found a well-marked coincidence between them. A comparison shows that the Traube-Hering curve descends lowest in that part of the group of knee jerks where the kicks are longest and at the place where the Traube-Hering curve is highest the knee jerks are much diminished. A rise in the Traube-Hering curve indicates increased blood pressure in the arm, and a fall in the curve corresponds to diminished blood pressure. On the theory that increased blood pressure in the extremities means lessened blood pressure in the central nervous system there is relative anaemia of the brain and cord when the Traube-Hering curve is at its height, and relative hyperaemia of the brain and cord when the curve is lowest. The diminished knee jerk would then follow from the lessened functional activity of the spinal cord at the height of the Traube-Hering wave, while an increased knee jerk from increased functional activity of the cord would follow at the low phase of the peripheral Traube-Hering curve. Although these results were obtained from a demented subject there is no reason for believing that the same relation would not 1 In a consideration of these variations Sommer says: "Es mussen demnach in den betreffenden Individuen Krafte wirksam sein, welche den Reflex hemmen oder verstarken. Diese Curvenreihen sind der einfachste Ausdruck eines variablen Momentes im Nervensystem, vermoge dessen bei gleichem Reiz verschiedene Wirkungen zustande kommen." Psychopatholog. Unter- suchungs-Methoden, 1899, p. 30. 2 American Journal of Psychology, Vol. IV, No. 3. 10 maintain between the two curves for a normal subject - provided all cerebral influences could be successfully removed or inhibited. Provided Noyes' inferences as to the influences affecting the knee jerk in the demented case are legitimate it is not evident why the same inferences do not apply to the normal subject. This fact may therefore be looked upon as one of the sources of the variation occurring in the extent of the jerks in a given group. In 1887 Lombard 1 made a study or the character and extent of this so-called normal knee jerk and in addition to variations occurring in successive kicks, found variations corresponding to the time of day at which the experiments were made. This study comprised a series of experiments made on one subject, during fourteen consecutive days in which the jerk of the right leg was examined seven times a day, twenty-five experiments being made at each examination. 2 The hours chosen were as follows, viz., 8.15 immediately upon arising; 9.15 soon after breakfast; 1.15 just before lunch; 2.15 just after lunch; 6.15 just before dinner; 8.00 soon after dinner, and 11 just before retiring. The averages of the results for the different periods were as follows: 25, 65, 43, 47, 30, 40, 27 mm. From these results Lombard concludes that there is a diurnal variation of the knee jerk. "The variation corresponds with the gradual loss of vigor which the body as a whole suffers from morning till bedtime. This decline is an interrupted one and further corresponds to the temporary and partial recoveries which the body undergoes as the result of the fresh supplies of nutriment and rest which it obtains at each meal." Lombard further observed that the average knee jerk for any given day did not always correspond with the average kick of other days in the series. In a comparison of the mean temperature and mean barometric records for the same period he found that there 1 American Journal of Psychology, Vol. I, No. 1. 2 For the assumed normal condition Lombard had the subject lie upon his left side upon a comfortable couch, the back and head being supported. The right thigh rested in a splint of plaster of Paris, shaped so as to conform to the inner and posterior surface, and of such a height as to hold the knee on a level with the hip joint. The right foot was supported at the same height by a swing suspended by a cord from the ceiling. In this position the muscles were passive, and the whole body was as far as possible in a state of rest. 11 was a marked correspondence in three curves. In general as the temperature rises the knee jerk becomes less and as the tempera- ture falls the knee jerk becomes larger. On the other hand the knee jerk rises and falls with the barometric curve. A few preliminary series of experiments upon several subjects is sufficient to establish the fact that the knee jerk is subject to marked individual differences. 1 The question as to what may be considered the normal jerk of the normal subject therefore presents itself. In order to study this problem, an extended series of experiments was performed upon seventeen subjects, seniors in college and graduate students, all of whom in a healthy and normal condition. One subject, however, during the period of time over which the experimentation extended, developed certain nervous symptoms which will be noted later. The experi- ments, divided into four groups, were made on four different days, a group each day, usually with an interval of one week, but in some few cases, at an interval of two or three weeks. The experi- ments on a given subject were made at the same hour each day and consequently always followed the same amount of preceding college work. A group consisted of either forty or forty-five experiments, which were divided into series containing five experiments each. The initial, middle and final series, i. e., the first, fifth, and eighth or ninth, were always made under the assumed normal conditions, the remaining series of the group involving various augmenting conditions, e. g., muscular contrac- tion, mild electrical shock to various portions of the body, pain stimulus, etc. In all of the experiments the force of the blows on the tendons and the intervals between the blows were kept constant. The subjects were directed to allow the body to relax and to avoid as far as possible, giving active attention either to the experiment or to any ideas which might come into mind. In order to eliminate as far as possible all variations due to the augmenting effect of accidental stimuli, the experiments were 1 Sommer views the knee jerk phenomenon as the motor expression of a definite disposition of the nervous system and maintains "dass aus dem Nachweis solcher Schwankungen von motorischen Reactionen ein Schluss auf eine gewisse generalle Disposition der Nervensubstanz eines Individuums gemacht werden konnte." x 12 performed in a room from which practically all noises from other parts of the building and streets were excluded. The room is partitioned off from a larger room on the top floor of a building by double walls, the space between being filled with sawdust. The entrance is closed by heavy double doors; the only other means of communication is a small opening in the wall directly behind the chair upon which the subject is seated, which permits the passage of two fine linen threads, attached to the heels of the subject in the experimenting room, to the recording apparatus in the room adjoining. The room is illuminated by a single incandescent lamp so placed that it does not shine directly on the eyes of the subject. The walls, ceiling and carpet are neutral gray in color. Ample ventila- tion is provided. The subject is comfortably seated in a Morris chair, the legs of which had been lengthened about 18 inches. The back of the chair is inclined at an angle of 45 degrees. In this semi-reclining position the greater portion of the weight of the body is supported upon the pelvis, thus reducing the pressure on the flexor muscles of the under part of the thigh to a minimum; the knees extend some distance beyond the edge of the seat; the lower legs are thus free to swing back and forth without striking the edge of the seat, the feet clearing the floor. The head is supported in a comfortable position and the arms are allowed to rest extended on the broad flat arms of the chair. The cushion is slightly hollowed out at the edge of the seat to conform to the contour of the legs,- thereby preventing them from being moved out of position. In this position the muscles of the body are thoroughly relaxed and the distracting effects of complicated harness and apparatus are avoided. The bell, the use of which is described later, was fastened to the wall just back of the subject in the median plane of the head and body and on a level with the head. Its ring consisted of a single tap of moderate intensity. The apparatus employed to give the blow on the tendons con- sists of two hammers swinging as pendulums from a supporting frame. This frame is constructed of two telescoping upright rods, secured in heavy iron bases, resting on the floor, and connected above by a horizontal brass bar. The frame is moveable and 13 after adjustment can be secured in position by braces clamped to the arms of the chair and the upper ends of the upright rods. The clamps on the horizontal bar furnish the point of support for the hammers. These clamps can be moved* to any position along the bar. The hammer is made of lead, cylindrical in form, 6 cm. long and 2.5 cm. in diameter; the pole coming in contact with the tendon is spherical in shape. It is suspended from the moveable clamp by a brass handle or arm made of two pieces which slide over each other and are fastened together by a thumb screw to give any desired length. When the hammer is adjusted for the experiment, the pole barely touches the ligamentum patellce. It is then drawn back by the experimenter until it is caught by an electro-magnet, also supported from the moveable clamp on the horizontal bar, and which can be further adjusted at any height within 90 degrees from the vertical position. When thus caught by the magnet it is held until the current is broken by the interrupting instrument in the recording room. This arrangement of the hammers provides for exact adjustment and makes it possible to keep the force of the blow, once determined upon, absolutely constant for all subjects. In the experiments herein reported, the length of the arm of the hammer was kept at 23 cm., the arc through which the hammers swung at 50 degrees. By means of an instrument (interrupter) in the recording room, the signal bell and the hammers were operated automatically at regular intervals. This instrument consists of a brass cylinder, 19 cm. in diameter, which is rotated at a constant rate about its vertical axis by means of a weight and escapement mechanism. The circumference of the cylinder is surrounded with a number of rows of small holes, placed at regular intervals. Brass pegs, 6 mm. long are inserted into these holes and project 5 mm. beyond the surface of the cylinder. The time interval required determines in which holes of a row the pegs are to be placed. During the rotation of the cylinder these projecting pegs come into contact with two contact pieces fastened on an upright. For the present experiment the upper contact piece was so adjusted that the upper row of pegs in the cylinder in passing closed momentarily a circuit thereby ringing the signal bell; in the lower contact piece the peg of the lower row broke a circuit momentarily, causing the electro-magnets to release the hammers. By the adjustment 14 of these two contact pieces, the time elapsing between the ringing of the bell and the fall of the hammers can be made any desired amount. For the experiments herein reported the interval was .5 second. The recording apparatus consists essentially of two pens to each of which are fastened the strings attached respectively to the right and left heels of the subject in the experimenting room. The pen carriage runs upon two steel wires drawn very taut arid consists of two parts : an ink reservoir and a pen which is supplied by it with a continuous flow of ink. The pen inscribes the excursion of the foot upon an endless roll of paper which passes over the tracing surface beneath the pen. The carriage is drawn back after the outward excursion by means of a small weight fastened to the rear end of the carriage by a string playing over a small pulley. As the total amount of the friction of the pen on the paper and the carriage on the wires is extremely small a weight of 17 grams is sufficient for the purpose and offers practically no resistance to the outward kick of the leg. The two rollers which receive the paper are connected by belts to a pulley which the person recording turns by hand ; by this means the rate of move- ment of the two rolls at any one time is the same and the two curves therefore correspond. The recorder begins to turn the pulley just before the tendons are struck and continues turning until the legs have come to rest. The first kick out of the legs and all the subsequent swings are therefore recorded. (See Record No. 1.) When the pens are at rest, a movement of the paper will cause a horizontal line to be inscribed. This base line is a broken line; its rise and fall indicates that the leg after the kick does not always drop back into exactly the same position. The extent of any given initial kick as well as any subsequent swing is taken to be the distance from the extremity of the line to the base line immediately preceding and in the tables given is expressed in millimeters. In order to determine the individual differences in the normal jerk an average of all the normal series in the four groups was obtained. Since no two kicks are of the same extent, even when taken under precisely similar conditions, an average of a deter- mined number of trials, must be taken as the index of the extent of the iSck under the given conditions. The mean variation is 15 frequently considerable but the averages obtained from several groups under similar conditions usually correspond very closely. Although the averages obtained in this manner may not be looked upon as absolute indexes of the extent of the kick, they, nevertheless, afford a satisfactory means of comparison between results obtained from different individuals and under varied conditions. The following table gives the averages for the seventeen subjects for both the right and left legs, each average representing the results of 60 experiments: TABLE I. Subject A|B C D E F G H I J K L M N P Q Right Leg ! 16.4 20.7 24.6 35.2 52.8 68.8 71.9 73.3 81.4 92 92.6 98.2 103.7 122.9 135.5 148.4 Left Leg.. 8.9 20.8 28 22.3 69.9 64.3 73.3 37.9 76.6 95.7 104.4 98 109.4 136.9 144.6 165.3 The subjects have been arranged in order with reference to the increase in the averages for the right leg. 1 It will be noticed that the increase in the averages for the left leg corresponds very closely. 2 This order is kept in the tables following. The average normal jerk for the subjects reported therefore varies from to 165 mm. These marked individual differences are not due to any of the sources of variation found by Lombard. 3 Although 1 The averages given for Subject D have little or no value. In Group I this subject gave large kicks with regularity, the average for all the normals being 92. 1 and 103.3 for the right and left legs respectively. In Group II the extent of the kicks was greatly diminished and in Groups III and IV no response was obtained under normal conditions. For the remaining subjects the averages fairly represent the actual results. 2 During the period covered by the experimentation Subject I, especially at the time when Groups III and IV were employed, was suffering from some slight sensory disturbances which were restricted entirely to the left side of the body. These disturbances manifested themselves in fugitive sensations of pressure along the left side of the lumbar and sacral regions of the cord and in numbness of the little finger, little toe and restricted areas on the abdomen. A medical examination revealed nothing further than that the subject was suffering from nerve exhaustion due to overwork. This, fact probably accounts for the marked discrepancy between the averages tf or the right and left legs. 3 American Journal of Psychology, Vol. I, No. 1 16 the subjects were experimented upon at three different periods of the day, no correspondence is found to exist between the extent of the kicks for the different subjects and the time of day at which the experiments were made. The experiments were performed on Subjects B, C, F, G, K and M between 9 and 11 A. M., the period at which Lombard found the largest kicks occurring with his subject. The averages for B and C are extremely small while no subjects in this number were among those giving very large kicks. The experiments were made on subjects A, E and N between 11 and 1 o'clock and on Subjects D, H, I, L, O, P and Q, most of whom were among those giving the largest kicks, between 3 and 5 P. M. Further the experiments were all made during the months of February, March and the early part of April, while the building was being heated, the temperature of the room being kept at about 70 degrees. Consequently these individual differences cannot be ascribed to this cause of variation. Finally the non-correspondence in the extent of the kicks obtained from subjects experimented upon at the same period of the same day is sufficient evidence that the barometric condi- tions are not responsible for these differences. From the results it appears impossible to fix upon any given amount as representing the extent of the normal knee jerk of the normal man, with a blow of given force. Nine of the seventeen subjects gave averages falling within wide limits, 50 to 110 mm., and in the absence of more definite knowledge we may arbitrarily designate an average falling within these limits as normal. There is no reason, however, for looking upon averages falling without these limits as indicative of an abnormal condition. There may even be a total absence of the jerk in apparently sound subjects (Subject A). Wide departures from the limits, thus arbitrarily determined, therefore have no significance. They may, however, direct the study of the cause of these individual differences to the cases in which they are most marked. A careful examination showed that the anatomical formation of the knees of the different subjects varied slightly, the hollow beneath the ligamentum patellae being more or less well marked. This of course allows a greater or less displacement of the tendon when struck, and in consequence the quadriceps is offered a 17 mechanical stimulus of varying intensity. No correspondence between this fact and the differences in the extent of the kick could be observed. On the assumption that the knee jerk is a muscular contraction, due to a mechanical stimulus, ^differences in the extent of the kick for different subjects, when the intensity of the stimulus is kept constant, must be referred to a difference in the tonicity of the quadriceps, i. e., its capacity to respond with a contraction of given energy to a stimulus. This tone is dependent upon certain relations of the muscle to the central nervous system. It may be assumed that in the normal subject, the tone of the various 1 BEEF.HILMN-F The dynamometer test was made only with the ten subjects indicated The knee jerk curve (the heavy line) is constructed from Table I. portions of the musculature of the body is relative uniform. We should then expect to find a correspondence between the work done by any two or more groups of muscles. A comparison between the contraction of the quadriceps in the knee jerk and the contraction of the muscles of the hands and arms in the dyna- mometer test gives no such correspondence. For many reasons the dynamometer test is unsatisfactory. The ability to make a high record involves mental factors quite as much as the mere tonic condition of the muscles. The test, in this instance, however, was unfamiliar to the subjects experimented upon and the results may therefore be considered more of an index of the muscular than of the mental conditions involved. 18 In the curve the heavy line represents the average normal jerk of the right leg for ten subjects. The broken line represents the average of five trials with the dynamometer with maximum energy. The wide divergence of the two curves suggests that the knee jerk is more dependent upon differences in the irritability or con- ductivity, or both, of the nervous structures involved in the knee jerk mechanism than upon differences in muscles tone. If this inference is legitimate some evidence is offered for the view that the normal jerk is more dependent upon the reflex influences initiated by the blow on the tendon than upon the response of the quadriceps muscle to a mechanical stimulus. A comparison of the results obtained in the initial, middle and final series of a given group shows that there is usually a marked decline in the extent of the normal kick during the course of the group. In the following table the averages represent the results obtained in the above-named series for all. four groups: TABLE II. Subject A B C D E F G H I J K L M N P Q Initial Series 17.7 24.4 37 57.4 71.7 69.6 63.5 91.4 92 90.6 109.1 103.9 120.4 111.5 163.3 152.2 Middle Series 18 21.7 34 26.4 45.9 75.5 81.6 76.6 74 97.6 93.1 106.8 99.6 127 133.4 146.7 Final Series.. 14 13.1 2.7 22 41 61.4 70.6 56.3 78.4 98.9 79.2 74.6 88.5 126 134.9 146.3 The initial series was always taken after one or two preliminary trials in the adjustment of the apparatus; the middle and final series were preceded by series in which augmenting stimuli of vari- ous kinds were employed. For three subjects (H, K, O) the average of the final series is slightly greater than the initial series. For the remaining subjects (except A, who gave no kicks at all) there is a decrease which is well marked in most instances. For ten subjects, the value representing the middle series falls, where we should expect to find it, between the values representing the initial and final series. This decrease may be due either to fatigue of the muscles or nerves, or both, or to an accommodation of the subject to the constantly recurring stimuli. Neither the nervous excitation nor 19 the muscular contractions seem to have been of sufficient intensity or duration to have produced an appreciable fatigue of the struc- tures involved. On the other hand, the fact that a series of experiments under slightly unusual conditions, when employed after the completion of a group, always resulted in kicks somewhat increased in extent, offers some evidence that the accommodation of the subject to the stimulus and the surrounding conditions is responsible for the decrease. It also indicates that the accommo- dation consists rather in a mental quiescence than in a purely physiological modification of the nervous system. The increase in the extent of the jerk when definite mental states are employed as augmenting stimuli and the decrease in the kick (frequently approximating zero) when consciousness is relatively emptied of content led Witmer 1 to look upon the kick as a function of its mental augmentation. This view finds corroboration in TABLE III. Subject A B C D E F G H I J K L M N P Q Group I 14.4 25.1 12.1 36.4 92.1 16.2 18.7 26.8 51.4 80.2 60.9 70.8 67.9 72.1 96.7 99.5 93.1 112.5 113.6 105.5 103.2 79.5 111.7 102.5 104.9 83.7 180.8 121.4 173.9 191.5 97.8 Group IV the fact that the subjects whose results are herein reported, gave testimony that mental activity seemed to wane during the experi- mentation and even frequently complained of drowsiness toward the end of the group. The absence of the reflex influences dependent upon vigorous mental activity may therefore account." for the observed decrease in the final series. Although the extent of the kick tends to decrease during any one group of experiments there is some evidence for believing that the experimentation from week to week tends to develop the reaction in certain subjects. Table III gives the averages of the normal kicks in Groups I and IV. Subjects G and Q gave decreased kicks in Group IV while Subject D failed to give any normal kicks in Group IV although reacting when augmenting stimuli were employed. In Group IV no response whatever could 1 A paper before the American Psychological Association, 1895. 20 be obtained, even with intense augmenting stimuli. Subjects B, C, F, I, M, O and P gave marked increases in Group IV while the results for the remaining subjects differ but little from the results of Group I. II. In addition to the differences in the extent of the initial kick out of the legs following blows of constant force on the tendons, the subsequent swings of the legs before coming to rest present certain variations. Record I 1 is the curve of one subject (Sub- ject M) for the right leg in a normal series of Group I. The tracing above the base line represents the actual excursion of the leg outward from the position at which it is at rest. 2 Experi- ments 1, 4 and 5 yield records which correspond very closely both in the extent of the initial kick and the number and extent of the subsequent swings. In Experiment 2 the height of the initial kick is equal to the initial kick of Experiment 5, but the first subsequent swing is considerably greater and an additional subse- quent swing appears. Experiment 3 also yielded three subsequent swings, but this may be due to the greater extent of the initial kick. This record is fairly typical of the tracings obtained from the other subjects experimented upon. When first observed the subsequent swings appear as merely the oscillations of the leg too and fro as a pendulum but the evidence furnished by the graphic record of the differences between the extent of the first secondary swings following initial kicks of equal extent disproves this view. That these subsequent swings are not merely the movements of the legs coming to rest as a pendulum is further established by a comparison which Sommer 3 made between the knee jerk curve and the curve obtained by raising the leg of a corpse, immediatelyafter death, releasing it and taking the record of the swinging leg in the same manner. The curve in the latter case with the given height consisted of five depressions below the base line and four elevations, the leg coming to rest as a pendulum in exactly the same place each trial. Equal elevations of the legs were followed by the same number of subsequent swings. On the other hand, fewer oscillations take place after the first kick 1 See pp. 38-39. * On account of the limitations of the recording apparatus used the tracings below the horizontal line have no meaning. 8 Psychopathologischen Methoden, p. 28. 21 out in the normal knee jerk. The curve obtained by Sommer, when the initial kick was about the same height as the elevated leg of the corpse, consists of an elevation, a relatively less depres- sion below the initial level, of a second elevation with return to the initial level. Record II is the result of a normal series for the same subject taken in Group IV. Eighty-five or more experiments were there- fore made between the two records. Although the conditions were exactly similar in the two instances, it will be .observed that together with the increase in the extent of the initial kick (see p. 19) there is a marked difference in the appearance of the subse- quent swings. Both the number and extent of the swings is greatly increased. In the last three experiments of the series the extent of the first secondary swing and in the third experiment the se c - ond secondary swing is greater than the initial kick. Although the secondary swings in the normal series of later experiments become actually greater than those in experiments at the beginning of experimentation, as in the case cited, for a limited number of subjects, there is a decided tendency for the extent of the secondary swings gradually to increase for all subjects as the experimentation proceeds. Some subjects, how- ever, show more marked variation in the number of subsequent swings. Record III (Subject H), which is the record of one experi- ment in a normal series in Group III, even though the initial kick is less than those of Record II (Subject M), shows a larger num- ber of subsequent swings. The record of this one experiment is typical of the records obtained in all the experiments in the normal series of this group for Subject H. The records of the normal series in Group I (Record IV) show two and three less subsequent swings following initial kicks nearly equal in extent. The increase in the number of swings develops gradually throughout the inter- vening experimentation. These same variations are much more apparent when some augmenting stimulus is employed to reinforce the normal jerk. Records V and VI were taken when the subject (Subject M) clinched both hands vigorously just before the blow on the ten- dons. Record V shows the result of a series in Group I, Record VI the result of a series in Group III for the same subject. The variations observed in the normal series are again presented. The subsequent swings increase in number and extent with the 22 increase in the total number of preceding experiments. A much greater number of subjects give secondary swings larger than the initial kick when some augmenting stimulus is employed than in the normal series; a marked increase is obtained in the remain- ing subjects. A comparison of Records VII and VIII (Subject H) taken in augmented series in Groups III and I respectively shows not only an increase in the extent of the subsequent swings but a remarkable increase in the number of swings. This record is fairly typical of the records obtained for the other experiments in the series mentioned. Sommer 1 concludes from the marked differences between the excursion of the leg of a corpse when elevated and allowed to swing until at rest and the curve of the knee jerk, that in the latter instance some energy is somewhere operative which inhibits the previous mechanical pendulum movements and that this inhibition is conditioned by " Innervationszustande " mainly cerebral in origin, and not by the mechanism of the knee joint. Many records of the knee jerk, presenting variations in both the form and number of oscillations following the initial kicl? out of the legs, for the most part obtained from subjects presenting neuropathological conditions, substantiates the view that the character of these oscillations as well as the initial kick are an expression of a variable disposition of the nervous system. This is certainly true as a general statement, but on the other hand the results of Sherrington, 2 which show that at the knee joint excitation of the afferent fibers coming from one set of antagonistic muscles induces reflex tonic contractions of the opposing set, despite the fact that the opponent muscles are not innervated from the same spinal segment, seem to offer a more definite and immediate explanation of the phenomenon. Severance of the great sciatic trunk in the lower animals pro- duces an increased briskness of the knee jerk. This Sherrington 3 found to depend solely upon the cutting of that portion of the trunk which is destined for the hamstring muscles. This sever- ance results in a loss of the tone of the hamstring muscles in consequence of which there is a relaxation of the flexor tension 1 Psychopathologischen Methoden. 1 Proceedings of the Royal Society, Vol. 52, p. 563. 3 Loc. cit. . 23 on the leg. This allows the leg to swing out without opposition. Certain experimental results obtained by Sherrington supplement this mechanical explanation with a more satisfactory physiological explanation. In this study he was able to establish three important facts concerning the contraction of the extensor muscles produced by stimulating the motor nerve to the flexors. 1 . If for the excitation of the motor root to the flexors a series of induced currents are employed, succeeding each other at a rate slow enough to produce not perfect tetanisation, but tremulent contraction of the muscles, the contraction obtained in the extensor muscles coincidently was, nevertheless, perfectly steady and tetanic, although not vigorous. 2. If the flexor muscles are severed from connection with the knee joint, so that their contraction cannot affect the joint, and if the knee jerk be elicited before, during and after stimulation of the motor root to the flexor muscles, during the excitation, when those flexor muscles were contracting, the knee jerk, brisk previously and brisk later, disappeared or almost disappeared. 3. If the sensory spinal roots belonging to the hamstring nerve are severed, the stimulation of a motor root to the hamstring muscles is no longer accompanied by contraction of the extensor muscles of the knee, even when strong stimulation is employed. From these facts Sherrington concludes that the degree of tension in one muscle of an antagonistic couple intimately affects the degree of tonicity in its opponent not only mechanically but also reflexly, through afferent and efferent channels and the spinal cord. It can be maintained, therefore, that a blow on the patella tendon primarily initiates a nervous excitation which is reflected in the second or third lumbar segment into the efferent tracts to the quadriceps muscle; as a secondary result an excitation passes down over the intervening segments of the cord and is reflected into the division of the great sciatic nerve distributed to the hamstring muscles. This nerve impulse momentarily increases the tone of the hamstring muscles, possibly produces mild contractions, consequently not only the initial kick but also the subsequent swings of the leg as well are opposed. In the corpse these reflex influences are totally absent and the leg is free to swing as a mechanical pendulum. 24 Upon this theory, how can the increase in the number and extent of the subsequent swings following the initial kick, observed to take place as the total number of experiments is increased be explained? Let us assume that the repeated transmission of reflex excitation along a reflex arc, including the spinal segment, an afferent and efferent conduction path or from a given segment in the cord to a segment somewhat removed, develops a pathway offering increasingly less resistance. Evidence bearing on this point is presented in Part I of this monograph. A blow of con- stant force on the tendons will then result in a more and more intense stimulation of both the quadriceps and its antagonist. This increase will be relatively constant, consequently the extent of the initial kick is still checked by the simultaneous stimula- tion of the hamstring muscles. The actual extent of the kick, however, tends to increase somewhat with the continuation of experimentation (see p. 19). With the increased intensity of the reflex influence, the flexors are thrown into more violent contrac- tion, which contraction now becomes an adequate stimulus to the sensory fibers terminating in the flexors and as a result there is a vigorous excitation to the cord and ultimately to the quad- riceps. This excitation therefore contributes its quota to the second outward excursion of the leg and frequently is sufficiently vigorous to produce a kick larger than the initial kick. This view is fully corroborated by Sherrington's 1 experiment in which he succeeded in greatly augmenting the extent of the knee jerk by merely compressing the hamstring muscle when it is com- pletely dissected away from its attachments, while the nerves were still intact. III. During the adjustment of the apparatus for an earlier group of experiments with one subject (Subject A) 2 a decided kick of both legs was observed to follow a tap of the signal bell occurring without the usual blow of the hammers on the tendons. It was at first believed that the subject had merely voluntarily kicked 1 Proceedings of the Royal Society, Vol. 52, p. 563. 2 Subjects A, B, etc., do not correspond with the subjects thus indicated in Parts I and II. The experiments reported here were made the year previous; the report includes the results for all subjects upon whom the series were performed. 25 out the legs, but upon being questioned, he stated that although quite conscious of the movement as it was taking place, it had not been caused by a volitional effort, and further, that the sub- jective feeling accompanying the movement was similar to the feeling of the movement following the blow on the tendons with the exception that he was quite conscious that the tendons had not been struck. Two alternatives presented themselves. Either (1) the subject was in error in his introspective obser- vation and had voluntarily moved his legs, or (2) the true knee jerk (or a movement resembling it in appearance) had been pro- duced by a stimulus other than the usual one. In a group of experiments, consisting of eight series, designed primarily to study the effects of motor augmentation, three series of five experiments each, were planned to investigate the phe- nomenon. In Series III the subject was directed to clinch both hands at the sound of the bell and twice in the series, the third and fifth experiments, the hammers were dropped as usual but were caught before they had struck the tendons. Again in Series VII the subject was directed to clinch both hands and the hammers were caught in the second, fourth and fifth experiments. In Series VIII, a normal series, the bell sounding as before, the hammers were caught in the third experiment. These experiments were performed one week subsequent to the first observation of the phenomenon. The subject w^as in abso- lute ignorance as to the nature of the experiments in the group and had no way of knowing when the hammers would not be allowed to strike the tendons. The following table gives in millimeters the results of the experiments when the hammers were not dropped: TABLE IV. Right Left Series III.... Exp. 3 5 130 255 193 195 Series VII... Exp. 2 4 5 244 145 183 291 204 240 Series VIII.. Exp. 3 105 168 26 It will be observed that in each of the six trials in which the tendons were not struck the subject responded with a decided kick. To all appearances the movement had the explosive, jerky character of the kick following the actual blow on the tendons. The subject again testified that the movement had not been pro- duced voluntarily. The same series of experiments was then performed with five other subjects who had not been made acquainted with the nature of the series and the results obtained from Subject A. The fol- lowing table gives the results of these trials for the six subjects: TABLE V. A 1 c I i E ] t Series Experiment Right Left Right Left Right Left Right Left Right Left Right Left III 3 130 193 40 45 60 5 255 195 2 244 291 61 70 98 5 VII 4 145 204 10 5 183 240 vm 3 105 168 Although results were obtained from two additional subjects under the conditions described, the results obtained from Sub- ject A were not considered to have been wholly confirmed. Subjects C, E and F gave no response when the tendons were not struck. Subjects B and D gave some kicks but not with the regularity of Subject A. They agreed, however, with Subject A in saying that the kicks which occurred were wholly involuntary. In order more fully to confirm the results obtained from Sub- ject A, two additional series were employed in which the bell sounded as usual and the hammers were caught each time before striking the tendons. In each series the bell struck every fifteen seconds with an interval of two minutes between the two series. For the first series the subject was given no directions what- ever; for the second series he was directed to clinch both hands 27 in all experiments except the fourth. In the latter series the subject, without the knowledge of the experimenter at the time, made an attempt to inhibit the kicks. The following was the result obtained: TABLE VI. Series I Series II Right Left Right Left 140 180 197 260 101 133 128 148 160 220 25 41 32 50 11 25 136 171 106 100 With the subject still remaining seated in position during the following five minutes there was no further movement of the legs after the tapping of the bell had ceased. One week later, no experiments having been performed on the subjects in the meantime, a new group was employed, four series of which contained experiments in which the hammers were not permitted to strike the tendons. In Series III the subject was as before directed to clinch both hands at the sound of the bell, tendons not struck in Experiments 3 and 5; Series IV, the sub- ject said "ah" when the bell struck, tendons not struck in Experiments 1, 3 and 4; Series VI, Experiment 4, slight pin prick on the left thigh; Experiment 5, same on right thigh, tendons not struck in either case; Series VIII the subject was directed to think vividly of clinching both hands without making any actual contractions; tendons not struck in Experiments 1 and 4. Table VII gives the results for the six subjects. Comparing the results of this group for all the subjects with the previous one it will be observed that more kicks were obtained in proportion to the number of experiments made. Kicks were recorded for Subjects C and F, both of whom had given no result in the first group. Subject A consistently kicked each time and Subject E alone failed to give any response. The week following a final group, consisting of eight series was employed, in which five of the series contained experiments with- out the blows on the tendons. Series II, the subject was directed 28 to clinch both hands at the tap of the bell, the tendons not struck in Experiments 3 and 5. Series IV the subject received a very mild electric shock from electrodes held in the hands. An irjduced current, resulting from an instantaneous make and break of the primary current of an induction coil was used to produce the shock which was not sufficiently strong to cause apparent con- tractions of the muscles of the hands and forearms. The cur- rent was closed by the experimenter reacting to the sound of TABLE VII. i I \ c | I > I E F Series Experiment Right Left Right Left Right Left Right Left Right Left Right Left n 3 108 175 20 11 63 57 5 84 104 20 26 1 130 151 127 110 35 35 IV 3 143 110 26 32 4 87 121 7 10 VI 4 89 302 264 330 22 65 5 171 169 232 178 41 VIII 1 125 190 4 174 360 50 44 the bell with a telegraph key; with probably large variations the shock occurred about .125 second after the stroke of the bell. The hammers did not strike the tendons in Experiments 2 and 5. In Series VI the subject clinched both hands at the sound of the bell and the shock, as in Series IV, was employed in addition. The hammers did not strike the tendons in Experiments 1, 3, 4 and 5. Series VII was a normal series, the hammers being caught in the second experiment. Series VIII, same conditions as Series IV, the tendons not struck in any of the five experiments. The following table gives the results for the six subjects: 29 TABLE VIII. 1 , I \ ( | I ) 1 B 1 p Series Experiment Right Left Right Left Right Left Right Left Right Left Right Left 3 142 192 128 162 12 9 33 36 5 90 128 18 5 TV 2 228 360 255 305 184 110 45 31 12 25 51 65 5 107 130 185 206 142 112 22 10 52 8 2 20 1 140 160 220 245 97 80 16 12 18 VT 3 160 215 328 360 23 15 55 30 4 156 184 240 295 60 49 72 80 5 145 152 280 310 58 27 53 30 vn 2 114 130 154 155 15 2 1 95 108 263 295 90 64 28 10 2 185 201 275 327 55 35 28 12 25 30 VIII 3 4 135 110 156 242 247 265 276 360 52 30 15 15 75 35 55 45 5 185 216 255 234 62 30 46 18 In the fourteen experiments of this final group in which the tendons were not struck, subjects A and F gave decided kicks every time, with the single exception of Subject F with the left leg in Experiment 1, Series VI. Subjects B and C failed to respond in only two experiments, while Subjects D and E gave three and four kicks respectively. A much greater proportion of kicks was therefore obtained in this group than in either of the two preceding groups. The results of these experiments warrant the opinion that the occurrence of the kick without the blow on the tendons cannot be explained as a mere accidental movement on the part of the subjects. On the contrary the phenomenon occurs with sufficient frequency and regularity to demand an inquiry as to its nature. Excepting in Series VI of the second group and Series IV, VI and VIII of the third group, the tap of the bell was clearly the 30 immediate stimulus to the movement. When the bell ceased striking no further kicks occurred without the blows on the ten- dons. The clinching of the hands, which was employed to pro- duce an augmentation of the normal jerk, even when performed with maximum energy on the part of the subject, elicited no movements of the legs when the bell was not striking. In Series VI of the second group the painful prick on the thigh and in Series IV, VI and VIII of the third group, the electric shock was employed as the augmenting stimulus. Subsequent attempts to produce the kicks with these stimuli when the bell was not striking were some- times successful and it may be questioned whether these stimuli did not immediately cause the movement in the experiments indicated rather than the stroke of the bell. Assuming this to be the case there is still no reason for believing that the move- ment differed in character from the movements following the stroke of the bell alone. The painful prick on the thigh was not sufficiently intense to cause the subjects consciously to react in an effort to remove, or get away from, the stimulus, and as a reflex movement in response to a pain stimulus, the reaction would naturally have manifested itself in withdrawal of the thigh from the stimulus or perhaps a movement of the entire body rather than in a simple kick out of the legs. The induced current employed was not strong enough to be painful and, excepting a few instances, produced no perceptible contractions of the muscles of the hands and arms, much less of the body and lower extremities. The movement in the first instance cannot, therefore, be viewed either as a voluntary effort or a general reflex movement in response to a painful stimulus nor in the second instance as a movement due to the diffusion of the current through the body to the muscles of the legs. We are therefore justified in looking upon the movements following these stimuli as identical in char- acter to the movements obtained with the bell alone or with the bell and some voluntary muscular contraction as a source of augmentation . Can these movements, then be looked upon as the true knee jerk phenomenon in which the appropriate stimulus, i. e., a blow on the tendons has been replaced by a new and unusual stimulus ? These movements are not the result of voluntary effort. Each subject gave unqualified testimony on this point. This testimony is further corroborated by the fact that the subjects 31 were kept in absolute ignorance as to the nature of the experi- ments about to be performed and consequently did not know when to expect the blow and when not to expect it. Further, the position in which the subjects were placed made it impossible for them to observe when the hammers were about to be caught before striking the tendons. The kinaesthetic sensations result- ing from these movements and the more general subjective feeling could not be distinguished from those aroused when the move- ments followed the actual. blows on the tendons. The subjects were, however, always aware of the fact when the tendons had not been struck. Considerable confidence may be placed in this testimony, for all the subjects had been students of psychology for two years and were consequently familiar with the method of introspective examination. Therefore, as far as the data of intro- spection is concerned, it can be definitely stated that there is no difference between these movements and the true knee jerk. An effort on the part of Subject A to inhibit the kicks in the supplementary series following Group II and in all of the experi- ments of Group III was wholly unsuccessful. (The attempt to inhibit the movement was the subject's own initiative and was not reported to the experimenter until the experimentation had been discontinued.) Later experiments on another subject com- pletely confirmed this result. This corresponds exactly with the well-known fact that the knee jerk cannot be voluntarily inhibited without actual contraction of the flexor muscles of the thigh. To the observer these movements resembled the knee jerk in every detail. The characteristic jerky or explosive appearance' was very evident. When the tracings on the record were com- pared no difference could be discovered. If differences really exist a much more rapid recording device will be required to detect them. In proportion to the extent of the kick, the number and appearance of the subsequent swings of the leg correspond to the number and appearance of the swings following the knee jerk. Except in a few cases the average extent of the kicks without the blow on the tendons was somewhat less than that of the kicks following the blow. The following tables make a comparison of the extent of the kicks in the two cases. The series in which Subjects A, B, C and F kicked every time, without a blow on the tendons, were selected for the tabulation. 32 TABLE IX. SUBJECT A Tendons Not Struck Tendons Struck Primary Secondary Primary Secondary Right Left Right Left Right Left Right Left Group I, Series iii and vii (subject clinch both hands) Av. 5 211.4 224.6 225.8 248 228.5 282.7 277 302 Group II, Series ii (subject clinch both hands) . Av. 2 96 139.5 65 91.5 164.5 255 224 243 Group II, Series iv (subject said "ah") ... . Av. 2 ' 136.5 130.5 161.5 193.5 99 134.5 180 273 Group II, Series viii (thinking of clinching hands) Av. 2 150 275 231 281 173.5 296.5 311.5 303.5 Supplementary (normal) Av. 5 113.8 150.8 111 143.4 118.2 123.4 201.8 274.4 Group III, Series vi (clinch hands and electric shock) Av 4 150 187.5 206.5 285.7 165 177 230 315 Group III, Series viii (electric shock, both hands) . . 142 184.6 197.6 267.2 154.3 253 313 356 TABLE X. SUBJECT B Tendons Not Struck Tendons Struck Primary Secondary Primary Secondary Right Left Right Left Right Left Right Left Group III, Series vi (clinch both hands and electric shock) Av. 4 267 302.5 227 286.5 287 312 357 360 Group III, Series viii (electric shock, both hands) Av. 5 261 298.4 204 250.4 235.6 243 281.7 289 SUBJECT C Group III Series vi Av 4 64.2 42.7 19.7 7.5 142 85 25 15 Group III, Series viii Av. 5 57.8 31.8 16.4 9 173.3 173 80.3 81.3 SUBJECT F Group III Series vi Av. 4 52 35 21.5 14.5 55 130 125 12 15 Group III, Series viii Av. 5 41.9 31.6 12.4 11.6 91.7 40 43 33 In a study of the tables it is observed that the relation between the extent of the kicks of the right and left legs corresponds almost exactly with the results obtained when the tendons are struck. Subjects A and B give a uniformly greater kick with the left leg than with the right when the tendons are struck both in normal and augmented series. Subject C gives larger kicks with the right leg. This relation maintains in the kicks obtained without the blows on the tendons. The results for Subject F are not as consistent as the results for the other subjects; this was, however, not unexpected, for in a long series of normals and augmented kicks, the subject frequently failed to give any response to a blow on the tendons, often kicked with one leg only and when results were obtained for both legs, neither was consistently larger than the other. Further, the relation between the extent of the initial kick out of the legs and the first secondary swing remains constant for each subject whether the movement follows the blow on the ten- dons or whether it follows the sound of the bell alone. Sub- jects A and B give secondary kicks somewhat larger than the primary kicks and Subjects C and F vice versa. The movement of the legs following the tap of the bell, with- out the blows on the tendons, has the characteristics of a simple, immediate reaction to a stimulus. Upon the unanimous testi- mony of the subjects, it was not produced voluntarily, i. e., there was no idea of the movement in consciousness, antecedent to the movement itself. It may, therefore, be held, tentatively at least, that the movement is a reflex action. The afferent excitation must therefore reach the cord at the level of the medulla (or in the case of the pain stimulus of the cervical cord) and then passes down to the second or third lumbar segment in which the cell bodies of the efferent conduction path are located. Here then we have a new and unusual reflex arc. With a stimulus of moderate intensity, a reflex movement is always restricted to the group of muscles most intimately connected with the sensitive part. The cell bodies of the efferent conduction path are situated in the same or immediately adjacent segments with the terminating fibers of the afferent paths. If the intensity of the stimulus is increased, the reflex movement may become more widely diffused but is still confined for some time to muscles in the neighborhood 34 of the organs stimulated. Increase the intensity of the stimulus still further and the response grows more and more general. In the case under consideration, however, the movements cannot be viewed as a general reflex response to an intensive stimulus. In no instance was the sound of the bell or the electrical shock suffi- ciently intense to produce diffused movements even of neighbor- ing parts; on the contrary, the only movement of the body was confined to a kick out of the legs which always displayed a definite character. All the experiments which had been performed in the investi- gation of other questions, previous to each of the three groups reported in this section of the monograph, may be considered as preliminary or preparatory. In these experiments the sound of the bell immediately preceded the blow on the tendons. One hundred and twenty-five such experiments had been performed on Subject A before the kick without the blow on the tendons was observed. After 150 trials the phenomenon was obtained from Subjects B and D, after 185 from E, after 220 from C and after 230 from F. These experiments were not in a continuous series but were performed at weekly intervals on five different days. Later two subjects, not reported in this study, were found who gave these kicks after thirty trials with the bell immediately preceding the blow. After thirty-five additional trials one of the subjects kicked regularly without the blows on the tendons. The occurrence of the phenomenon, therefore, depends upon the pre- liminary simultaneous occurrence of the sound of the bell with the kick produced in the usual way, i. e., a blow on the tendon. After a certain number of such trials, the number varying for different subjects, the association of the sound of the bell and the kick becomes so fixed that the bell itself is capable of serving as a stimulus to the movement. Physiologically the repeated association of the functioning of the motor cells in the lumbar segment of the cord, -upon which the kick immediately depends, with the excitation of centers in the nuclei of the medulla con- nected with the auditory conduction path, has resulted in develop- ing a fixed relationship between them. The impulse entering the latter therefore finds an accustomed channel to the former. "Considerations of embryology and comparative anatomy" in the opinion of Dercum, "point to the conclusion that the nervous 35 system, though inextricably complex and composed of an almost infinite number of parts, acts as a whole, and that its parts are so closely related and interdependent that no one part can move unless every other part, no matter how slightly or how profoundly, moves also." According to this view every incoming impulse, or whatever origin, besides effecting a change in the appropriate central cells, diffuses itself over the entire central system. The excitation aroused by the tap of the bell, therefore, always in- fluences, to a greater or less degree, the motor centers of the cord. The results of the experiments herein reported, however, would seem to indicate that it is only after a habit of interaction between the two involved centers has been developed by repetition, i. e., when the connecting pathway of discharge has become well worn, that the sound of the bell alone is an adequate stimulus to the movement. A comparison of Tables V, VII and VIII shows that the number of kicks obtained without the blows on the tendons for each sub- ject gradually increases during the three series. In the first group containing experiments definitely planned to study the phe- nomenon, Subject A did not fail to respond in any trial. An equal number of preliminary experiments had been performed on the remaining five subjects, but only two gave kicks and these with no regularity. For all the subjects, in 36 possible times, 11 kicks, or 30 per cent, were obtained. In Group II, after the addi- tional series of Group I had been performed under precisely similar conditions, the six subjects gave 21 kicks out of a pos- sible 54, or 40 per cent. In Group III, for which all the pre- ceding experiments can be considered as preliminary, the six subjects gave kicks in 87 per cent of the times in which the' tendons were not struck. Further, two of the subjects reacted each trial and two in all trials but two. Hence, not only the frequency but also the regularity of the occurrence increases with the total number of experiments. It may be possible that the employment of the more exciting electric stimulus in the last group partly accounts for the marked increase. It does not wholly explain it however. When this stimulus was tried on a subject upon whom no previous experiments had been performed no responses were obtained without the blows on the tendons. If the knee jerk following a blow on the tendons is a true reflex 36 it differs from the movements of the legs following the tap of the bell merely in that in the latter an auditory excitation (in some instances possibly an electrical shock) has been substituted for the excitation of the peripheral sensory fibers terminating in the quadriceps muscle and tendon. There is consequently ah asso- ciation in a reflex arc of centers in the medulla or cervical cord with the motor cells of the third lumbar segment instead of two centers represented in the same level, i. e., the third lumbar segment. On the other hand if the knee jerk is not a true reflex but merely the result of an immediate response to a mechanical stimulation of the muscle, and the movement of the legs without the blow is a true reflex, a comparison of the latent times of the two move- ments should show a marked difference. In the mere observation of the two phenomena no marked difference seems to exist. The matter can be settled only by an exact measurement. This problem will be undertaken by the writer in the near future. The results of the experiments herein reported seem to give considerable support to the theory held by Lombard, DeWatt- ville, Mills and others that the knee jerk is due first to direct stimulation of the muscle and secondly to reflex influence. The results show that the extent of the kick following a blow on the tendons, with very few exceptions, is greater than the kick fol- lowing the sound of the bell alone, all other conditions being kept constant. If in the knee jerk, the movement is initiated by the contraction of the muscle due to the mechanical stimulation and then reinforced or continued by a reflex excitation coming from the spinal center, the difference between the extent of the kicks following the stroke of the bell or the electrical shock and those following the blows on the tendons may be taken to represent that amount of the movement which is due to the mechanical stimu- lation of the muscle. If this is true the extent of the normal knee jerk depends largely upon the reflex influence, the mechanical stimulation being responsible for only the initial stage of the movement. LIST OF RECORDS The photographs are approximately one- third size of original records. 1. Record of five experiments under normal conditions in Group I for Subject M, right leg. 2. Record of five experiments under normal conditions in Group IV for Subject M, right leg. Eighty-five experiments were performed between Groups I and IV. 3. Record of one experiment under normal conditions in Group IV for Subject H, right leg. 4. Record of three experiments for Subject H, right leg in Group I. 5. Record of five experiments with motor augmentation in Group I for Sub- ject M, right leg. 6. Record of five experiments with motor augmentation in Group IV for Subject M, right leg. 7. Record of one experiment with motor augmentation in Group IV for Subject H, right leg. 8. Record of one experiment with motor augmentation in Group IV for Subject H, right leg. 7 GENERAL LIBRARY UNIVERSITY OF CALIFORNIA BERKELEY RETURN TO DESK FROM WHICH BORROWED This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. MOV 41955 OCT251955 LD 21-100m-l,'54(1887sl6)476 .'2 UNIVERSITY OF CALIFORNIA LIBRARY