PATELLAR REFLEX AND ANKLE CLONUS. 643 



by volitional acts directed to other parts of the body, e.g., by exercising voluntary 

 pressure with the hand (Jendrdssik).] [A "jaw-jerk" is obtained by suddenly 

 depressing the lower jaw (Gowers, Beevor, and De Watteville), and the last ob- 

 server finds that the latent period is 0"02 second, and if this be the case, it is an 

 argument against these so-called " tendon reflexes " being true reflexes, and that they 

 are direct contractions of the muscles due to sudden stimulation by extension.] 



[Method. The knee-jerk is easily elicited by striking the patellar tendon with 

 the edge of the hand or a percussion-hammer when the leg is semi-flexed, as when 

 the legs are hanging over the edge of a table or when one leg is crossed over the 

 other. It is almost invariably present in health, but it becomes greatly exaggerated 

 in descending degeneration of the lateral columns and lateral sclerosis.] 



[Ankle clonus is another tendon reflex, and it is never present in health. If the 

 leg be nearly extended, and pressure made upon the sole of the foot so as suddenly 

 to flex the foot at the ankle, a series of (5 to 7 per second) rhythmical contractions 

 of the muscles of the calf takes place. Gowers describes a modification elicited by 

 tapping the muscles of the front of the leg, the "front-tap contraction." Ankle 

 clonus is excessive in sclerosis of the lateral columns and spastic paralysis.] 



[In "ankle clonus" excited by sudden passive flexion of the foot, there is a multiple spasm 

 of the gastrocnemius. Here also the latent period is about 0*03 to 0*04 second, and the rhythm 

 8 to 10 per second. This short latent period has led some observers to doubt the essentially 

 reflex nature of this act.] 



When we are about to sleep ( 374), there is first of all a temporary increase of the reflexes ; 

 in the first sleep the reflexes are diminished, and the pupils are contracted. In deep sleep the 

 abdominal, cremasteric, and patellar reflexes are absent ; while tickling the soles of the feet and 

 the nose only^acts when the stimulus is of a certain intensity. In narcosis, e.g., chloroform or 

 morphia, the abdominal, then the conjunctival and patellar reflexes disappear; lastly, the pupils 

 contract {O. Rosenbach). 



Abnormal increase of the reflex activity usually indicates an increase of the excitability of 

 the reflex centre, although an abnormal sensibility of the afferent nerve may be the cause. As 

 the harmonious equilibrium of the voluntary movements is largely dependent upon and regulated 

 by the reflexes, it is evident that in affections of the spinal cord, there are frequent disturbances 

 of the voluntary movements, e.g., the characteristic disturbance of motion in attempting to 

 walk, and in grasping movements exhibited by persons suffering from ataxic tabes dorsalis [or, 

 as it is more generally called, locomotor ataxia]. 



[The organic reflexes include a consideration of the acts of micturition, erection, 

 ejaculation, defalcation, and those connected with the motor and secretory digestive 

 processes, respiration, and circulation.] 



362. CENTRES IN THE SPINAL CORD. Centres capable of being 

 excited reflexly, and which can bring about the discharge of certain complicated, 

 yet well-co-ordinated, motor acts exist in various parts of the spinal cord. They 

 still retain their activity after the spinal cord is separated from the medulla 

 oblongata ; further, those centres lying in the lower part of the spinal cord still 

 retain their activity after being separated from the higher centres, but in the normal 

 intact body, they are subjected to the control of higher reflex centres in the medulla 

 oblongata. Hence, we may speak of them as subordinate spinal centres. The 

 cerebrum also, partly by the production of perceptions, and partly as the organ of 

 volition, can excite or suppress the action of certain of these subordinate spinal 

 centres. [For the significance of the term " Centre," see p. 625.] 



1. The cilio-spinal centre connected with the dilatation of the pupil lies in 

 the lower cervical part of the cord, and extends downwards to the region of the 1st 

 to the 3rd dorsal vertebra. It is excited by diminution of light ; both pupils always 

 react simultaneously, when one retina is shaded. Unilateral extirpation of this part 

 of the spinal cord causes contraction of the pupil on the same side. The motor 

 fibres pass out by the anterior roots of the two lower cervical and two upper dorsal 

 nerves, into the cervical sympathetic ( 392). Even the idea of darkness may some- 

 times, though rarely, cause dilatation of the pupil {Budge). 



