v SPINAL CORD AND NERVES 329 



other afferent nerve paths to the limb are severed. The integrity 

 of the reiiex arc seems a necessity, either because the stimulus 

 mechanically excited from the patellar ligament traverses this arc 

 in order to throw the muscle into contraction, or because it main- 

 tains the mild tonic tension in the muscle which is the sine qua 

 non of the slight passive extension this again acting on the 

 muscle as a direct stimulus. The dependence of the knee-jerk on 

 the excitability of the spinal centres is also shown by the fact that 

 it is favoured by the waking state and by voluntary activity ; it 

 is depressed during sleep, anaesthesia, and spinal anaemia ; and it 

 is abolished by the inhibitory excitation of the afferent nerves of 

 the antagonist muscles (Sherrington supra). 



Speaking generally, it may be said that the patellar reflex 

 faithfully follows the oscillations in spinal excitability, showing 

 now a rise and now a fall. Hence it may almost be taken as a 

 very delicate physiological indicator of the tone of the nerve- 

 centres in general, and those of the cord in particular. In this 

 lies its great clinical value. Its disappearance is a characteristic 

 symptom in locomotor ataxy ; its exaggeration is indicative of those 

 descending processes of degeneration in the cord which are associated 

 with the pronounced exaggeration of muscular tone, clinically known 

 as spasticity. 



Experimentally the knee-jerk has been the object of much 

 study, and some of the experiments bear directly on the physio- 

 logy of the spinal cord. The conditions which intensify the 

 reflex are : electrical excitation of the central end of the sciatic on 

 the opposite side ; stimulation of the skin or mucous membrane 

 0-2-04 sec, before the jerk is elicited ; a flash of light ; a sudden 

 sound preceding the jerk by 0'2-0'3 sec. ; two taps on the tendon 

 at a short interval ; lastly, rest, food, etc. Other conditions depress 

 or abolish the phenomenon either immediately or after a brief re- 

 inforcement ; as local fatigue of the extensor muscles, general 

 fatigue, local anaemia produced by an Esmarch's bandage, arrest 

 of circulation in the lumbar region, inhalation of chloroform or 

 ether, etc. 



Sherrington says that in the monkey spinal transection usually 

 abolishes the jerk for a week or so. In the dog and cat it can be 

 evoked in a quarter of an hour or less from the time of the opera- 

 tion, while Barbe stated that he obtained the phenomenon in man 

 immediately after decapitation. On the other hand, complete 

 destruction of the cord in the thoracic region usually seems to 

 abolish the knee-jerk permanently. 



The reflex spinal mechanism connected with the knee-jerk of 

 each side is unilateral and lies in its own half of the cord. As 

 shown by the diagram (Fig. 191), the reflex centre in the monkey 

 lies in the fourth and fifth lumbar segments (chiefly the fourth in 

 man). If the cord be split in the median sagittal plane the jerk 



