THE NERVOUS SYSTEM 45 



Tendon Reflexes. — One of the best known of the tendon 

 reflexes in man is the knee-jerk, a jerking forward of the leg 

 when the straight ligament of the patella is struck. This is 

 caused by a momentary single spasm of the extensor muscles 

 of the knee, and although often called a reflex act, cannot truly 

 be so, because the time between the blow and the jerk is too 

 short for any reflex act. It is well seen in the dog, cat, rabbit, 

 etc. Although not a reflex action, it is dependent on the reflex 

 tonus that is maintained in the muscles by the spinal arcs con- 

 nected with them ; if that tonus be much lowered, as by severance 

 of the nervous reflex arc, the jerk can no longer be elicited. The 

 jerk is a good index of the condition of the reflex arc, and there- 

 fore of the condition of the activity or depression of the segments 

 of the cord by which the extensor muscles are innervated. It is 

 depressed during sleep or anaesthesia, and by anaemia of the 

 cord ; it is intensified when the cerebral restraint is removed 

 from the lumbar spinal segments by diversion or attention to 

 another part, or by severance of the cord in the dorsal region. 

 Another brisk ' jerk ' in the dog is the ischial, obtained from the 

 hamstring muscles by tapping the tuberosity of the ischium. 



Tendon reflexes have not, so far as we are aware, been studied 

 in the ungulates ; nor is it known whether the existence of* any 

 reflexes has been demonstrated, if, perhaps, we except the im- 

 mediate lifting up of the foot, which generally follows pressure on 

 the so-called ' chestnut ' found on the inside of the fore-arm of 

 the horse. 



Reflex Inhibition. — In the reflex movements of ' stepping ' and 

 • scratching ' in the spinal dog, attention has been drawn to 

 acts of inhibition ; for instance, it was shown that the stimulus 

 which excites the flexors to contract causes the extensors to 

 relax. The stimulus to produce contraction in the extensors is 

 still present, but it is inoperative, owing to inhibition. It was 

 also shown that the scratch reflex, having been started on one 

 side of the body, could be inhibited by starting it on the opposite 

 side. In this case the stimulus starts one set of flexor muscles 

 going, and inhibits the corresponding set of flexors in the opposite 

 limb. Similarly, in the conflict between the two reflexes of 

 scratching and stepping, all the conditions needful to maintain 

 the scratch reflex in operation are present, but they are inhibited 

 so long as the pad of the foot is pressed upon ; when that 

 stimulation is withdrawn, the scratch reflex returns. We have 

 therefore to consider the question of reflex inhibition. 



The visceral muscles receive, as we have already seen, a double 

 source of efferent nerve supply. The heart-muscle and that of 

 the intestines and bloodvessels can be stimulated or inhibited ; 

 this is effected by one efferent conveying impulses of an ex- 



