CH. XLIX.] REFLEX ACTION IN MAN 713 



nerve, or spinal grey matter, abolishes them. Thus they cannot be 

 obtained in locomotor ataxy (damage to the posterior nerve-roots), 

 or in infantile paralysis (damage to the anterior horns of grey 

 matter). 



They are excessive in those conditions which increase reflex 

 irritability, such as severance of brain from cord, and in lateral 

 sclerosis, that is, a degenerative condition of the pyramidal tract. 



In order that the muscle may respond, it is necessary that it 

 be in an irritable condition; this is accomplished by putting it 

 slightly on the stretch, and so calling forth the condition called tonus, 

 and thus a readiness to contract on slight provocation. Until the 

 last few years, considerable doubt existed as to whether the tendon- 

 reflexes were true reflex actions, because it was asserted that the 

 time intervening between the stimulus and the muscular response 

 was too short. Now that we know that nerve-impulses are in man 

 propagated at the rate of 120 metres per second (and not at the rate 

 of 30 metres per second as was formerly supposed), this difficulty 

 has disappeared. Jolly has made careful time measurements, and 

 demonstrated that in the knee-jerk there is sufficient time for the 

 nerve impulse to travel to the spinal cord and back again, but that 

 the time occupied in the cord itself is only about half that which is 

 necessary in the case of ordinary coordinated reflex actions. This is 

 explicable on the assumption that the tendon reflexes are subserved 

 by the collaterals of the entering afferent fibres which go direct to 

 the anterior horn cells, whereas ordinary reflexes are worked through 

 intermediate neurons, and therefore have to pass through additional 

 synapses. 



The increased rapidity of a tendon reflex is useful, for a sudden 

 strain on a ligament would rupture some of its fibres or lead to 

 injury of the joint surfaces if too great a time intervened before the 

 muscles could contract and so save the joint. 



The exact course of the reflex arc concerned in the knee-jerk has 

 been worked out by Sherrington in the monkey. The nerve-fibres 

 are mainly those which pass (1) to and from the crureus by the 

 anterior crural nerve, and (2) to and from the hamstrings by the 

 sciatic nerve. The fibres which supply the crureus arise from the 

 spinal nerve-roots which in man correspond to the 3rd and 4th 

 lumbar ; the hamstring supply is from the 5th lumbar and 1st and 

 2nd sacral roots. 



Lombard's experiments upon the knee-jerk indicate that it is 

 sometimes more readily obtained even in the same person than at 

 other times. It varies with changes in mental activity, and during 

 sleep may be entirely absent. It is increased and diminished by 

 whatever increases or diminishes the relative state of irritability of 

 the nervous system as a whole. 



