CH. XLVII.] TENDON REFLEXES 673 



be obtained in locomotor ataxy (damage to the posterior nerve- 

 roots), or in infantile paralysis (damage to the anterior horns of 

 grey matter). 



3. They are excessive in those conditions that increase the true 

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

 lateral sclerosis. 



How, then, is it possible^ to reconcile these two sets of facts? 

 The explanation advanced by' Sir "William Gowers does so best ; it is 

 briefly as follows : 



(1) The tendon reflexes are not reflexes, but are due to direct 

 stimulation of the muscle itself. 



(2) 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 

 (see p. 130), a readiness to contract on slight provocation. 



(3) Muscular tonus depends on the integrity of the reflex arc. 

 The sensory stimulus for this reflex muscular tone arises either in 

 the muscle itself, or more probably in the condition of the antagon- 

 istic muscles. (See more fully, next paragraph but two). 



(4) Hence injury to any part of the reflex arc, by abolishing the 

 healthy tone of a muscle, deprives it of that irritable condition 

 necessary for the production of these so-called reflex actions. 



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 ham- 

 string supply is from the 5th lumbar and 1st and 2nd sacral roots. 



Reciprocal Action of Antagonistic Muscles. This is an 

 interesting branch of muscle physiology related to the question of 

 tendon reflexes, which we owe to the researches of Sherrington. 

 In brief, he shows that the inhibition of the tonus of a 

 voluntary muscle may be brought about by excitation of its 

 antagonist. 



Movement at a joint in any direction involves the shortening 

 of one set of muscles and the elongation of another (antagonistic) 

 set. The stretching of a muscle produced by the contraction of 

 its antagonist may excite (mechanically) the sensorial organs 

 (probably the muscle-spindles, see p. 86) in the muscle that is 

 under extension ; in this way a reflex of pure muscular initiation may 

 be started. Experiments show that electrical excitation of the 

 central end of an exclusively muscular nerve produces inhibition 

 of the tonus of its antagonist. For instance, the central end of the 

 severed hamstring nerve is faradised. This nerve contains in the 

 cat 4510 nerve-fibres, and of these about 1810 are sensory in 



2 U 



