664 FUNCTIONS OF THE SPINAL CORD. [CH. XLVII. 



The explanation advanced by Sir William Gowers does so best 

 it is briefly as follows : 



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

 stimulation of the muscle itself. 



(ii) 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. 136), a readiness to contract on slight provocation. 



(iii) 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 condi- 

 tion of the antagonistic muscles. (See more fully, next para- 

 graph but one.) 



(iv) 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. 



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. 93) 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 inhibi- 

 tion 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 function*; these come from the flexor muscles of the knee, not 

 from the skin. The effect of the stimulation of the nerve on the 

 tonus of the extensor muscles of the knee is seen (a) in elongation 

 of those muscles, (b) in temporary diminution of the knee-jerk. 

 The experiment may be varied as follows : the exposed flexor 

 muscles detached from the knee, and therefore incapable of 

 mechanically affecting the position of the joint, are stretched or 

 kneaded. This produces a reflex elongation of the extensor 

 muscles of the knee and a temporary diminution of the knee-jerk. 



* The number of sensory nerve-fibres is determined by counting the 

 degenerated fibres in the nerves after section of the posterior nerve-roots. 



