CH. XLIX.] ANTAGONISTIC MUSCLES 715 



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 mechanically stimulates the sensory nerve-endings 

 in the muscle - spindles of the muscle that is under exten- 

 sion; 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 

 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 

 effects are in fact the same as those produced by faradisation of the 

 central end of the nerve supplying them. It may therefore be that 

 reciprocal innervation, which is a common form of coordination of 

 antagonistic muscles, is secured by a simple reflex mechanism, an 

 important factor in its execution being the tendency for the action of 

 a muscle to produce its own inhibition reflexly by mechanical stimu- 

 lation of the sensory apparatus in its antagonist. 



We have in our description of the anatomical path of the entering 

 posterior roots drawn attention to what may be termed the three 

 " nervous circles " by which an afferent impulse may affect the motor 

 discharge from the anterior horn-cells of the cord ; there is the short 

 path by the collaterals of the entering fibre which pass directly to 

 these cells, and there are the two longer paths, via the cerebellum 

 and cerebrum respectively. In the execution of a voluntary action 

 all three circles are in activity to produce the coordination and due 

 contraction and elongation of antagonistic muscles which characterise 

 an effective muscular act. Section of the posterior roots produces 

 not only an inability to carry out reflex actions, but also leads to an 

 inability to carry out effectively those more complicated reflex actions 

 which are called voluntary, and in which the brain participates. 

 Locomotor ataxy, or tabes dorsalis, is a slowly progressive disease, 

 the anatomical basis of which is a degeneration of the nerve-units of 

 tho spinal ganglia. It is, therefore, analogous to a physiological 



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

 fibres in the nerve after section of the anterior nerve-roots. 



