674 FUNCTIONS OF THE SPINAL CORD [cil. XLVII. 



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 co-ordination 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. 



On p. 661 we have drawn attention to 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, vid the cerebellum and cerebrum 

 respectively. In the execution of a voluntary action all three circles 

 are in activity to produce the co-ordination 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 the spinal 

 ganglia. It is, therefore, analogous to a physiological experiment in 

 which the posterior roots are divided, and although fibres may remain 

 which still allow of the passage of nervous impulses, the action of the 

 three circles is greatly interfered with; the spinal reflex arc is at 

 fault ; this is shown by the loss of reflex action, the disappearance of 

 the tendon reflexes, and the want of tonus in antagonistic muscles ; 

 the main symptom of the disease is want of muscular co-ordination, 

 and this is produced not only by the lesion in the spinal cord, but is 

 accentuated by the want of continuity in the other two circles, so 

 that the brain is unable to effectively control the motor discharge 

 from the anterior cornual cells. 



The tendon phenomena are important to the pathologist; they 

 furnish him with a valuable means of diagnosis in nervous disorders. 



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

 fibres in the nerves after section of the anterior nerve-roots, 



