CH. XLVII.J ANTAGONISTIC MUSCLES. 665 



The effects are in fact the same as those produced by faradisa- 

 tion 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 stimulation of the sensory apparatus in its 

 antagonist. 



On p. 652 we have drawn attention to the three "nervous 

 circles " by which an afferent impulse may affect the motor dis- 

 charge 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 cerebel- 

 lum 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 compli- 

 cated 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-ordina- 

 tion, 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. 



Reaction Time in Man. The term reaction time is applied to the time 

 occupied in the centre in that complex response to a pre-arranged stimulus in 

 whicn the brain as well as the cord comes into play. It is sometimes called 

 the perxonal equation. It may be most readily measured by the electrical 

 method, and the accompanying diagram (fig. 492) will illustrate one of the 

 numerous arrangements which have been proposed for the purpose. 



In the primary circuit two keys A and B are included, and a chrono- 

 graph (i), arranged to write on a revolving cylinder (fast rate). Another 

 chronograph (2). marking i-icoths of a second, is placed below this. The 

 experiment is performed by two persons rand D. The key A, under the 

 control of C is opened. The key B, under the control of D is closed. The 

 electrodes E are applied to some part of J5's body. C closes A. The 



