330 THE BODY AT WORK 



history of the race have established their right to dominance 

 not only seize and hold a route through the nervous system, to 

 the exclusion of all competitors, but, as we have already shown 

 in the case of the swallowing impulse, the traffic in neighbouring 

 routes is suspended for their benefit. At the other end of the 

 scale we find reflexes which may be termed " occasional," in 

 that, although of frequent occurrence, they exhibit illimitable 

 variability in form. Occasional reflexes require, as a pre- 

 liminary to their transmission, that the afferent impulses which 

 give rise to them should secure for a time the exclusive use 

 of the motor neurones by which they are carried out. The 

 receptors bring the motor neurones into tune with themselves, 

 and while in tune they will respond to impulses from no 

 others. But the tuning lasts for a short time only. Either 

 receptor or neurone, or both, soon tire. There is no danger of 

 a particular reflex being prolonged to the detriment of the 

 organism as a whole. As an illustration of an occasional reflex, 

 we may cite the scratching movement of a dog. Its skin is 

 punctured by a flea. It scratches the place. A second flea 

 bites it somewhere in the same neighbourhood. The dog 

 does not shift its hind-foot so as to scratch midway between 

 the two bites. It finishes out one scratch before paying atten- 

 tion to its second tormentor. The exact position to which the 

 hind-foot is raised depends upon the position of the irritant ; 

 and since this may be shifted over a very considerable surface, 

 the form of the reflex varies equally widely. Each of the very 

 numerous receptors in the skin tunes a slightly different group of 

 motor neurones ; and since a second irritant may reinforce the 

 first, instead of making an alteration in the group of neurones 

 which the reflex is discharging, it is clear that there is no fixed 

 path uniting receptor A with neurones X, Y, Z and receptor B 

 with neurones W, X, Y. If, however, the second irritation 

 occurs at a spot lying at a considerable distance from A, in 

 place of reinforcing the scratching movement which A has set 

 going, it weakens and shortens it. The receptor C, which is 

 calling for the discharge of a markedly different set of motor 

 neurones, tends to inhibit those which are already active. 

 These results are tested with precision upon a " spinal dog " 

 and with the aid of an electric needle, the other pole from the 

 battery being a large flat plate placed in contact with the 



