Section 2. 

 Reflex Action. 



Nerve-fibres do not under natural circumstances generate 

 impulses ; they transmit them, but without modifying them. 

 Modification can only occur in nerve centres, such as the brain 

 and spinal cord, and these centres always consist largely of 

 nerve-cells, of which, as we have seen, the nerve-fibres leaving 

 or entering the centre are simply processes or branches. The 

 spinal cord may be described, not as one long centre, but a series 

 of centres lying end to end, each capable to a greater or less 

 extent of acting independently of its neighbour, and each centre 

 possessing its afferent and efferent roots. 



In these segments of spinal cord complex acts can be initiated 

 by the arrival of simple afferent impulses. Such acts may be 

 carried out without any assistance from the brain, for they can 

 readily be demonstrated in an animal where the brain has been 

 destroyed. These acts are known by the name of ' reflex,' from 

 which it must not be inferred that an afferent impulse is simply 

 reflected into an efferent channel, but rather that an afferent 

 impulse reaches the cord, and, passing into the grey matter, 

 stimulates the ganglionic cells which generate the efferent im- 

 pulse. The structures necessary for a simple reflex act are — 



(1) an afferent nerve to convey the impression to a nerve centre ; 



(2) a nerve centre in which the outgoing impulses are generated ; 



(3) an efferent channel for their transmission (Fig. 132). More 

 complex acts may need more afferent nerves, a larger number of 

 excitable centres, and a greater number of efferent fibres. The 

 nervous chain is known as a Reflex Arc, and can never consist of 

 less than two neurones. 



A classical example of a reflex act is exhibited when a frog 

 from which the brain has been entirely removed draws up its 

 leg when the foot is pinched. Depending upon the degree of 

 pressure applied to the foot, it draws up either one leg or both — 

 i.e., the reflex movements are unilateral or symmetrical, accord- 

 ing to the number of ganglionic centres in the cord which have 

 been stimulated. Still greater violence applied to the foot of 

 this brainless frog will affect a larger number of centres further 

 forward in the cord, so that the fore-limbs may share in the 

 reflex. The brainless frog reacts more regularly to this experi- 

 ment than one possessing a brain, which is evidence that the 

 brain is capable of exercising a controlling influence or inhibitory 



441 



