124 INTRODUCTION TO GENERAL PHYSIOLOGY 



receptors. In this process their outer ends become elongated into 

 protrusions of various kinds, so that the nucleated cell body recedes 

 from the surface somewhat. But this does not proceed far until 

 we arrive at the vertebrate, in which the cell bodies of the receptor 

 neurones have receded nearly as far as the spinal cord itself, 

 forming what are known as the dorsal root ganglia. In this 

 way the ends of their axons are either merely situated between 

 cells at the periphery, forming pain receptors, or are connected 

 with the cells of specially developed receptors, such as were 

 described in the preceding chapter. 



Those association neurones which form the arcs extending 

 through parts of the brain itself represent the complex co-ordinated 

 activities in which thought and memory take part. 



We must now return to consider some aspects of reflex action 

 hitherto unmentioned. First of all, we should realise, by the 

 examination of some of these, that they take place without the 

 necessary participation of consciousness. The spinal frog (E., p. 

 216) serves well for certain experiments. In ourselves, the quick 

 withdrawal of the hand, when it touches a hot object, is done with- 

 out the conscious intention of doing so, although the sensation and 

 the fact of the reflex taking place are present in our consciousness. 

 Although the afferent impulses from the skin receptors have 

 travelled across by short arcs, branches from them have also pro- 

 ceeded to the brain by the long arcs. 



The variety of reflexes in which the same muscle or group of 

 muscles take part will probably be noticed in the experiments on 

 the spinal frog. The importance of the final common path is 

 shown here. Instead of having the receptors for each of these 

 reflexes separately joined up to the muscle, they have merely to be 

 connected to the motor centres of the muscles, directly or through 

 intermediate neurones, and one set of out-going or efferent fibres 

 suffices. But it is clear that the same final common path cannot 

 be used for different reflexes at the same time, and if it is to be 

 used quickly for a new reflex, the preceding one must be cut short. 

 The discharge of a reflex arc lasts longer than the stimulus pro- 

 ducing it, and it is frequently necessary to stop it more rapidly 

 than it would cease if left to itself. This is done by a process of 

 inhibition. We do not know what this actually consists in, beyond 

 the fact that certain nerve fibres end on a muscle or nerve cell in 

 such a way as to lessen or stop its activity, instead of increasing 

 it, as the ordinary motor or excitatory fibres do. We have seen 

 an example of it in the case of the intestinal muscle, and a very 

 important one is that of the action of the vagus nerve on the heart 

 (E,p. 217). 



It must be understood that the process of inhibition is an actual 



