THE BRAIN, AND COMPARATIVE BEHAVIOUR 373 



interchangeable standard units (the neurons), capable of an 

 infinite variety of combinations is one of the main factors of 

 the success of the higher vertebrates. An animal possessing 

 this type of nervous system can make many kinds of response, 

 and indeed by suitable connexions and adjustments there is 

 no limit to the number of combinations which may be formed 

 between receptors and effectors. These adjustments are made 

 in the central nervous system, and they are its function, just 

 as that of a telephone exchange is to make adjustments between 

 calling and answering subscribers. The key to the whole 

 system is the neuron, which is not rigidly fused on to any 

 other cell, but which can make synaptic connexions with a 

 great number of other cells and pass impulses on to them. 

 New connexions can be made, and new kinds of response can 

 be evolved, which become " conditioned " reflexes, or habits. 



The places in the central nervous system where these 

 adjustments are made are called centres, and they lie in the 

 grey matter. When the skin of a dog is stimulated by a small 

 irritation, the receptor in the skin sends an impulse through 

 an afferent neuron which runs into the spinal cord by the 

 dorsal root. This neuron makes a synaptic connexion with 

 an association-neuron in the grey matter of the spinal cord. 

 The fibre of this association-neuron runs down the spinal 

 cord in the white matter to the segment of the body where 

 the hind leg is situated. There it makes a synaptic connexion 

 with an efferent neuron (in the grey matter) which passes out 

 through the ventral root to the muscle of the leg. The result 

 of the stimulus is a jerk or " scratch " on the part of the leg. 

 This reflex arc illustrates the fact that the function of the 

 spinal cord is twofold. It contains a number of reflex adjust- 

 ment-centres (in the grey matter), and it conducts impulses 

 up or down the cord to different levels (in the white matter). 



In the brain there are the primary centres, connected with 

 the different functional systems of components. These are 

 the " skin brain," " ear-brain," " taste-brain " (in the medulla 

 oblongata), the " eye-brain " (in the midbrain) and the " nose- 

 brain " (in the forebrain). Each of these is a centre where 

 impulses are received of a particular type (from a particular 



