CHAPTER XXX 



THE BRAIN, AND COMPARATIVE BEHAVIOUR 



The brain is the anterior region of the spinal cord, modified, 

 specialised, and enlarged in connexion with the development 

 of special sense-organs in the anterior region of the body. 

 That these sense-organs should be accumulated here rather 

 than elsewhere is due to the fact that chordate animals are 

 bilaterally symmetrical and move along a definite axis with 

 one end constantly leading. This end is the first to come into 

 contact with new surroundings, information concerning which 

 is of the highest value to the animal. 



In order to understand the evolution of the spinal cord and 

 brain, it is necessary to consider what is known as a reflex arc. 

 An afferent fibre brings an impulse from a receptor, and if this 

 afferent fibre were to connect with only one efferent fibre going 

 to a particular muscle, whenever the receptor was stimulated 

 the response would be the contraction of this muscle. Nothing 

 else in the way of response would be possible. But actually 

 the afferent fibre when it has run into the brain or spinal cord 

 makes a large number of connexions with other neurons. 

 Some of these may be efferent neurons and connected with 

 various effectors ; others may be neurons which carry the 

 impulse to other parts of the spinal cord or brain : the so- 

 called association-neurons. By this means a receptor can be 

 connected up with several effectors, or one effector may be 

 stimulated by impulses coming from several different receptors. 

 This possibility of one efferent neuron being used by impulses 

 coming from several afferent neurons, as a " final common 

 path " for their reflex circuits, is of the greatest importance. 

 The efficiency and economy of using what may be called 



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