THE BRAIN, AND COMPARATIVE BEHAVIOUR 379 



the same time, the end-brain has been evolving in another 

 direction, in that the cerebral hemispheres are formed as 

 outgrowths containing each a cavity (the lateral ventricles) 

 communicating with that of the between-brain through the 

 foramina of Monro. Cerebral hemispheres first appear in the 

 Dipnoi, and it is possible that they are an adaptation to deficient 

 oxygen-supply : a matter of great importance, for the brain 

 requires the purest arterial blood in the body. The formation 

 of cerebral hemispheres increases the surface of the brain- 

 tissue relatively to its volume, not only on the outside in contact 

 with the vascular pia mater, but also on the inside which is 

 bathed by the cerebro-spinal fluid, itself oxygenated by the 

 choroid plexus. The migration of the neurons to the surface 

 to form a cortex may also be an adaptation to oxygen require- 

 ments, for solid masses of neurons would require large arteries 

 to enter the brain, and there are indications that the pulse 

 of large arteries is injurious to the delicate workings of the 

 neurons. 



Another advantage of the cortex type of structure is that 

 it allows of the arrangement of centres on its surface after 

 the fashion of a chequer board. The cortex deals with 

 impulses from the outside world, in animals with sense-organs 

 sufficiently well developed to give them good representations 

 of the relations of different objects and events in space. It 

 is apparently necessary that these representations of objects 

 in space should remain separate in the brain until finally co- 

 ordinated. In the same way it would be impossible to judge 

 which of a number of threads was which, if they were all 

 tangled up together in a ball. This analogy also introduces 

 the fact that the function of the cerebral cortex is to receive the 

 impulses which have already been sorted out in the correlation- 

 centres, and to judge which of many possible is the best 

 response to make. The cortex introduces hesitancy and 

 arbitration into behaviour, which, on the level of the reflex arc, 

 is immediate and determined. 



Another factor to be borne in mind is that the cerebral 

 cortex is principally concerned with impulses coming from 

 the exteroceptors, and especially those which, like the eye, 



