THE BRAIN, AND COMPARATIVE BEHAVIOUR 381 



is developed to link together the neopallium of the two hemi- 

 spheres ; this is the corpus callosum. The dorsal commissure 

 of the reptiles, which links together the hippocampal archi- 

 pallia, persists in the mammals as the hippocampal commissure. 



The volume of the neopallium is increased in higher 

 mammals without much increasing its thickness by throwing 

 it into folds. 



The various regions of the neopallium are connected with 

 the other centres by projection-fibres, and in addition, these 

 regions are interconnected by association-fibres. The number 

 of possible combinations between the neurons is so large that 

 it baffles the power of the mind to grasp it. As an example, 

 one million neurons connected together in all possible ways 

 in groups of two neurons each, gives a number of combinations 

 with nearly three million figures in it. There are not far off 

 ten million neurons in the human cerebral cortex. 



The neopallium is therefore well fitted to correlate all the 

 stimuli which the animal receives and to make delicately 

 adjusted responses to them. It also serves as a storehouse 

 for impressions which are collected during experience, and an 

 animal which, in determining the response to be made to a 

 set of stimuli, considers the results of experience, is said to 

 show intelligent behaviour. Such an animal has the power of 

 learning, which is not the same thing as the establishment of 

 a habit. Habits can be formed in the lower simple correlation- 

 centres, by means of neurons between certain afferent and 

 certain efferent neurons. The oftener an impulse passes 

 along a reflex arc the easier does its passage become, with the 

 result that the " habitual " response is given to a stimulus. 

 Some habits so formed may be quite complicated, as when 

 a piece of music is " learned by heart." This learning is, 

 however, not necessarily intelligent, because it often happens 

 that when the musician breaks down he is unable to adapt 

 himself to the immediate circumstances and continue, but 

 has to start again at the beginning. 



In a similar way animals can be trained to do tricks, or to 

 thread the " Hampton Court " maze without going down any 

 of the blind alleys. If a rat be so trained as to " know " a 



