178 ARBOREAL MAN 



now of only minor importance. It has increased the 

 possibilities of both these senses by the greater mobility 

 of its head, and, in the case of sight, by the recession of 

 the snout region and the bringing of the eyes to the 

 front. It is easy to see that these things can only in- 

 crease the educational adaptability of these two senses, 

 and add to the possibilities of the association of their 

 impressions with (1) each other, and (2), in the case of 

 sight, with the sense of touch. 



This is exactly the condition which the neopallial 

 development of arboreal animals would lead us to expect 

 from pure anatomical and experimental evidences. When 

 the auditory area is first laid down in the neopallium, it 

 is situated at the hinder and lower portion of the cerebral 

 hemisphere; the visual area lies immediately above it, 

 and the kinsesthetic area lies in front of both. When 

 the cerebral hemispheres expand with the increased 

 demand for neopallial representation, the expansion does 

 not take place equally. One portion of the cortex is 

 in lateral apposition with the ganglionic mass of the 

 corpus striatum, and this portion does not share in the 

 expansion which affects those parts of the cerebral 

 vesicle which have no such rigid anchoring mass in rela- 

 tion with them. This portion consequently remains more 

 or less fixed (as the island of Reil), while the rest of the 

 hollow hemispheres enlarge around it. As the cerebral 

 hemispheres expand, the main direction of their growth 

 is backwards, and in this way it comes about that the 

 enlarging hollow hemispheres revolver ound this fixed point 

 as they grow (see Figs. 64-69). 



The auditory portion of the neopallium, which was 

 situated at the posterior inferior extremity of the primitive 

 brain, grows downwards and forwards as it revolves 

 below the island of Reil, pushed on from above by the 

 enlarging visual field. Not only this, but, as it expands 

 round the island of Reil, it becomes separated from the 

 visual field by an intervening cortical area. The visual 



