THE LIVING BIRD 19 



taneously and so enlarging the anterior end of the 

 brain. Ultimately, in the vertebrates above the 

 fishes, we get true cerebral hemispheres established, 

 which begin to overshadow the old olfactory sec- 

 tions. This arrangement reaches its climax in 

 mammals where the hemispheres are not only enor- 

 mous, but the top layer develops convolutions 

 which greatly increase the surface area. An exami- 

 nation of the hemisphere of a mammal gives us a 

 picture of a thick outer layer of nerve cells which 

 constitute the well-known grey matter of the brain, 

 or cortex. Here, as neurologists have demonstrated 

 in a thousand ways, is the centre of the higher 

 mental processes and, in man, the seat of the mind. 

 Here then in mammals we find the master organ of 

 the vertebrate brain. Except for the characteristic 

 arrangement of its cell-layers — there are many 

 millions of neurones in the cortex — and the fact 

 that they occur at the surface, in close juxtaposition 

 to the richly vascular pia mater, a most advantage- 

 ous site that also permits expansion, there is nothing 

 remarkable about the cortex, but there is something 

 remarkable in its relationship to the rest of the brain. 

 The various primitive connections of the fish's 

 brain (already described) still exist, but there is a 

 far larger number and the arrangement of them is 

 such that every other part of the brain and the 



