XIII MAMMALIA 447 



placed transverse commissure (pons varolii). In the mid-brain the optic 

 lobes are small and each is divided into an anterior and a posterior 

 portion (corpora quadrigemina). 



But the great characteristic of the mammalian brain lies in the 

 immense development of the cerebral hemispheres, in which is concen- 

 trated the general control of the subsidiary nerve-centres as well as the 

 psychic activity. This high development of the hemispheres is indicated 

 in the first place by their size and by the great area of their surface 

 layers. Their size is such that they extend right back to the cerebellum, 

 covering over the thalamencephalon and mesencephalon so that these 

 parts of the brain are no longer visible in surface view. Where the size 

 of the body is relatively great the surface layers of the pallium, growing 

 rapidly within the confined space limited by the cranium, become thrown 

 into characteristic folds or convolutions (gyri) separated by deep fissures 

 or clefts (sulci). Thus while in an adult Opossum the hemisphere is 

 smooth, in the Kangaroo it is convoluted. Or amongst the Primates 

 the small Marmosets (Hapale) have smooth hemispheres while in the 

 larger Primates they are convoluted. 



The high degree of development of the mammalian hemisphere is 

 still more apparent in the elaboration of its minute detail. The study 

 of this shows that it is above all the pallium that is highly developed. 

 The cortex in a lowly organized vertebrate like a lung-fish occupies only 

 a restricted portion of the pallium and is concerned with the olfactory 

 sense. In the mammal on the other hand its outer and inner (i.e. lateral 

 and mesial) portions, now known respectively as the pyriform lobe and 

 the hippocampus, are caused to recede from one another by an immense 

 increase in the intervening portion which in the mammal comes to form by 

 far the greater part of the cortex and is known as the neopallium. Whereas 

 the cortex appears originally to have been associated entirely with the 

 sense of smell the neopallium on the other hand is associated with the 

 three other senses — vision, hearing, and touch — a special area of cortex 

 being devoted to each of these. The tactile area is especially large 

 and a particular portion of it shows a further specialization for the control 

 of the movements of the body — each portion of the body having its own 

 special part of this motor area of the cortex allocated to it. It will 

 readily be understood how this localization of function is of the greatest 

 importance in medicine, for it renders possible the exact location of a 

 disturbing factor within the cortex— such as for example the presence of 

 a tumour — by a study of its disturbing effect upon the movements of the 

 particular parts of the body. 



The cerebral cortex would appear also to be the seat of the psychic 



