338 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XIX. 

 THE NEOPALLIUM. 



The evolution and general morphology of the cerebral hemi- 

 spheres have been described in the last chapter and as full a 

 description of the archipallium given as the limits of this book 

 will permit. The neopallium, although it occupies the greater 

 part of the mammalian hemisphere, is essentially dorso-lateral 

 in position, lying between the pyriform lobe ventro-laterally and 

 the hippocampal formation dorso-mesially. It is connected 

 with the lower parts of the brain by large bundles of fibers 

 which run down through the corpus striatum, forming the cap- 

 sula interna (Figs. 162 and 166). By means of the corpus 

 callosum the neopallial areas of the two hemispheres are connected 

 with one another. 



STRUCTURE OF THE CORTEX. The neopallium everywhere 

 consists of a thick internal layer of fibers and of a superfical layer 

 of cells, the cortex. This cortical layer varies in thickness from 

 about 1.70 mm. to about 3.50 mm. Its total volume increases 

 enormously from lower to higher mammals on account of the 

 increasing size of the hemisphere as a whole, and still more on 

 account of the superficial folding in the higher forms. The 

 cortex consists of several zones of cells of different forms which 

 are to be recognized with certain modifications in all parts of the 

 pallium. The typical structure of the cortex as seen in Golgi 

 sections is illustrated in the accompanying Figure 171, which is 

 combined from numerous figures of Cajal representing sections 

 of the frontal and parietal cortex of the young child. As indicated 

 by arabic numerals at the left of the figure, seven layers are dis- 

 tinguished according to the size and form of their cells and the 

 disposition of their neurites. 



i. Plexiform layer. This layer contains (a) small and medium 

 sized cells with short neurites and (b) large horizontal cells whose 



