Mammalia: Nervous System 707 



and sulci, thereby greatly expanding the surface of contact between 

 the cortex and pia. This makes it possible for the cortex to obtain a 

 greater proportion of its necessary blood from an external source 

 rather than from vessels inside the brain. It has been suggested that 

 the presence of large pulsating arteries within the nervous tissue might 

 interfere with nervous functions, although this is by no means certain. 

 A second advantage derives from the fact that the folding increases the 

 superficial extent of the region where the "gray " layer merges into the 

 "white." Millions of nerve-cells in the cortex must be reached by 

 nerve-fibers which course through the "white" substance in the form 

 of great, but not sharply delimited, bundles or "tracts," each com- 

 prising vast numbers of fibers. Many of these tracts connect with 

 centers in the more posterior parts of the brain (Fig. 525). Many others 

 pass, by way of the "white" substance, from one region of the cortex 

 to another (Fig. 526). The convoluted arrangement of the cortex in- 

 creases the space available for the inconceivably great number of 

 connections between the cortical cells and the nerve-fibers of the 

 subcortical mass. It seems probable, further, that for spatial localiza- 

 tion of nervous functions in the cortex a thinner layer having greater 

 superficial extent is more favorable than a thicker layer of less extent. 

 That such localization exists has been proved (Fig. 527). The sorting 

 out of fiber-tracts so that certain tracts go to a particular group of 

 cortical cells is more readily done if the cortical centers are spaced 

 apart laterally in a thin layer instead of bei-ng superimposed one above 

 another in a thick layer. 



In monotremes and marsupials and in insectivores, bats, and 

 rodents, convolutions of the cortex are, at most, few and not promi- 

 nently developed, and in many cases the external surfaces of the 

 hemispheres are quite smooth (Figs. 5215, 52 1A). 



Next to the cerebral cortex itself, and important in relation to the 

 function of the cortex, the most conspicuous "new" feature of the 

 mammalian brain is a great system of nerve-fibers making transverse 

 connection between right and left cerebral hemispheres. The central 

 nervous organs, like most other vertebrate organs, are bilaterally 

 symmetric. For perfect action and control of the motor mechanism of 

 the body, right and left coordination is just as important as longi- 

 tudinal. Correlation of right and left nervous centers in the brain is 

 effected by transverse systems of fibers — commissures. For correla- 

 tion of centers in the tremendously enlarged hemispheres of mammals, 

 a "new" commissure, vastly larger than any of the several commis- 

 sures of the palaeencephalon, appears. It extends transversely between 

 the hemispheres, but its position is extraordinary. The two hemi- 



