CENTRAL NERVOUS SYSTEM. 



729 



Here, as in the frog, the apparent increase must be looked upon as due to 

 the gradual development of elements present from an early date. 



Increase in the Fibres of the Cortex. The area of the cerebral cortex 

 (see Fig. 205) varies according to several conditions, but in general the more 

 voluminous the cerebral hemispheres the greater its extent. That which cov- 

 ers the walls of the sulci has in man about twice the extent of that directly 

 exposed on the surface of the hemispheres. 



FIG. 205. Diagram illustrating the extent of the cerebral cortex. The outer square (B) shows a sur- 

 face approximately & of 2352 sq. cm. in extent ; the inner square (A) has two-thirds of this area, and is 

 the proportion of the cortex sunken in the fissures. 2352 sq. cm. is approximately the area of the entire 

 cortex in a male brain weighing 1360 grams. . 



In the cortex of the human cerebral hemispheres it has been shown by 

 Vulpius 1 that the number of fibres in the different layers is greater at the 

 thirty-third year than at earlier periods, and in old age the number is again 

 decreased. At exactly what age decrease sets in is not to be determined from 

 these observations. They show, simply, that in general the number of fibres 

 was less at seventy-nine years than at thirty-three years. 



In a similar way Kaes 2 has compared the development of the thickness of 

 the cortical fibre-layers in a youth of eighteen years as contrasted with a man 

 of thirty-eight years, and found them thicker in the latter. 



The relation of the cell-bodies in the cerebral cortex at different ages is 

 illustrated by Figure 206. 



Significance of Medullation. Two sorts of nerve-fibres are described 

 those with and those without a medullary sheath. Both have the power of 

 isolated conduction, but in the peripheral system the non-medullated fibres are 

 found in connection with the sympathetic system, where less specialized func- 

 tions are carried on, and also in a large but varying degree in the central sys- 



1 Vulpins: Archivfiir Psychiatric und Nervenkrankheiten, 1892. 

 * Neurologische Centralblati, 1891. 



