THE CEREBRAL FIBRES. 



From the preceding description, it will be seen that the free convo- 

 luted surface of the cerebral hemispheres, is of enormous extent, in 

 comparison with the small surfaces, which are connected below with 

 the peduncles, and across, from side to side, with the corpus callosum. 

 This free surface, indeed, occupies at least five-sixths of the internal 

 surface of the hemispheres corresponding with the parts turned towards 

 the longitudinal fissure; it extends likewise all over the outer surface, 

 and over the base, with the exception of the comparatively small part 

 connected with the peduncles. It corresponds with the internal sur- 

 faces of the frontal, temporal, and parietal bones, with a portion of the 

 occipital bone, and with the upper surface of the tentorium. 



The whole of this highly complex free surface of the hemispheres, 

 which appear like efflorescences upon the summit of a stalk, Fig. 59, d, 

 is composed essentially of cineritious or gray nervous matter, here 

 named the cortical substance, Fig. 58, 6, consisting chiefly of nerve cells, 

 but also traversed by a number of very fine white fibres, and extremely 

 well supplied with bloodvessels. The two principal purposes fulfilled 

 by the superficial position of the gray matter, and by the great compli- 

 cation of its surface, are that of allowing a more perfect communication 

 with the numerous fibres connected with it, and that of obtaining the 

 greatest possible amount of gray nervous matter within a comparatively 

 small space; moreover, this large surface facilitates the free and abun- 

 dant supply of blood to the important gray cortical substance of the 

 cerebrum. The surface of the human cerebrum has been computed by 

 Baillarger to be equal to about 670 square inches. The thickness of 

 the cortical substance is usually about one-fifth of an inch ; but this 

 varies, as well as the depth of the sulci, which is a measure of the 

 height of the convolutions; in both these respects, the adult human 

 brain exceeds that of the infant, or of the aged ; and a deficiency in 

 both, has been observed in idiots, and in the less civilized races of men. 

 The color and structure of the cortical substance are not uniform in 

 its whole depth ; but it presents three different zones, viz., an outer pale 

 zone, in which two narrower ones may be recognized; a middle grayish 

 zone, and an internal reddish-yellow zone, in which four narrow ones, 

 two reddish-yellow, and two white, may be discerned, thus making 

 seven zones, or layers, in all. The nerve cells, also arranged in layers, 

 are most abundant in the middle zone, and least so in the external 

 one, in which the white fibres are very numerous. The nerve cells 

 are provided with numerous branching offsets, i. e., are markedly mul- 

 tipolar. The white fibres of the deepest zone are chiefly radiating in 

 their direction, but some also pass parallel to the surface of the cere- 

 brum, and so give rise to the appearance of zones. As the white fibres 

 proceed through the gray matter, they become finer, and many of them 

 end in the processes of the nerve cells. In the external zone, they 

 are finest of all, and many of them again spread out parallel with the 

 surface, in the paler streaks of the outer zone, where they form loops, 

 or are connected with nerve cells. The object of the intricate arrange- 

 ment here described, appears to be to multiply the nerve fibres, which, 

 by successive connection with layers of branched cells, are rendered 

 finer, at the same time that they are enormously increased in number, 



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