CHAPTER V. 

 BRA IX. 



THE brain consists of various deposits of gray substance, and of 

 tracts of white substance serving as commissures between its 

 different regions, or as means of communication with the spinal cord. 

 Its principal divisions are the cerebral hemispheres, the cerebellum, the 

 tuber annulare, and the medulla oblongata. Of these the hemispheres 

 are by far the largest j forming, in man, nearly four-fifths of the entire 

 brain, 



The Hemispheres, 



The hemispheres are two ovoidal masses, flattened against each 

 other at the median line, where they are separated by the great longi- 

 tudinal fissure, and presenting on their lateral surfaces a rounded or 

 hemispherical form, whence their name is derived. They consist 

 externally of a layer of gray substance from two to three millimetres 

 in thickness, covering a mass of white substance, the fibres of which 

 in general radiate from within toward the cortical layer. Their 

 surface is thrown into numerous convolutions, separated from each 

 other by fissures generally from 10 to 25 millimetres deep. These 

 fissures, like the great longitudinal fissure, are the spaces where oppo- 

 site surfaces of adjacent convolutions lie in contact with each other ; 

 and they indicate the points at which the layer of gray substance 

 folds inward, to return upon itself again and form the next convolu- 

 tion. The larger quantity of gray substance' is, therefore, situated at 

 the fissures rather than at the convolutions ; and the more numerous 

 and deeper the fissures on the surface of a brain, the greater the 

 amount of gray substance which it contains. 



Although the cerebral fissures and convolutions are not all the same 

 in different brains, nor even exactly symmetrical in the two hemispheres, 

 yet many of them are sufficiently constant to be regarded as essential 

 features of the organ ; and the remainder, while varying within certain 

 limits, exhibit a general arrangement characteristic of the species to 

 which they belong. In man they attain a very high degree of devel- 

 opment ; and their nomenclature is useful for designating different 

 parts of the cerebral surface. 



Next in importance to the great' longitudinal fissure, which separates 

 the hemispheres at the median line, is the Fissure of Sylvius (Fig. 

 110, S). ThisJs a much, deeper cleft than the others, and exists, 

 according to Wilder, in all animals whose brains are fissured at all. 

 In man it is the first to appear during embryonic life, being visible as 



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