CHAPTEE Y. 



THE BRAIN. 



THE brain, or encephalon, comprises all that portion of the cerebro- 

 spinal axis which is contained within the cavity of the cranium. It 

 consists of a variety of nervous centres, or collections of gray substance, 

 connected with each other and with that of the spinal cord by tracts of 

 longitudinal, transverse, oblique, and radiating nerve fibres. The results 

 of experimental investigation leave no doubt that each one of these 

 different nervous centres has a special function, more or less independent 

 of the others in its immediate action, though necessarily connected with 

 the rest in the production and external manifestation of the nervous 

 phenomena. They are situated upon both sides of the median line, and 

 are, for the most part, evidently arranged in symmetrical pairs, like the 

 hemispheres of the cerebrum, the cerebral ganglia, the olfactory lobes, 

 the tubercula quadrigemina, and the two halves of the cerebellum. The 

 largest of these nervous centres, forming in man nearly four-fifths of the 

 mass of the entire brain, are the two convoluted masses known as the 

 "hemispheres" of the cerebrum. 



The Hemispheres. 



The hemispheres form two ovoidal masses of nervous matter, flattened 

 against each other at the median line, where they are separated by the 

 great longitudinal fissure, corresponding to the posterior median fissure 

 of the spinal cord, and presenting on their lateral surfaces a general 

 rounded or hemispherical form, whence their name is derived. They 

 consist externally of a layer of gray nervous substance, and internally 

 of a mass of white substance, the fibres of which may be said in general 

 terms to radiate from the cerebral ganglia (corpora striata and optic 

 thalami) toward the cortical layer of the hemispheres. The external 

 layer of gray substance, and consequently the surface of the hemis- 

 pheres, is thrown into numerous folds or convolutions, which are sepa- 

 rated from each other by fissures, generally from 10 to 25 millimetres 

 deep. These fissures, like the great longitudinal fissure in the median 

 line, are simply spaces where the opposite surfaces of two adjacent con- 

 volutions lie in contact with each other ; and they indicate the points 

 at which the external layer of gray matter dips down toward the interior, 

 to return upon itself and form the next convolution. The larger quan- 

 tity of gray substance is, therefore, situated at the fissures rather than 

 at the projecting edges of the convolutions between them ; and the more 



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