

THE HEMISPHERES. 479 



with absolute certainty. Although a few isolated instances have been 

 reported in which a cell prolongation has been seen to become con- 

 tinuous with a medullated nerve fibre, it is usually impossible to 

 demonstrate this by direct observation, and some of the best micro- 

 scopists have been unable to see it in a single instance. The probability 

 that such a connection exists is assumed from the fact that many of the 

 nerve fibres in the gray substance become so slender and pale as to 

 resemble very closely the cell prolongations ; and, on the other hand, 

 the cell prolongations frequently run in the same direction with the 

 nerve fibres. According to Henle 1 the prolongations, given off from 

 the bases of the pyramidal cells, are so often seen to lose themselves in 

 the bundles of nerve fibres coming from the white substance as to justify 

 the assumption that the fibres in these instances terminate in the cells. 

 The delicacy of texture of the gray substance and the distance through 

 which a cell prolongation runs before it attains the character of a nerve 

 fibre may be sufficient reason whj r the connection is not more frequently 

 seen in microscopic preparations. 



Physiological Properties of the Hemispheres. The importance of 

 the hemispheres, in connection with the higher manifestations of nervous 

 action, is sufficiently indicated by their excessive development in man, 

 as compared with the other portions of the encephalon. For while in 

 the lower mammalians they are of medium size, and often smooth or 

 sparingly convoluted upon their surface, and in reptiles and fish are 

 sometimes hardly larger than the other nervous centres of the brain, in 

 man they acquire such an extension as to cover and surround almost 

 completely every other part of the encephalic mass, their superficial 

 layer of gray substance being at the same time still further increased by 

 the multiplied convolutions of its surface. 



Notwithstanding, however, the evident importance of the hemispheres 

 as special parts of the nervous system, the first fact certainly known in 

 regard to them is that they are not, even in man, directly essential to 

 life. That is to sa} r , they do not hold under their immediate control 

 any of the physiological acts, like those of respiration and circulation, 

 which are necessary to the continuance of vitality. They often influ- 

 ence these acts, in an indirect manner, by the sympathetic connections 

 of the nervous system; but life will continue for a certain period under 

 the influence of other nervous centres, without the aid of the cerebral 

 hemispheres. 



This is readily demonstrated in some of the lower animals by the 

 entire removal of the hemispheres on both sides. In man, extensive 

 morbid changes may take place in these parts, or severe mechanical 

 injuries, accompanied by greater or less loss of substance, may be in- 

 flicted upon them without producing a fatal result. In a case reported 

 by Prof. Detmold, 2 of abscess in the anterior lobe of the brain, a knife 



1 Anatomie des Menschen. Braunschweig, 1871, Nervenlehre, p. 270. 



2 American Journal of the Medical Sciences. Philadelphia, January, 1850. 



