CHAP. X.] THE CEREBRAL HEMISPHERES. 279 



after seven years of age, and is in a great degree peculiar to the 

 human subject. 



The structure of the pineal body is very imperfectly known; 

 and although its office has been a theme for some of the wildest 

 speculators in physiological theories, we are still utterly in the dark 

 respecting it. 



Of the Cerebral Hemispheres. The hemispheres of the brain are 

 ovoid masses, which in man constitute by far the largest portion 

 of the encephalon. All that mass of nervous matter which is 

 external and superficial to the optic thalami and corpora striata 

 constitutes the hemispheres properly so called. A vertical fissure 

 separates the right and left hemispheres, which, although not per- 

 fectly symmetrical, very closely resemble each other. This fissure 

 contains the great falciform process of the dura mater, which thus 

 forms a septum between the cerebral hemispheres. 



When a horizontal section is made through either hemisphere, 

 an oval surface is exposed (centrum ovale of Vieussens), which 

 consists of an area of white or .fibrous matter, bounded by a 

 waving margin of gray. The latter is about an eighth of an inch 

 in thickness: it is covered on its exterior by pia mater, from which 

 innumerable minute vessels penetrate it; and within it adheres 

 intimately to the white matter, the fibres of which extend into it, 

 and mingle with its elements. 



In examining the surface of a hemisphere from which the pia 

 mater has been stripped, the peculiar folded arrangement of it 

 is manifest. These folds, commonly known as the convolutions of 

 the brain, resemble the rugse which are produced in the mucous 

 membrane of the stomach when its muscular coat is very much 

 contracted. They are evidently destined to pack into a small 

 compass a large surface of vesicular matter. A sulcus separates 

 each convolution from the neighbouring one. The gray matter 

 is found at the bottom of the sulci, as well as upon the promi- 

 nences of the folds, and its union with the fibrous matter takes 

 place equally in the one as in the other situation. A sulcus, 

 therefore, contains the gray (vesicular) and white (fibrous) ele- 

 ments as distinctly as a fold or convolution. It is evident, that if 

 the surface of gray did not exceed that of the white matter, folds 

 or convolutions would not be necessary, but a simple expanse of 

 the former would suffice to cover the surface of the latter. The 

 convoluted arrangement increases the vesicular surface to an im- 

 mense extent, without occupying much additional space ; and, by 

 the prolongation of the fibres, which correspond to the concavities 



