NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHAI.ON.) 097 



mispbere. Again, Nos. 2, on opposite sides, 

 resemble each other so closely that their sym- 

 metrical relation cannot be doubted. The like- 

 ness, however, is impaired by slight fissures in 

 the convolution on the left which do not exist in 

 that on the right side. Nos. 3 and 3 evidently 

 correspond, but that of the right side is the larger 

 and more undulating. And it may here be 

 remarked that this great developement of the 

 convolution marked 3 on the right side affects 

 materially the position, relations, and shape of 

 those in its neighbourhood, by throwing them 

 backwards or outwards and altering their 

 form. Thus the position and shape of con- 

 volution 4 seems evidently modified by the 

 large posterior undulations of convolution 3. 

 In the brain from which the figure was taken, 

 the convolutions on the right side are evidently 

 larger and more highly developed than those 

 of the left. It does not appear that there is 

 any constancy with respect to the relative size 

 of the convolutions of the right and left side, 

 sometimes one side predominating, sometimes 

 the other; nor have we any clue to discover 

 the cause of the difference between the two 

 hemispheres, or the reason of the variation as 

 regards predominance of size. 



In the imperfectly developed brains of the 

 infant or young child, the convolutions are 

 quite symmetrical. They are so likewise in 

 idiots, or persons of very inferior intellect, 

 and, as has been already stated, in some Negro 

 brains. 



The following convolutions of the human 

 brain are constant in their position, although 

 they differ much in different brains in size 

 and developement. 



1. The internal convolution, or that of the 

 corpus callosum, called by Foville convolution 

 d\>urlet (jirocexso cristato, Rolando). The 

 principal portion of this convolution is above 

 and parallel to the corpus callosum : in front it 

 curves down parallel to the anterior reflection 

 of the corpus callosum, as far as the locus per- 

 foratus, connecting itself with some of the ante- 

 rior convolutions. Behind it passes in a similar 

 manner round the posterior reflexion, connecting 

 itself with some of the posterior convolutions, 

 and in the middle lobe forming the hippocam- 

 pus major, the anterior extremity of which is 

 situate immediately behind the fissure of Sylvius 

 and locus perforatus. Its horizontal portion 

 appears to be connected with some nearly ver- 

 tical ones, which seem indeed to branch off 

 from it. (Fig. 395, O.) 



This is the most constant and regular con- 

 volution of the brain. It exhibits with its fel- 

 low of the opposite side very exact symmetry. 

 Its inferior or concave border is smooth and 

 uninterrupted, and foims the superior boun- 

 dary of a sulcus, which intervenes between it 

 and the surface of the corpus callosum. It 

 forms, to use Foville's expression, a hem or 

 selvage to the cortical layer of the cerebral 

 hemisphere. The fibrous matter which is in- 

 closed by the cortical layer of this convolu- 

 tion consists of longitudinal fibres following 

 the same general direction, a large number of 

 them no doubt bending inwards into the cor- 

 tical layer. These fibres are evidently com- 

 missurul in their office, and will be referred to 

 by-and-bye as constituting the superior longi- 

 tudinal commissure. 



The free margin of this convolution varies in 



Fig. 395. 



Internal surface of the left hemisphere of the brain, shewing the connections of the internal convolution and the 

 band of longitudinal Jibres by which it is formed (d'ourlet). 



C, C, corpus callosum ; O, 0, 0, internal convolution ; b. septum lucidum ; a, anterior commissure ; 

 /, fornix ; c, superior layer of the crus cerebri ; d, inferior layer of the same separated from the forme r 

 by the locus nigpr. 

 The fibres of the internal convolution arc seen in the middle lobe extending to the hippocampus major. 



