CHAP.x.] CONVOLUTIONS OF THE BRAIN. 281 



Taking the brain of the fox as the standard of comparison, 

 M. Leuret* describes in it six obvious convolutions, prominently 

 marked on the surface of its hemispheres. Four of these are ex- 

 ternal, the uppermost of which occupies also the principal portion 

 of the superior surface; one is internal, and situate immediately 

 above and parallel to the corpus callosum ; while the sixth is on the 

 inferior surface of the anterior lobe, and rests upon the orbit, 

 whence it is named supra-orbitar. Of the four external, the in- 

 ferior one bounds the fissure of Sylvius above, in front, and behind ; 

 and its relation to that fissure enables the observer to distinguish 

 it very readily. The three remaining ones, curved similarly and 

 parallel to the first, and to each other, occupy the remainder of the 

 external, and, in pari, the superior surface of the hemisphere. 



M. Leuret distinctly traces these convolutions in other groups of 

 animals in which the general organization of the brain has mani- 

 festly acquired a considerable increase. Some of them, however, 

 are fissured, or exhibit a tortuous appearance; or one or more 

 small folds unite neighbouring convolutions at one or more points ; 

 or a fissure may be of such depth as to divide a convolution at 

 one extremity into two, either or both of which may form a junc- 

 ture with others. Thus, from a few primary or fundamental con- 

 volutions, a highly complicated surface of the brain may be formed, 

 by their subdivision, by their tortuosity, and by their junction 

 at various points, through the intervention of straight or tortuous 

 secondary folds. 



In some animals, however, the primary convolutions may even 

 be less numerous than those above mentioned ; and yet the surface 

 of the brain may appear more complex, owing to the tortuosity 

 of those which do exist, and their subdivision into, or junction 

 with, numerous secondary folds. This is the case in that group of 

 which the sheep forms the type. There are but two primary 

 convolutions on the external surface, one of which corresponds to 

 that of the fissure of Sylvius in the fox, the other to the one imme- 

 diately above it ; and there are the internal convolution and the 

 supra-orbitar one, making in all only four primary convolutions. 

 Yet the surface of the sheep's brain exhibits a much greater num- 

 ber of folds than that of the fox. 



On the other hand, when the brain ha's acquired an enormous 

 increase of size, as in the elephant and in man, new convolutions 



* Professor Owen has pursued the same subject extensively, and has given 

 his results in his lectures at the Royal College of Surgeons ; but we believe 

 these have not yet been published. 



