282 INNERVATION. [CHAP. X. 



seem to be added to the primary ones met with in inferior groups, 

 and the secondary folds are greatly increased in number. The 

 additional convolutions are found chiefly at the superior and an- 

 terior part of the hemisphere. 



In the human brain the following convolutions are constantly 

 present, and resemble the primitive ones, which have been 

 already referred to in the brains of the inferior animals. The 

 internal one is always well marked ; it lies parallel to the corpus 

 callosum, overlapping it slightly on either side. In front it winds 

 round the anterior margin of the corpus callosum, and is con- 

 nected with the convolutions of the anterior lobe ; posteriorly it 

 divides, appears to be continuous with some posterior convolu- 

 tions, and passes into the middle lobe, forming the hippocampus 

 major. Numerous small folds pass from its upper edge to the 

 superior convolutions. The supra-orbitar convolution is well de- 

 veloped, and bears a constant relation to the fissure for the olfactory 

 process. The fissure of Sylvius is bounded by a tortuous external 

 convolution, which forms numerous connexions with others on the 

 external surface of the brain. In this fissure is found constantly a 

 group of shallow convolutions, which forms what is been called by 

 Reil, the island, insula, from their isolated position, having only 

 deep-seated connexions in the vicinity of the corpus striatum. 

 Some longitudinal convolutions are found on the superior and on 

 the inner surfaces of the hemispheres, uniting with neighbouring 

 ones by means of numerous transverse folds. 



According to Leuret, that only can be properly called a convo- 

 lution which is primary; and these, for the most part, take a 

 direction in the length of the brain. Those which form angles 

 with the primary convolutions are, in his estimation, mere folds 

 derived from them, and connecting them to others. When the 

 convolutions are very highly developed, as in man and the ele- 

 phant, their numerous undulations obscure in a great degree their 

 real direction. Hence many of the primary convolutions in the 

 human brain seem to take a vertical direction. 



There are, however, other differences in the convolutions, whether 

 of the brains of the same or of different groups, besides those de- 

 pendent on form and degree of undulation. These are referrible 

 to their depth and their thickness. Animals, even of the same 

 group, or of the same species, exhibit much variety with respect 

 to these points. The wolf has precisely the same convolutions 

 as the fox; but those of the former are deeper, and thicker, 

 as well as more undulating than those of the latter. Much 



