Animals, whose brains have nearly the sarne degree of developement as that of the fox, have exactly the same convolutions, differing, however, somewhat in point of size. This in crease of size is denoted by undulations formed it) the course of convolutions throughout more or less of their extent. The dog may be taken as an example. M. Leuret states that, in com paring the brains of several dogs together, he found with all of them the same convolutions, differing only in the extent of undulations and the number of depressions, both of which were greatest in the largest bmins. The brain of a large mastiff ((hien dope), a good watch-dog, of such great ferocity that he attacked the person who fed him, had all the convolutions very large and much undulated, with numerous depressions in them.
A group of animals, consisting of the cats and the hyena, exhibits another stage of in crease in the developement of convolutions. The same type prevails as in the fox and dog ; four external convolutions, one internal, and a supra-orbitar. These convolutions, however, are united to each other at numerous points by means of small folds crossing the sulci. These uniting folds form the secondary or supple mentary convolutions. Nearly all the primary convolutions have supplementary ones con nected with them.
A group, which includes the sheep and other ruminant animals, exhibits inuch more com plication in the cerebral convolutions, but still preserves the same type. The undulations and the supplementary convolutions are more numerous. The primary appear less nu merous because less distinct. The anterior part of the internal convolution is much in creased in developement, and the supra-orbitar is much more complex. In the fissure of Sylvius some small convolutions are found which are the first developement of those which in the human subject constitute tile insula of Reil.
In the brain of the elephant new convolu tions are added. These consist of folds pass ing in a perpendicular direction; the primitive convolutions always taking a longitudinal course. These latter are divided by the former into an anterior and a posterior set. Others are found above and in front of the fissure of Sylvius; three superior convolutions are found, the continuations of which backwards are situate above the internal convolution. All the convolutions of the elephant are remark ably undulating and exhibit numerous depres sions. The brain of the whale is very similar to it in this respect, and both resemble that of man.
Monkeys have not the tortuous or com plicated convolutions which are found in the whale and elephant. Yet the developement of the hemispheres at their posterior part, the general form of the brain, the extent and in clination of the fissure of Sylvius approximate the brain of monkeys to that of man much naore nearly than the whale's or elephant's, which, notwithstanding their complicated con volutions, are generally inferior in orpnization, and resemble the brains of other Mammalia.
The internal convolution in monkeys is simple; below and behind it forms the hippocampus, from Is.hich convolutions are prolonged back wards, forming the posterior lobe. Two supe rior convolutions are met with above the fissure of Sylvius, between which is placed a trans verse fissure very constant, called the fissure of Rolando. The orbitar convolutions are largely developed.
In comparing the human brain with that of the inferior animals, we notice great exactness of symmetry between the convolutions of op posite hemispheres in the latter, and the want of it in the former. It cannot, however, be said that the convolutions of opposite hemispheres in the human subject are absolutely unsymmetri cal. A careful examination will show that the same convolutions exist on each side, but appa rently of d i fferent si zes, and not closely correspon ding as regards situation. My meaning will be more readily understood by referring to fig. 381, p. 671, where the same numbers have been affixed to corresponding convolutions. No. 1 on the right has a certain general re semblance with No. 1 on the left, which would be much more perfect but for the fissure which marks the convolution of the right he misphere. 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 forrn. 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.