Изображения страниц
PDF
EPUB

But the presence of the thumb on the lower extremities suggests the use which it serves in the animal's ordinary life, in grasping the branches along which it moves. If from the similarity of outward configuration, we pass to contemplate the brain, we find here also great similarity of structure. And indeed if the relations of muscle, nerve, and brain be as already indicated, it follows from the resemblances of outward form that there must be a greater resemblance between the brain of man and the brain of the monkey and of the ape, than between the human brain and that of any other animal known to us. And so it proves to be. The brain of the monkey has its subdivisions and convolutions very similar to those of the human brain, only the convolutions are simpler in arrangement. In outline it is deficient only in the diminished bulk of the front part, and also the back part of the organ; but in its expansion it resembles the human brain in this, that to the rear it spreads back over the cerebellum, so as to cover it. The brain of the ape, including under this designation the orang, gorilla, and chimpanzee, is in still closer resemblance to the human, being still, however, somewhat

[graphic][merged small][merged small][graphic][merged small]

simpler in the arrangement of its convolutions, but so closely approximating that the exact state of the case is as nearly as possible described, if we say that the brain of the ape, while it is decidedly smaller, appears like a miniature of the human brain in a slightly undeveloped state.*

The human brain is an elaborate organ, exceedingly complicated in its convolutions. We can not, indeed, describe it as the most convoluted, for the brain of the elephant is at least as distinguished for the beauty and complication of its folding, and the brain of the whale is far more minute and detailed, presenting quite a multitude of minute convolutions. For descriptive purposes, the human brain is divided into four superficial areas, known as lobes, and pretty clearly defined by certain natural boundaries. From the lower part of the organ, entering at a point scarcely half way back is a fissure or cutting running up into the mass in a direction uniformly inclining towards the rear, known as the Sylvian fissure; while coming over the summit, at a point near the middle, and inclining down. towards that just described, is another fissure, known as the fissure of Rolando. By these * See Appendix XI.

two deeply cut hollows, the brain is marked off into four separate areas superficially, a front and a rear lobe; and two central lobes, the one upper and the other under. Besides this there is a concealed and isolated lobe, described on account of its situation as an island, which is covered from view by the overlapping of the two sides of the Sylvian fissure. Such is a description in outline of the configuration of the human brain, to which must be added the statement that each lobe is filled in with its own special arrangement of convolutions, each one having at least three well defined lines of convolution. Each of the hemispheres is similarly arranged, though not by any means quite identical in disposal of convolutions, yet the general description now given is strictly applicable to both. The two hemispheres, connected mainly with the ramification of nerve fibre running to the opposite sides of the body, are united together a considerable way down by a transverse band of nerve fibres, which at once unite the two into one organ, and make the union so effected a living efficient union by carrying a multitude of lines of communication from the one side to the other. Just below this, in the interior

« ПредыдущаяПродолжить »