In man the brain is supposed to constitute about 1-35th part o' the weight of his body. In the dog, averaging the different breeds it is 1-120th part; in the horse it is only the 450th part, in the sheet the 750th part, and in the ox the 800th part.
As an illustration of the greater size and development of the nerve' of sense in animals, the olfactory one may be selected. In man, who has other means of judging of the qualities of his food, and of sur -ounding objects, than by the sense of smell, the olfactory nerve is not me-fourth the size of that of the horse ; in the ox, which is not so ouch domesticated as the horse, and oftener sent into the field to drift for itself, it is considerably larger ; it is larger still in the ovine, which has to search for a portion of its food buried in the earth, or deeply immersed in refuse or filth ; and it is largest of all in the log, whose acuteness of scent renders him so useful a servant to man. The different development of the medulla oblongata in different animals may be adduced as another proof of the admirable adaptation of each to the situation which it occupies and the functions which it discharges. The medulla oblongata is the prolongation and conden sation of the medullary matter of the brain, and it is the origin of that portion of the spinal cord which is devoted to organic life. In the human being, the breadth of it is only a seventh part of that of the brain ; in the horse and the ox, it is nearly a third ; and in tho dog it is more than a half. In every part of the brain of the quadruped the medullary portion preponderates, and the cineritious is deficient.
In the smaller quadrupeds the comparative size of the brain approaches nearer to that of the human being. In the mouse it is a 43rd part of the weight of the animal.
The brain of the larger birds agrees with that of the marnmalia in the smallness of its bulk, compared with the development of the same organ in the human being. The brain of the eagle is not more than a 260th part of the weight of the bird. The brain of the goose is not more than a 360th part. If in some of the lesser birds, as in the chaffinch and the redbreast, it approaches to the proportionate size of that of the human being, it is, as in the smaller quadrupeds, on account of the quantity of medullary matter required for the origins of the nerves ; and the cineritious matter forms only a very small part of the brain. The brain of the bird has no convolutions on its surface ; no cor
pora etriata in the ventricles; no pens varolii between the brain and the spinal cord ; and the origins of the optic nerves are separate from the brain, and lie behind and below it.
In fishes the brain is yet more diminished in proportionate size. In some species it does not constitute a 2000th part of the bulk of the fish. It scarcely half fills the cranial cavity, but is surrounded by a cellular tissue containing a transparent semifluid mass. It singularly varies in different species. It consists of at least four or more rounded eminences, placed iu pairs opposite to each other and forming two parallel lines ; and there is often only a very slight con nection between these lines, or the eminences of which either of them is composed. The two principal hemispheres of the brain and the optic thalami are always present. The olfactory nerves often form a third pair of tubercles anterior to these and the cerebellum, and are always found posteriorly on the mesian line. The optic nerves usually. cross each other without any intermingling of medullary matter. The cineritious substance is found in an exceedingly small proportion in the brain of fishes.
There is no brain properly speaking in the Inrertebrata. In the worm, the upper ganglion of the nervous system, which represents the brain, is placed near to, or may be said to be perforated by, the superior por tion of the oesophagus, and thence proceed little white threads or cords, which run along the course of the digestive canal. In insects the upper ganglion usually surrounds the oesophagus, and a ganglionic system of nerves can generally be traced proceeding from it. In the larvae of insects the brain is inclosed in a horny cavity. The spinal cord proceeding from it pursues its course through the whole of the abdomen, presenting evident ganglia at different points, from which nerves are distributed ; while from the intermediate spaces are given out other nerves without ganglia, presenting a rude but satisfactory sketch of the combined systems of sensitive and motor nerves dis covered by inoderu physiologists.
(Quain, Elements of Anatomy ; Grant, Outlines of Comparative Anatomy ; Carpenter, Principles of Physiology.)