All the simize, says Siknmerring, are placed far behind man in this respect. Although the brain in some instances, particularly among the smaller kinds, which have prehensile tails, is larger in proportion to their body than that of the human subject ; yet a very large share of that brain is required for the immense nerves which supply their organs of sense and mastication. Let us remove that por tion of the brain, and a very small quan tity will remain.
The researches of the same author on animals in general have led him to con clude, that the quantity of brain, over and above that which is necessary for a mere animal existence ; that part, in short, which is devoted to the faculties of the mind, bears a direct ratio to the docility of the animal, to the rank which it would hold in a comparative scale of mental powers.
The largest brain, which Sonimerring has found in a horse, weighed 1112. 4oz. and the smallest, which he has seen in an adult man; was 21b. 5ioz. Yet the nerves arising from the former brain were at least ten times larger than those of the latter.
Generally speaking, small animals have a larger brain in proportion to their body than larger ones. The pachydermata have it very small ; and in red-blooded animals, its size is very trifling when compared with the body.
It forms in man from Q to of the body.
In some simix - - the Mole - - - - - 74 Bear - - - - - via Dog 167 Cat - - - - Hare- - - - Rat - - - - - Mouse - - - Elephant - - - Tt--u Pig - Horse Dolphin - Eagle - - - - Sparrow - - - Chaffinch - Redbreast - - Blackbird - - A Canary bird - - Cock Duck - - - - 7+7 Goose - - - - .sh Tortoise - - -are, Turtle - - - TAI-r Coluber natrix - Frog - Shark - - - Pike - i o3 Carp -gwo Many mammalia possess a bony tento rium cerebelli. It is difficult to give a physiological explanation of the use of this bony tentorium. The opinion which has been generally adopted by anatomists, that the structure in question belongs to such animals only as jump far, or run with great velocity, and that it serves the purpose of protecting the cerebellum from the pressure of the cerebrum in these quick motions, is obviously unsatisfactory. It exists in the bear, which is not dis tinguished for its activity ; while several animals which excel in jumping or spring ing do not possess it ; viz. the wild goat, (capra ibex.) Cheselden ascribes it to predaceous animals only, (" Anat. of the Bones," cap. 8;) but it exists in several others.
We have given these remarks on the generally assigned use of the bony tento rium, because a similar mechanical ex planation has been assigned of the falx, and the tentorium of the human subject ; viz. that the former protects the hemi spheres from mutual pressure, when the person lies with his head resting on one side ; and that the latter provides against the compression of the cerebellum by the superincumbent cerebrum. These ex planations are assigned in the present day by anatomists of such distinguished reputation as Sommerring and Cuvier (" de Corporis Humani Fabrics," vol. 4, p. 27. " Ldcons d'Anat. compar." tom. 2, p. 178.) If the futility of this piece of physiology were not sufficiently proved, by considering that the cranium is accu rately filled, and that there is conse quently no room for its contents to fall from one side to the other, it must im mediately be rendered manifest by Mr. Carlisle's case ; in which the falx was entirely absent, and the two hemispheres united throughout in one mass, without any perceptible inconvenience during the patient's life. (" Transactions of a So ciety for the Improvement of Medical and Chirurgical knowledge," vol. ii. p. 212.) ‘Ve have met with four instances, in which the anterior half of the falx was deficient. This production of the dues mater commenced in a narrow form about the middle of the sagittal suture ; and gradually expanding, had acquired the usual breadth at its termination in the tentorium. The two hemispheres ad hered by the pia mater covering their opposed plane surfaces, but were form ed naturally in other respects. A want of the falx has also been recorded by Garengeot, (" Splauchnologie," tom. ii. p. 24.) The brain of the mammalia wants the digital cavity of the lateral ventricle, and in general the acervulus of the pineal gland. Its anterior lobes are elongated into a process called the mammillary, giv ing rise to the olfactory nerves. In birds, reptiles, and fishes, there is a successive and gradual change towards a more simple structure ; the brain in these classes consisting merely of tubercular eminences. In the lower orders the brain seems to be really wanting. A nervous chord runs along the body, and possesses ganglia at different distances, from which the nerves arise. In insects and vermes the upper ganglion of the ner vous chord, which represents the brain, is placed near the mouth, or cesopliagus, and very generally surrounds that tube by a kind of collar.