On cutting into the cerebral hemispheres, cavities are found in their interior, the lateral ventricles. In the bat and rodent animals, as in the rabbit (fig. 358, c), they are of small size, but large in proportion to the hemi spheres; in the pig, sheep, stoat, and cat they are larger and broader, but smaller in propor tion to the hemispheres. In all these animals the anterior and descending cornua are obser vable; the posterior are found only in the mon key (fig. 359, c), where the lateral ventricles quite approach the characters of the same parts in the human adult brain. In the interior of these ventricles are to be observed the corpora striata, twnia (for the first time observable), optic thalami, and fornix. In the bat genus and Rodentia, the corpora striata are very large, forming, indeed, the greater pans of the hemispheres of the brain, and the twnia very narrow (fig. 358, d, e); in the pig, sheep, and cat they are oblong and smooth ; in the mon key they were also oblong (fig. 359 d, e), and though in reality large, appear smaller, when compared with the hemispheres, than in the preceding classes,— which apparent defects of relation Tiedemann considers evidently to depend on the greater augmentation of the hemispheres. The fornix, with its appendages, is for the first time observable in this class of animals, and exists in the brains of all the animals before mentioned ; in the lower orders, its relative size, particularly of the hippocampus major, is somewhat considerable.
From the anterior part of these cerebral hernispheres the olfactory nerves arise, which still possess many points of extreme interest. In the dolphin and other Cetacea, they are entirely wanting. In all the marnmiferous animals before enumerated, except the Oiradru mana, they consist of oblong or rounded me dullary masses, situated on the cribriform plate of the ethinoid bone, from which filaments are given off to be distributed on the pituitary membrane. In the lower orders, as in the bat, rabbit, rit, and mouse, these masses 7anglia of the olfactory nerves are situated on"'a plane directly anterior to the cerebral hemispheres, and may be seen on looking, upon the superior face of the brain, these latter not being yet sufficiently developed anteriorly to cover them ; in the pig they are nearly covered by the hemispheres ; in the horse, ass, sheep, and deer, they are quite covered by them, and are only to be seen on the inferior surface of the brain ; in the cat they are similarly situated, but the anterior edge of the hemispheres pro jects still further beyond them. In all these animals a medullary band or tract ( h ) con nects them with the median lobes of the hemi spheres, and in all they contain cavities (i), which cominunicate with the lateral ventricles. In the monkey the olfactory nerves (processes) consist of free, flattened, medullary bands situ ated on the inferior surface oF the anterior lobes of the brain, precisely the same as in the human adult brain.
2dly. The optic lobes, or second cerebral mass, or, as they are now to be called, the tubercula quadrigemina, consist of an anterior and posterior pair of ganglia, in which cavi ties are no longer perceptible. They differ in size, relatively to each other as well as to the cerebral hemispheres, and in position. In the bat, rabbit (leg. 358, k), rat, and mouse, the anterior pair are the larger, and, compared with the cerebral hemispheres, are very voluminous ; in the pig, horse, ass, sheep, and deer, the anterior pair are also the larger, but they are of less proportional size with the brain ; in the cat and stoat the pos terior pair are the larger ; in the monkey they are nearly of equal size and present less relative volume, thus approaching very much the cha racters of the tubercula quadrigemina in the human adult brain. With regard to their po sition, as before observed, in the lower orders they are situated behind the cerebral hemi spheres and are quite exposed, while in the higher orders they are situated underneath the hemispheres, and quite covered by them, as in the human adult brain.
3dly. The cerebellum, or third cerebral mass (figs. 358, 359, /), is remarkable for its great developement; nevertheless, it passes through many grades in the different orders. In the animals before enumerated It is marked externally by transverse strix and small con volutions and presents a division into me dian and' lateral lobes. The relative size of the mass itself, and of its different parts, and the number of external stri, differ accord ing as the animal examined is high or low in the class. In the bat it is within half a line as long as the cerebral hemispheres, the proportions being as 100 : 125; the lateral lobes are just observable, smooth on their sur face, but on the large median portion there are two transverse striw. In the rabbit (fig. 358, /) its proportional length in the median portion to that of the cerebral hemispheres is as 100 : 207; in the rat, as 100 : 166. The lateral lobes in both are more distinctly developed, and the strim are better marked. In the horse its pro portional length is as 100: 256. In the sheep, as 100 : 232. In the deer, as 100 : 233. The lateral lobes are very evident in all, and con volutions are observable on the external sur face, particularly iu the horse. In the cat its proportional length is as 100 : 200 ; in the stoat as 100 : 228. The external convolutions in both are numerous : in the monkey (fig. 359, I), the proportional leng,th is as 100 : 305 ; the laminx are numerous and small, thus ap proaching very much the characters of the same part in man.