NERVOUS SYSTEM. (('OMPAKATIVI: ANATOMY.) 



rous than in the preceding classes ; the fissure 

 of Sylvius is a deep groove, marking the divi- 

 sion into anterior and median lobes, and here, 

 for the first time, are observed the posterior 

 lobes (a*), as yet but of small size, narrowed 

 posteriorly, and free from convolutions. In 

 the orang-outang they are altogether larger, and 

 more approaching the form and character of 

 the human bruin, covering the cerebellum en- 

 tirely, and convoluted on their posterior lobes.* 

 These cerebral hemispheres are united by an 

 important commissure, which makes its first 

 appearance in mammiferious animals, the cor- 

 pus cu/loitum ; in the lower orders, as in the 

 bat, rabbit (Jig. 358, b), rat, and mouse, it 

 is very short, shorter even than the tubercula 

 quadrigemina ; in the pig, ass, and sheep, it is 

 longer and broader; in the stoat, cat, and 

 monkey (jig. 359, b) it is increased in length 

 and width, approaching the characters of the 

 corpus callosum in the human adult brain. 



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 (Jig. 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, taenia (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 parts of the 

 hemispheres of the brain, and the taenia very 

 narrow (Jig. 358, d, e) ; in the pig, sheep, and 

 cat they are oblong and smooth ; in the mon- 

 key they were also oblong (Jig. 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 

 hemispheres 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 mammiferous 

 animals before enumerated, except the Quadru- 

 mana, they consist of oblong or rounded me- 

 dullary masses, situated on the cribriform plate 

 of the ethmoid bone, from which filaments are 

 given oft" to be distributed on the pituitary 

 membrane. In the lower orders, as in the bat, 



' For the length, by measurement, of the cere- 

 bral hemispheres in these different animals, see 

 the table. 



VOL. in. 



625 



rabbit, r.it, and mouse, these masses or ganglia 

 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 communicate 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 (Jig. 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 (Jigs. 358, 359, I), 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 striae 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 striae, 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 striae. In the rabbit (Jig. 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 

 striae are better marked. In the horse its pro- 



2 s 



