AVES Pigeon . as 100 : 30 MAMMALIA Mouse . .... as 100 : 22 The spinal cord passes lower down the ver tebral column than in man, but terminates by a true cauda equina, as in the bat and mouse, in which latter animal it is continued into the sacrum, but not into the caudal vertebrx, as in the preceding classes. In the bat the spinal cord descends no lower than the eleventh dorsal vertebra, a conformation rather un usual :* the fissure on its posterior surface was deep in those animals, but it becomes less evi dent as we approach the human species. It presents three distinct enlargements in its course : a superior one, the medulla oblongata; a median, and a posterior one, where the nerves for the extremities are given off: this is the case in the mouse and bat, though in the for mer animal the superior and median enlarge ments are so closely approximated as to render the spinal cord of great thickness in the tho racic region of the body.f The following is a table sliewing the relative proportions of the body and brain in the four classes of Vertebrata:— On reviewing these statements of the ner vous system in the birds, we observe that the brain and spinal marrow are no lorrrer situated on the same horizontal plane, and that the prepondemnce is now in favour of the brain: In the brain of the Mammalia we shall find the same parts as heretofore to occupy our attention, though at an extraordinarily in creased degree of developement: this, however, varying greatly in the different orders. Its direction, with regard to the spinal marrow, is no longer horizontal, as we found in the fishes and reptiles, but approaching more or less to a right angle ; the first traces of which inflection were perceptible in the birds. In the bat ( Vespertstio murinus); mouse (Mus mascutus); rat ( rattus); rabbit ( Lepus cuniculus, fig. 358); pig (Sus scrofia domes tica); horse ( Equus caballus); ass (Equus minus); sheep (Ovis ammon) ; deer (Cervus dam); mole ( Talpa Europrea); stoat plus tele euninea); cat (Fells catus); and monkey ( Callithrix —? fig. 359); the brain exactly fills the cranial cavity, that cavity correspond ing with the shape and size of the head. The size and bulk of the brain are greater than in any of the preceding classes, as shown by its relative weight compared with the body. In a sheep weighing, as near as could be calcu lated, 7466 drachtns, the brain weighed 33 drachms; .the proportion of the brain to the body being as 100:22600. In a pig weighing about 7116 drachms, the brain weighed 22 drachms; the proportion being as 100:32350. The brain of a horse weighed 156 drachms. In a mouse weighing 327 grains, the brain weighed 61 grains, the proportions being as 100: 3,500.
On taking a review of the structure of the brain in Mammalia, we find that it presents a great variety of form and developement in its different parts. 1. The cerebral hemispheres, or first cerebral mass, which vary greatly in their size and extent, and are united in the median line by a commissure, the corpus cal losum. 2. The optic lobes, or second cerebral mass, which are here small and divided into two pairs, presenting more particularly the characters of the tubercula quadrigemina in the human brain, under which name they will in future be noticed. 3. The cerebellum, or
third cerebral mass, which is greatly increased in developement, and presents a division into median and lateral portions.
1st. The cerebral hemispheres, or first ce rebral mass (figs. 357, 353, a) are of large size, but this varies according to the order in which they are examined. In the lower ones they resemble very much the same parts in birds, with regard 'to their small size and their want of convolutions. In the dol phin they are very short and broad ; in the ornithorynchus they are oval, and narrowed anteriorly. In both these animals their sur faces are smooth and unconvoluted. The same occurs in the opossum and myrmecophaga di dactyla, amongst the Marsupialia. In the bat they are no longer than wide (2f lines each way), leaving the tubercula quadrigemina quite exposed ; they are of a triangular form, and perfectly smooth on their surface. In the rabbit (fig. 358, a), rat, and mouse, rodent animals, they are oblong ovate, but much nar rowed anteriorly. The tubercula quadrigemina are quite exposed, but scarcely so much so as in the bat ; their surfaces are smooth and uncon voluted, though in the rabbit there are a few slight furrows; on their inferior surface there is a faint groove, dividing them into lobes, the rudiments of the fissura Sylvii. In the pig, horse, ass, sheep, and deer, the hemispheres are more oval in form, more convex, and less narrowed anteriorly ; they extend backwards, so as quite to cover the tubercula quadrigemina, and their surfaces are marked with numerous convolutions ; the fissures of Sylvius are more strongly marked, and the division into lobes is more apparent. In the stoat and cat they are similarly shaped and eonvolutedon their surface, and they extend backwards, covering the tuber cula quadrigemina and a portion of the cerebel lum. In the monkey (fig.359, a) they are more rounded, very much elevated, broader m the middle, and extend backwards, covering the cerebellum. The convolutions are more flume rous than in the preceding classes ; tlie fissure of Sylvius is a deep groove, marking the divi sion into anterior and median lobes, and here, for the tirst time, are observed the posterior lobes (0), 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 brain, covering the cerebellum en tirely, and convoluted on their posterior lobes.* These cerebral hemispheres are united by an important commissure, which rnakes its first appearance in mammiferious animals, the cor pus callosum ; in the lower orders, as in the bat, rabbit (fig 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 (fig. 359, b) it is increased in length and width, approaching the characters of the corpus callosum in the human adult brain.