624 
Brain. Body. 
Carp...... as 100: 80,600 
; Chub. .... as 100: 84,200 
Pt) we. a8 100: 52,900 
Lompeey .. as 100 : 142,500 
4 Triton..... as 100: 27,300 
REPritta.. > Turtle. |... as 100 : 454,500 
AVES,.... . Pigeon.... as 100: 9,100 
Sheep..... as 100: 22,600 
Mammatiay Pig. ...... as 100 : 32,300 
Mouse..... as 100: 3,500 
Tn 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 ( Vespertilio murinus ); mouse (Mus 
musculus); rat (Mus rattus); rabbit ( Lepus 
cuniculus, fig. 358); pig (Sus scrofa domes- 
tica); horse ( Equus caballus); ass ( Equus 
asinus ); sheep ( Ovis ammon) ; deer ( Cervus 
dama ) ; mole ( Talpa Europea); stoat ( Mus- 
tela euninea ) ; cat ( Felis 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 drachms, 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 6} 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 
NERVOUS SYSTEM. (Comparative Anatomy.) 
’ 
anteriorly. In both these animals their sur- 
faces are smooth and unconvoluted. The same — 
occurs in the opossum and m cooper ‘ 
dactyla, manger Woe Marsu inte. n the bat — 
they are no longer than wide (2% lines each 
way), leaving the ap gas quadrig em na 
quite exposed ; they are of a triangu orm, 
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 
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 ; 
narrowed anteriorly ; they extend ba 
as quite to cover the tubercula quadrigeming 
and their surfaces are marked with numerou 
convolutions ; the fissures of Sylvius are me 
strongly marked, and the division into lobe: 
more apparent. In the stoat and cat they are 
similarly shaped and convoluted on 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 
+ 
vad 
ara: 
HHL 
| 
RAncaniell > | 
Brain of Callithrix ? (Me Dy 
size, right lateral ventricle exposed. — 
a, First cerebral mass or cerebral i 
elevated and broad, and extending back 
covering the cerebellum. a*, posterior lobe 
rebrum, free from convolutions. 6, corpus 
sum. c¢, cavity of lateral ventricle. d, | 0 
of corpus striatum, e, tenia semicireularis 
third cerebral mass or cerebellum. d*, § 
cord, eal 
rounded, very much elevated, broader mn 
middle, and extend backwards, covering: 
cerebellum. The convolutions are more 
