NERVOUS SYSTEM. (Comparative Anatomy.) 
receding classes) analogous to the optic tha- 
ini of the brain of man: between them is the 
canal leading to the infundibulum. . 
3d. The cerebellum, or third cerebral mass, 
(fig. 355, ¢), is particularly well developed, 
exhibiting an amazingly increased degree of 
organization when compared with the preced- 
ing classes, and bearing great analogy to the 
cerebellum of the higher animals. In the be- 
fore-mentioned species it consists of a more or 
less rounded median lobe, with very small 
lateral appendages; its external surface is 
marked by transverse sulci, varying in number, 
that extend a short distance into the interior of 
its substance. On cutting into it in the sea- 
gull, fowl, and pigeon, the appearance of the 
“arbor vite is slightly perceptible. In the em- 
o of the chick on the sixteenth day, however, 
the cerebellum is very small, not yet suffici- 
ently developed to separate the optic lobes; 
very slight traces only of grooves were a 
_ parent on its surface (fig. 356, c). On the 
‘twentieth day it presents all the characters of the 
full-grown bird, both as regards relative size, 
‘position, and external strie (fig. 357, c). 
! 
Fig. 358. 
Brain and portion of spinal cord of Lepus cuniculus 
( ook petal size, left hemisphere sliced. 
a, first cerebral mass or cerebral hemispheres, 
ightly grooved on the external surface. 6, corpus 
, short. c, cavity of lateral ventricle. d, 
ogy of corpus striatum, e, tenia semicircu- 
aris. f, hippocampus major, the superior surface 
ES + g» masses or ganglia of the olfactory nerves, 
with their cavities. h, bristle passed, shewing the 
communication between the cavity of the olfactory 
nerve and the lateral ventricle. k, second cere- 
bral mass or tubercula quadrigemina,—the anterior 
gg largest. 1,1, third cerebral mass or cere- 
llum, very much grooved. d*, spinal cord, with 
its posterior longitudinal fissure. 
On reviewing these statements of the ner- 
vous system in the birds, we observe that the 
brain and spinal marrow are no longer situated 
on the same horizontal plane, and that the 
preponderance is now in favour of the brain: 
623 
its weight, too, when compared with the body, 
is greater; and the ganglia composing\t are 
more above, and less behind each other. The 
axiigdd cerebral mass has now acquired so 
igh a degree of developement as to surpass 
the others in size; no convolutions are, how- 
ever, yet apparent on its surface; no large 
commissure yet exists to unite them. The 
optic lobes, or median cerebral mass, are small, 
separated from each other, and their cavities 
have decreased. The cerebellum, or third 
cerebral mass, is large; traces of lateral lobes 
are evident, and external striz are perceptible. 
Mammatia.—lIn the last and highest class, 
the Mammalia, will be found some most in- 
teresting grades of developement and structural 
forms of the cerebral mass to arrest our atten- 
tion, and we shall observe how rapidly the 
different parts are added, and those already 
formed are more highly developed, to consti- 
tute the complex brain of the human species. 
The spinal cord (figs. 358, 321, 322, 323, d*) 
is of still less relative size than in the pre- 
ceding classes; it has an anterior and pos- 
terior longitudinal fissure. In a full-grown 
mouse, weighing 227 grains, the spinal 
marrow weighed one grain and a half, the 
brain six grains and a half—the proportions 
being as 100:22. We thus observe that the 
former is of much less relative size than the 
latter. 
The following is a table shewing the relative 
proportions of the brain and spinal marrow in 
the four classes of Vertebrata :— , 
Spinal 
Brain. Marrow. 
Lamprey.... as 100 : 750 
Triton...... as 100 : 180 
Pigeon..... as 100 : 30 
Mouse. ..6. a8 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 vertebra, 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.+ 
The following is a table shewing the relative 
proportions of the body and brain in the four 
classes of Vertebrata ;— 
Pisces .... 
Reptivia.. 
AVES .is\s.5' 
MamMAtia 
* Meckel (Archiv. fiir Physiologie) also states, 
that in the hedgehog the spinal cord terminated in 
the thoracic vertebra. 
+ Carus remarks the same thing in many of the 
Mammalia with a short neck; the Rodentia, for 
instance. 
