172 
COMPARATIVE ZOOLOGY. 
The medulla oblongata is the connecting link between 
the cerebrum and cerebellum and the spinal cord. In 
structure, it resembles the spinal cord—the white matter 
being external and the gray internal. The former lies 
beneath or behind the brain, passing through the foramen 
magnum of the skull, and merging imperceptibly into the 
cord. The latter is a continuous tract of gray matter en¬ 
closed within strands of white fibres. It usually ends in 
the lumbar region of the vertebral column, but in Fishes 
it reaches to the end of the tail. In Fishes, Amphibians, 
and Reptiles, the cord outweighs the brain: in Birds and 
Mammals, the brain is heavier than the cord. In Man, 
the cord weighs about an ounce and a half. 
Besides these parts, there are also the olfactory and the 
optic lobes, which give rise respectively to the nerves of 
smell and sight. 
The parts of the brain are always in pairs; but in rela¬ 
tive development and po¬ 
sition they differ widely in 
the several classes of Ver¬ 
tebrates. In Fishes and 
Reptiles, they are arranged 
in a horizontal line; in 
Birds and Mammals, the 
axis of the spinal cord 
bends to nearly a right an- 
P, , 9 hePe7ch!'t n ^ S le in passing through the 
view: a, cerebei- brain, so that the lobes no 
lum; b, optic . . 
lobes; c, cere- longer Jie in a straight line. 
In Man, the fore-brain is Fig. ho.— 
brum; i, olfacto¬ 
ry lobes; g, me¬ 
dulla oblongata. g0 developed that it cov. 
ers all the other lobes. In looking down 
upon the brain of a Perch, we see in 
front a pair of olfactory lobes (which 
send forth the nerves of smell), behind 
Brain of the 
Frog, upper view, x 4: 
/.olfactorynerves; Lol, 
olfactory lobes; He, cer¬ 
ebral hemispheres; Pn, 
pineal gland ; Fho and 
Srh, third and fourth 
ventricles; Lop, optic 
lobes; C, cerebellum; 
Mo, medulla oblongata. 
