THE NERVOUS SYSTEM. 169 
The cerebellum, or “little brain,” lies behind the cere- 
‘brum, and, like it, presents an external gray layer, with a 
white interior. In Mammals, it is likewise finely convo- 
luted, consisting of gray and white lamine, and is divided 
into two lobes, or hemispheres. In the rest of the Verte- 
brates, the cerebellum is nearly or quite smooth; and in 
the lowest Fishes it is merely a thin plate of nervous mat- 
ter. In many Vertebrates, however, it is larger, compared 
with the cerebrum, than in Man, since in Man the cere- 
brum is extraordinarily developed. 
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 in- 
closed within strands of white fibres, and corresponds to 
the ventral cord in Insects. It usually ends in the lumbar 
region of the vertebral column; but in Fishes it reaches 
to the end of the tail. In Fishes and Reptiles, the cord 
outweighs the brain; in Birds and Mammals, the brain is 
heavier than the cord. In Man, it weighs about an ounce 
and a half. 
The parts of the brain are always in pairs; but in rela- 
tive development and position they differ widely in the 
several classes of Vertebrates. In Fishes and Reptiles, 
they are arranged in a horizontal line; in Birds and Mam- 
mals, they lie on top of each other, till, in Man, the axis of 
the brain is at right angles with the spinal cord. In look- 
ing down upon the brain of a Cod, we see in front a pair 
of olfactory lobes (which send forth the nerves of smell), 
behind them the small cerebral hemispheres, then the 
large optic lobes (in which originate the nerves of sight), 
and, last of all, the thin cerebellum. Not till we reach 
