166 COMPARATIVE ZOOLOGY. 
are distinguished by an irregularly scattered nervous sys- 
tem. They have two or more ganglia around the gullet, 
and one or two more in the posterior region; all are 
united by threads, and send off nerves to the various or- 
gans. The articulated animals generally have a double 
nervous cord lying along the ventral side, and studded 
with ganglia of nearly uniform size, except the first, which 
is the largest of all, and represents the brain. In the sim- 
ple Earth-worm there is no trace of ganglia; in the Centi- 
pede and Caterpillar there is a ganglion for each segment ; 
but in the higher forms, as the Bee, several ganglia are 
fused together in the head and thorax, indicating a con- 
centration of organs for sensation and locomotion. 
In Vertebrates, the nervous system is more highly de- 
veloped, more complex, and more concentrated than in 
the lower forms. In fact, there are some parts, as the 
brain, to which we find nothing analogous in the Inverte- 
brates; and while the actions of the latter are mainly, if 
not wholly, automatic, those of backboned animals are 
voluntary. Its position, moreover, is peculiar, the great 
mass of the nervous matter being accumulated on the 
dorsal side, and inclosed by the neural arches of the skele- 
ton. 
The brain and spinal cord lie in the cavity of the skull 
and spinal column, wrapped in three membranes. Both 
consist of gray and white nervous matter; but in the brain 
the gray is on the outside, and the white within; while the 
white of the spinal cord is external, and the gray internal. 
Both are double, a deep fissure running from the forehead 
backward, dividing the brain into two hemispheres, and 
the spinal cord resembling two columns welded together ; 
even the nerves come forth in pairs to the right and left. 
So that a person may be said to consist of two individuals 
acting simultaneously. If the two halves of the brain do 
not act in concert, the man is said to be insane; if one 
