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THE ANIMAL SYSTEM. 123
artificial instrument; the artist, who flings the expres
sion of beauty and passion upon the canvas, and fastens
it there for centuries.
The apparatus by which these effects are produced is,
of necessity, complicated, and it is also obscure, both in
its structure and its action. The nerves of motion which
govern the movements of the muscles of the face, eyes,
tongue, etc., pass through several openings in the base
of the cranium ; but they probably have their origin in
the upper part of the spinal cord, or medulla oblongata;
and are connected with all parts of the cerebrum and
cerebellum. The nerves of motion, that, supply the
trunk and extremities, are given off" in pairs from the
spinal cord, and seem to have in that continuation of the
brain nervous centers which supply them with the
power of action, though their movements are usually
under the control of the will. This, however, is not
always the case. There are involuntary actions, both
constant and occasional, which seem to center in the
grey matter of the spinal cord. Respiration, performed
both by the muscles of the chest, and the diaphragm and
abdominal muscles, goes on from the beginning to the
end of life. It may be controlled by the will, but does
not. depend upon it. The sphincters of the bladder and
rectum are also in constant action. The muscles of the
pharynx, employed in swallowing, act the moment any
substance passes the fauces. The muscles which aid in
the erection of the penis, and which produce the forci
ble ejaculation of the semen, act in a similar manner.
So of the acts of coughing, sneezing, and, to some
extent, that of laughing. Many of these acts are per
formed when the brain is quiet in sleep, or in a state of