lse passes from the nerves of touch in the fingers to
the brain, which at once hurries off its order along another set of nerves
for the hand to drop the burning object. These examples, so common in
daily life, may be multiplied to any extent. Almost every voluntary act we
perform is executed under the direction of the nervous system, although
the time occupied is so small that it is beyond our power to estimate it.
The very frequency with which the nerves act tends to make us forget their
beneficent work.

265. Divisions of the Nervous System. This system in man consists of
two great divisions. The first is the great nerve center of the body, the
cerebro-spinal system, which rules the organs of animal life. This
includes the brain, the spinal cord, and the cerebro-spinal
nerves. Nerves are given off from the brain and the cord, and form the
mediums of communication between the external parts of the body, the
muscles or the sense organs, and the brain.

The second part is the sympathetic system, which regulates the
organic life. This consists of numerous small nerve centers arranged in
oval masses varying greatly in size, called ganglia or knots. These
are either scattered irregularly through the body, or arranged in a double
chain of knots lying on the front of the spine, within the chest and
abdomen. From this chain large numbers of nerves are given off, which end
chiefly in the organs of digestion, circulation, and respiration. The
sympathetic system serves to bring all portions of the animal economy into
direct sympathy with one another.

266. The Brain as a Whole. The brain is the seat of the
intellect, the will, the affections, the emotions, the memory, and
sensation. It has also many other and complex functions. In it are
established many reflex, automatic, and coordinating centers, which are as
independent of consciousness as are those of the spinal cord.

The brain is the largest and most complex mass of nerve tissue in the
body, made up of an enormous collection of gray cells and nerve fibers.
This organ consists of a vast number of distinct ganglia, or separate
masses of nerve matter, each capable of performing separate functions, but
united through the cerebral action into a harmonious whole.

[Illustration: Fig. 114.--The Upper Surface of the Cerebrum. (Showing its
division into two hemispheres, and also the convolutions)]

The average weight of the adult human brain is about 50 ounces for men and
45