itting and in
standing, in the position of the hands for different kinds of work, in the
variations of the circulation of the blood in the skin, and in the
movements for protecting the body.(95)

*Work of the Nervous System.*--How are the different activities of the body
controlled and cooerdinated? How is the body adjusted to its surroundings?
The answer is found in the study of the nervous system. Briefly speaking,
the nervous system controls, cooerdinates, and adjusts the different parts
of the body by fulfilling two conditions:

1. It provides a complete system of connections throughout the body,
thereby bringing all parts into communication.

2. It supplies a means of controlling action (the so-called impulse) which
it passes along the nervous connections from one part of the body to
another.

The present chapter deals with the first of these conditions; the chapter
following, with the second.

*The Nerve Skeleton.*--If all the other tissues are removed, leaving only
the nervous tissue, a complete skeleton outline of the body still remains.
This nerve skeleton, as it has been called, has the general form of the
framework of bones, but differs from it greatly in the fineness of its
structures and the extent to which it represents every portion of the
body. An examination of a nerve skeleton, or a diagram of one (Fig. 125),
shows the main structures of the nervous system and their connection with
the different parts of the body.

Corresponding to the skull and the spinal column is a central nervous
axis, made up of two parts, the _brain_ and the _spinal cord_. From this
central axis white, cord-like bodies emerge and pass to different parts of
the body. These are called _nerve trunks_, and the smaller branches into
which they divide are called _nerves_. The nerves also undergo division
until they terminate as fine thread-like structures in all parts of the
body. The distribution of nerve terminations, however, is not uniform, as
might be supposed, but the skin and important organs like the heart,
stomach, and muscles are the more abundantly supplied. On many of the
nerves are small rounded masses, called _ganglia_, and from many of these
small nerves also emerge. At certain places the nerves and ganglia are so
numerous as to form a kind of network, known as a _plexus_.

[Fig. 125]

Fig. 125--*Diagram of nerve skeleton.* The illustration shows the extent
and general arrang