for demonstrating the brain (dissectible).

A prepared nervous system of a frog (such as may be obtained from supply
houses) should also be examined. Observe the appearance and general
distribution of the nerves and their connection with the brain and spinal
cord. If such a preparation is not at hand, some small animal may be
dissected to show the main divisions of the nervous system, as follows:

*Dissection of the Nervous System* (by the teacher).--For this purpose a
half-grown cat is generally the best available material. This should be
killed with chloroform and secured to a board as in the dissection of the
abdomen (page 169). Open the abdominal cavity and remove the contents,
tying the alimentary canal where it is cut, and washing out any blood
which may escape. Dissect for the nervous system in the following order:

1. Cut away the front of the chest, exposing the heart and lungs. Find on
each side of the heart a nerve which passes by the side of the pericardium
to the diaphragm. These nerves assist in controlling respiration and are
called the _phrenic_ nerves. Find other nerves going to different parts of
the thorax.

2. Remove the heart and lungs. Find in the back part of the thoracic
cavity, on each side of the spinal column, a number of small "knots" of
nervous matter joined together by a single nerve. These are sympathetic
ganglia. Where the neck joins the thorax, find two sympathetic ganglia
much larger than the others.

3. Cut away the skin from the shoulder and upper side of the fore leg. By
separating the muscles and connective tissue where the leg joins the
thorax, find several nerves of considerable size. These connect with each
other, forming a network called the _brachial plexus_. From here nerves
pass to the thorax and to the fore leg.

4. From the brachial plexus trace out the nerves which pass to different
parts of the fore leg. In doing this separate the muscles with the fingers
and use the knife only where it is necessary to expose the nerves. Note
that some of the branches pass into the muscles, while others connect with
the skin.

5. Remove the skin from the upper portion of one of the hind legs and
separate the muscles carefully until a large nerve is found. This is one
of the divisions of the _sciatic_ nerve. Carefully trace it to the spinal
cord, cutting away the bone where necessary, and find the connections of
its branches with the cord. Then trace it toward the foot, discovering