s in
sight, smell, taste, or hearing,--every instinct, every act of the will,
and every thought are possible only through the action of the nerve
centers.

261. Nerve Cells. However complicated the structure of nerve tissue
in man seems to be, it is found to consist of only two different elements,
nerve cells and nerve fibers. These are associated and combined
in many ways. They are arranged in distinct masses called nerve
centers, or in the form of cords known as nerves. The former are
made up of nerve fibers; the latter of both cells and fibers.

[Illustration: Fig. 110. Nerve Cells from the Spinal Cord.]

Nerve cells, which may be regarded as the central organs of the nerve
fibers, consist of masses of cell protoplasm, with a large _nucleus_ and
_nucleolus_. They bear a general resemblance to other cells, but vary much
in size and shape. Nerve cells grow, become active, and die, as do other
cells. A number of processes branch off from them, some cells giving one
or two, others many. The various kinds of nerve cells differ much in the
shape and number of processes. One of the processes is a strand which
becomes continuous with the axis cylinder of the nerve fibers; that is,
the axis cylinders of all nerve fibers are joined in one place or another
with at least one cell.

Each part of this system has its own characteristic cell. Thus we have in
the spinal cord the large, irregular cells with many processes, and in the
brain proper the three-sided cells with a process jutting out from each
corner. So characteristic are these forms of cells, that any particular
part of nerve structure may be identified by the kind of cells seen under
the microscope. Nerve cells and nerve fibers are often arranged in
groups, the various cells of the groups communicating with one another.
This clustered arrangement is called a nerve center.

262. Nerve Fibers. The nerve fibers, the essential elements of
the nerves, somewhat resemble tubes filled with a clear, jelly-like
substance. They consist of a rod, or central core, continuous throughout
the whole length of the nerve, called the axis cylinder. This core is
surrounded by the white substance of Schwann, or medullary sheath, which
gives the nerve its characteristic ivory-white appearance. The whole is
enclosed in a thin, delicate sheath, known as neurilemma.

[Illustration: Fig. 111.--Nerve Cells from the Gray Matter of the Brain.]

The axis cylinder generally passes without any br