ement of the nervous tissue. _A._ Brain. _B._ Spinal
cord. _N._ Nerve trunks and nerves. _G._ Ganglia.

It is through these structures--brain and spinal cord, nerve trunks and
nerves, ganglia and nerve terminations--that connections are established
between all parts of the body, but more especially between the surface of
the body and the organs within.

*The Neurons, or Nerve Cells.*--While a hasty examination of the nerve
skeleton is sufficient to show the connection of the nervous system with
all parts of the body, no amount of study of its gross structures reveals
the nature of its connections or suggests its method of operation. Insight
into the real nature of the nervous system is obtained only through a
study of its minute structural elements. These, instead of being called
cells, as in the case of the other tissues, are called _neurons_. The use
of this term, instead of the simpler one of nerve cell, is the result of
recent advances in our knowledge of the nervous system.(96)

[Fig. 126]

Fig. 126--*Diagram of a mon-axonic neuron* (greatly enlarged except as to
length). The central thread in the axon is the axis cylinder.

The neurons are in all respects cells. They differ widely, however, from
all the other cells of the body and are, in some respects, the most
remarkable of all cells. They are characterized by minute extensions, or
prolongations, which in some instances extend to great distances. Though
the neurons in certain parts of the body differ greatly in form and size
from those in other parts of the body, most of them may be included in one
or the other of two classes, known as _mon-axonic_ neurons and _di-axonic_
neurons.

*Mon-axonic Neurons.*--Neurons of this class consist of three distinct
parts, known as the cell-body, the dendrites, and the axon (Fig. 126).

The _cell-body_ has in itself the form of a complete cell and was at one
time so described. It consists of a rounded mass of protoplasm, containing
a well-defined nucleus. The protoplasm is similar to that of other cells,
but is characterized by the presence of many small granules and has a
slightly grayish color.

The _dendrites_ are short extensions from the cell-body. They branch
somewhat as the roots of a tree and form in many instances a complex
network of tiny rootlets. Their protoplasm, like that of the cell-body, is
more or less granular. The dendrites increase greatly the