l nervous system is about
seventeen inches in length and two thirds of an inch in diameter. It does
not extend the entire length of the spinal cavity, as might be supposed,
but terminates at the lower margin of the first lumbar vertebra.(100) It
connects at the upper end with the bulb, and terminates at the lower
extremity in a number of large nerve roots, which are continuous with the
nerves of the hips and legs (Fig. 133). Two deep fissures, one in front
and the other at the back, extend the entire length of the cord, and
separate it into two similar divisions. These are connected, however,
along their entire length by a central band consisting of both gray and
white matter.

[Fig. 133]

Fig. 133--*Spinal cord*, showing on one side the nerves and ganglia with
which it is closely related in function. _A._ Bulb. _B._ Cervical
enlargement. _C._ Lumbar enlargement. _D._ Termination of cord. _E._ Nerve
roots that occupy the spinal cavity below the cord. _P._ Pons. _D.G._
Dorsal root ganglia. _S.G._ Sympathetic ganglia. _N._ Nerve trunks to
upper and lower extremities.

The arrangement of the neurons of the spinal cord is just the reverse of
that in the cerebrum--the center being occupied by a double column of
cell-bodies, which give it a grayish appearance, while the fibers occupy
the outer portion of the cord, giving it a whitish appearance.

The spinal cord is not uniform in thickness, but tapers slightly, though
not uniformly, from the upper toward the lower end. At the places where
the nerves from the arms and legs enter the cord two enlargements are to
be found, the upper being called the _cervical_ and the lower the _lumbar
enlargement_. These, on account of the difference in length between the
cord and the spinal cavity, are above--the lower one considerably above--the
places where the limbs which they supply join the trunk (Fig. 133).

*Arrangement of the Neurons of the Brain and Cord.*--The cell-bodies in the
brain and spinal cord are collected into groups, and their fibers extend
from these groups to places that may be near or remote. Guided by the
white and gray colors of the nervous tissue, and also by the structures
revealed by the microscope, physiologists have made out three general
schemes in the grouping of cell-bodies, as follows:

1. _That of surface distribution_, the cell-bodies forming a thin but
continuous layer over a given sur