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 surface. This is the plan in the cerebrum
and cerebellum, and here are found devices for increasing the surface: the
cerebrum having convolutions, the cerebellum transverse ridges.
2. _That of collections of cell-bodies into rounded masses._ Such masses
are found in the bulb, the pons, the midbrain, and the base of the
cerebrum.
3. _That of arrangement in a continuous column._ This is the plan in the
spinal cord. It matters not at what place the spinal cord be cut, a
central area of gray matter, resembling in form the capital letter H, is
always found.
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