throughout the cell body and larger dendrites, but never in the
axone or the axone hillock. These subdivisions are shown in
Fig. 22. In the embryonic or developing nervous system the cell
body which is called a neuroblast,
first appears. If one were to watch
the neuroblast develop into a
partially complete neurone (as has
actually been done), he would see
the axone hillock first form and from
it would develop the slender axone
process, and later the dendrites.

The axone, ax, Fig. 21, is a
slender outgrowth from the cell body
varying in length from a fraction of
a mm. to more than a meter. Under
the high power microscope it is seen
to be made up of elementary neuro-
fibrils. There is usually only one
axone to a cell body. It is to be
distinguished from the dendrites by
its straighter course, its uniform
diameter and smooth outline, Fig.
24. Shortly after leaving the cell
day the axone may become myeli-
nated, i.e., covered by a fatty sheath
whose function may be insulating
or nutritive. It may possibly also
be concerned with conduction. In
addition to the medullary sheath,
many of the axones are covered also
with the more primitive sheath of
Schwann or neurilemma. The sheath
of Schwann is probably not found
in the central nervous system. Cer-
tain axones, for example, those of
the sympathetic neurones and of the

FIG. 21.--Scheme of peripheral
motor neurone. The cell body,
dendrites, axone, collaterals, and
terminal arborisations in the muscle
are seen to be parts of a single cell
the neurone. c, cytoplasm of cell
body containing chromophilic bodies,
neurofibrils and perifibrillar sub-
stance; n, nucleus; n', nucleolus:
d, dendrites; ah, axone hillock, free
from chromophilic bodies; ax, axone;
sf, collateral; m, medullary sheath;
nR, node of Ravier where branch
is given off: sl, neurilemma (not
present in central nervous system);
m', striated muscle fiber; tel, motor
end plate. (From Bailey's Text-Book
of Histology.) Courtesy Wm. Wood
& Co.

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