HISTOLOGY. Ig 
Nervous Tissues. 
Nervous tissue has for its function the correlation of the animal with 
its environment. In order to accomplish this it must provide for the 
recognition of stimuli from without, the inauguration of other impulses 
within itself and the transfer of both to other parts. The essential 
constituent of the tissue is the nerve cell, ganglion cell or neuron, 
to which are added others of a supportive (glia cells) or nutritive 
character. As the parts to be connected by the nervous tissue are often 
remote from each other the neuron is not compact like most other cells, 
but gives off long processes from the central mass, these processes differ- 
ing in their terminations. Some end in places where they can only 
Fic. 12.—Various kinds of nerve cells. A, multipolar cells; B, portion of nerve fibre 
with sheaths; C, unipolar cell; D, pyramidal cell; a, axon; c, collateral; d, dendrites; cb, 
cell body; m, medullary sheath; 2, nucleus of cell of Schwann’s sheath; s, sheath of Schwann; 
t, telodendron. . 
receive stimuli, others where the stimuli can only cause parts to act. 
Thus the processes are physiologically divisible into afferent and 
efferent tracts, the body of the cell being the place for the regulation and 
correlation of the impulses, and apparently in many cells for the inau- 
guration of new impulses. 
A nerve cell (fig. 12) is uni-, bi- or multipolar accordingly as it has 
one, two or more of these processes. In the case of unipolar cells (C) 
the single process sooner or later divides, so that the cell in reality is at 
least bipolar. At the ends the processes may either break up in minute 
twigs (dendrites, ¢) or may end, as in muscles and sense organs, in 
special end organs. The part connecting the efferent termination and 
the central cell body is called the axon (a). Axons and cell bodies are 
gray in color, but usually the axons are surrounded by a medullary 
sheath (m) of a peculiar white substance (myelin) rich in fat, which 
