198 ELEMENTS OF MAMMALIAN ANATOMY. 
Each posterior root fiber after passing into the cord sepa- 
rates into two parts one of which extends craniad, the other 
caudad (Fig. 102). Both give off branches at right angles, 
called collaterals, which terminate in arborizations about the 
cells of the cord. 
Each nerve cell with all its processes is called a neurone. 
It presents two kinds of processes, protoplasmic processes 
or dendrites and an axis-cylinder process or axone (Fig. 
tor). The dendrites, except in the ganglia outside of the 
central nervous system, are usually several in number and 
comparatively short, while there is but one axone from each 
cell, which may be more than a foot in length. A number 
of axones (nerve fibers), each of which is surrounded by a 
sheath, the neurilemma, constitutes a nerve bundle or nerve. 
Every axone or nerve fiber originates in a cell, but termi- 
nates freely either within the central nervous system or 
in some other part of the body (Figs. 101, 102). Within 
the central nervous system a fiber usually ends in an 
arborescence which may be contiguous but not continuous 
with the dendrites of another cell. The nerve processes 
have the power of conducting impulses whether derived 
from the cell itself or an external stimulus. The dendrites 
conduct impulses toward the cell, while the axone conducts 
them from the cell. 
The sensory fiber r (Fig. 102) leads from the dermis of 
the cat’s paw. A pin-prick in the paw causes an impulse 
to be transmitted along the fiber to the cell g, and thence by 
its axone, +, to the point v within the cord where the fiber 
splits. From the point v the impulse will proceed both 
through the ascending portion of the fiber, ir, and the col- 
lateral, w. By the latter route it will stimulate the cell m, 
whose axone terminates in the foreleg muscles, which are 
thereby made to contract and pull the paw away from the 
irritating object. This process may take place without con- 
