230 ELEMENTS OF MAMMALIAN ANATOMY 



show that the ventral root fibers are, for the most part, the 

 axones of the cells in the ventral horn (Fig. 113). 



Each dorsal root fiber after passing into the cord separates 

 into two parts one of which extends craniad, the other caudad 

 (Fig. 113). 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 den- 

 drites and an axis-cylinder process or axone (Fig. 112). The 

 dendrites, except in the gangha 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 terminates freely either within 

 the central nervous system or in some other part of the body 

 (Figs. 112, 113). 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 den- 

 drites conduct impulses toward the cell, while the axone con- 

 ducts them from the cell. 



The sensory fiber r (Fig. 113), 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, x, 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 collateral, w. By 

 the latter route it will stimulate the cell w, whose axone termi- 

 nates in the foreleg muscles, which are thereby made to con- 

 tract and pull the paw away from the irritating object. This 

 process may take place without consciousness, and is then 



