19 8 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 ax one (Fig. 

 101). 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, 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 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- 



