210 ELEMENTS OF MAMMALIAN ANATOMY. 



nervous system or in some other part of the body (Figs. 

 97, 98). 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 to- 

 ward the cell, while the axis-cylinder conducts them 

 from the cell. 



The sensory fiber r (Fig. 98) 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 axis-cylinder, 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 m, whose axis- 

 cylinder 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 consciousness, and is then known as reflex action. 

 If, however, the impulse travels along the fiber ir, and 

 thence through the fiber s to the brain, whence an im- 

 pulse descends through the fiber t' , is, the process is known 

 as voluntary reaction. 



THE FIBER TRACTS OF THE CENTRAL NERVOUS 



SYSTEM. 



As before stated, the white matter of each half of the 

 cord is divided by the exit and entrance of the nerve 

 roots into three columns anterior, lateral, and posterior. 

 Each of these columns is subdivided into tracts which 

 have special names and special functions (Fig. 99). 



