THE NERVE FIBER 



139 



The entire nervous 

 system may be considered 

 as an enormous tangle, 

 formed by the interlacing 

 processes of an innumer- 

 able number of neurons 

 whose complex fiber paths 

 place all portions of the 

 body in communication 

 with all other portions. 



Nerve cells are un- 

 equally distributed 

 throughout the central 

 division of the nervous 

 system ; they therefore oc- 

 cur in more or less dis- 

 tinct groups or nuclei, 

 from each cell of which 

 an axon is frequently dis- 

 tributed along the same 

 path. The larger bundles 

 thus formed are called 

 tracts; the smaller ones, 

 funiculi, fasciculi, or fiber 

 bundles. 



Since each fiber of 

 such a tract is dependent 

 for nutrition upon the 

 nerve cell from which it 

 arises, the tract as a 

 whole must depend upon 

 its nucleus of origin for 

 its nutrition. Each nu- 

 cleus therefore becomes 

 the trophic center for the 

 fiber tract to which it 

 gives origin. 



It may be readily 

 demonstrated that if any 

 such group of axons be 

 cut or otherwise separated 

 from its trophic center, 

 that tract will promptly 



X M.tt. 



t 



2 



,C 1 



FIG. 152. REGENERATIVE STAGES IN THE PROXIMAL 

 STUMP OF THE CUT SCIATIC NERVE OF THE DOG, 

 SEVERAL MILLIMETERS ABOVE THE LEVEL OF 



SECTION. 



p, toward the periphery; c, toward the center. 

 (1) on the nineteenth day after section; a, point in 

 the old medullated axon from which arises an 

 extremely short branch which at once divides 

 into two. (2) on the twenty-fifth day. (3) five 

 protoplasmic strands down which a new axon 

 is growing. (Ranson.) (Pyridin-silver prepara- 

 tions.) 



