i6o 



NERVOUS SYSTEM OF VERTEBRATES. 



lobes many of them divide into terminal branches immediately, 

 and in many fishes the visceral lobe is strongly thickened opposite 

 each nerve root so that it appears somewhat Hke a string of beads 

 (Figs. 2, 3, ii). The bifurcation of the fibers and the formation 

 of a longitudinal tract are not such prominent features in the 

 visceral system as in the cutaneous. A part of the fibers, however, 

 do turn caudad and in fishes form a diffuse bundle which extends 

 to the nucleus commissurahs, where a part of the fibers cross 



Fig. 8i. — Four transverse sections through the medulla oblongata of the frog to 

 show the position and ending of the fasciculus communis and the nucleus commis- 

 surahs. C. inf. H., commissura infima; nuc. of C, nucleus commissurahs; /. c, 

 fasciculus communis; d. h., dorsal horn. 



to the opposite side in the commissura infima. In amphibia 

 (Fig. 8i) these fibers form a well defined bundle which on account 

 of its relation to the three nerves, VII, IX and X has been given 

 the name fasciculus communis. In terrestrial vertebrates the 

 loss of the gills has led to a great reduction in size of this system, 

 and the centers no longer form conspicuous ridges projecting into 

 the ventricle. Instead, there is in mammals only a small, >sharply 

 defined longitudinal bundle of fibers comparable to the fasciculus 

 communis, which is known as the fasciculus solitarius, and a 



