410 ^Journal of Comparative Neurology and Psychology. 



purpose of this paper is the examination of a number of types of 

 fishes in which the various elements of the commissure are dif- 

 ferently developed, for the purpose of reaching a more complete 

 understanding of this critical region of the brain. We shall begin 

 with a review of the conditions in Ameiurus, where the visceral 

 and somatic elements are about equally developed, and then 

 examine other species showing various deviations from this type. 



The visceral commissura infima of Ameiurus. — The visceral 

 commissural nucleus is a dorsal unpaired structure forming a pro- 

 tuberance in the triangular space behind the vagal lobes and 

 between the funicular nuclei. The cells of the nucleus, which are 

 rather small, are more thickly arranged near the external borders of 

 the lobe (fig. i). Their dendrites ramify through its whole sub- 

 stance and freely cross the median line. The fibers of the descend- 

 ing vagus root, which enter the nucleus at its lateral borders, ter- 

 minate chiefly uncrossed by widely branched aborizations, but 

 some of these endings cross to the opposite side before terminating 

 (fig. 3). This commissure also receives short secondary tracts, 

 either unmedullated or with slight meduUation, from the vagal 

 lobe, which end partly crossed and partly uncrossed (cf. Herrick 

 1906, fig. 7). The cells of its micleus therefore may receive vis- 

 ceral impulses either directly from the periphery by way of the 

 descending vagus root or indirectly by way of the descending sec- 

 ondary visceral tract from the vagal lobes. Both types of affer- 

 ent fibers, as well as dendrites of the cells of the nucleus, partici- 

 pate in the formation of the commissure, which is difi^use and 

 chiefly unmedullated. The efferent tract from this nucleus is by 

 tolerably compact bundles of unmedullated fibers which curve 

 downward near the median line around the cephalic end of the 

 median funicular nucleus to effect connection with the adjacent 

 formatio reticularis under the vagal lobe (fig. i). 



The somatic commissura infima of Ameiurus. — This commis- 

 sure lies ventrally of the preceding and dorsally of the ventricle. 

 The two commissures are for the most part very distinct anatom- 

 ically, though at the cephalic end there is some mingling of their 

 fibers. The somatic commissure is chiefly, though by no means 

 wholly, a commissure of the funicular nuclei (fig. 2). Its most 

 dorsal part is a strong compact bundle from the dorso-lateral fas- 

 ciculus and lateral funicular nucleus (fig. 2, y; fig. 3). Among 

 these fibers and ventrally of them are dendrites of the cells of the 



