I92 Journal of Comparative Neurology. 



Fiber Tracts. 



It seems best to collect under this head disconnected ob- 

 servations that have been made on the fiber relations in various 

 parts of the brain, and which, due to inadequate study of the 

 structure of related parts must be treated separately. 



LoHJiisais. — As far caudad in the oblongata as the exit of 

 the tenth nerve (X^) a denser arrangement of some of the fine 

 fibers in the ventral region of the oblongata is apparent. This 

 becomes more marked at the ninth and in the region of the 

 eighth a more or less distinct bundle is formed in the alba (Fig. 

 17) midway between the meson and lateral aspect. This grad- 

 ually becomes more marked and migrated laterad to pass dors- 

 ad into the mesencephal and there become lost. It is shown 

 in the figures 15-17 ; 19-21 ; and 35. A large portion of the 

 fibers which form this this tract in the oblongata cross in the 

 raphe, and undoubtedly come from the dorsal regions as arcu- 

 ate fibers, as may be seen from horizontal sections. This pre- 

 sumably represents the lemniscus or a portion of it as Herrick 

 has already stated. It appears to be much better developed, 

 as a myelinic tract at least, in Nectmus, than in Ainbly stoma or 

 Diemyctybis. 



Posterior longitudinal fasciculus ( Fasciculus longitudinalis 

 dorsalis, Hintere Langsbiindel). — The course and relations of 

 the fibers of this tract in the caudal portion of the oblongata 

 have been already described. Cephalad of the fifth nerve the 

 tracts are represented by a few coarse fibers with which are as- 

 sociated finer ones. Just caudad of the mesencephalic pit 

 other longitudinal fibers decussate and become closely associated 

 with these bundles. Slightly cephalad of the exit of the third 

 nerve, this tract is no longer recognizable ; its disappearance in 



the ventral region of the oblongata are motor, those arising from the dorsal por- 

 tion and having relation to small-celled nidi are considered sensory. There 

 would seem to be no doubt of the correctness of the function ascribed, since it 

 is confirmed by the study of the distribution of the nerves, and, in the case of 

 certain of them (e. g., fifth and seventh) in which the ental origin is much 

 as in higher forms, a double confirmation can be had. 



