Kappers, Tehostcaii and Selachian Brain. ii 



extend backward, always lying at a higher level than the tractus 

 strio-thalamicus, of which I shall have occasion to treat later. This 

 bundle ends after decussation in the post-infundibular region of 

 the diencephalon in the neighborhood of the ventriculus lobi 

 inferioris partly under the ventricle, and has never before been 

 described in the teleostean brain, unless perhaps by C. L. Her- 

 RiCK as the "dorsal peduncle." Johnston observed a similar 

 connection in the ganoids, as mentioned in 1898 in his first publi- 

 cation on the brain of Acipenser, where it is present as two bundles, 

 a more medial and a more lateral one, of which one has a more 

 ventral and the other a more dorsal course. He did not, however, 

 observe a decussation of the fibers. That such a tertiary lateral 

 olfactory connection is present in all fishes appears, moreover, 

 from the fact that this author describes it in Petromyzon also as 

 "gathering from the lateral expansions of the fore-brain and end- 

 ing in all parts of the inferior lobes." Its exact course in the 

 'tween-brain I shall take up in the second chapter. 



The third tertiary olfactory connection originates in almost the 

 same region but more ventrally and in a smaller group of cells 

 which lie in the area posterior lateralis and have been designated 

 nucleus icsnice. The neurones of this nucleus have medullated 

 fibers which run mingled with the tr. olfacto-hypothalamicus 

 lateralis, but separate from it where thev pass under the ganglia 

 habenulae of the thalamus. The medullated fibers go into the 

 ganglia habenulne, where they end in the opposite ganglion for the 

 most part, constituting in this way a part of the habenular com- 

 missure, while the olfacto-hypothalamicus fibers run farther 

 backw^ard without giving off fibers to these ganglia. This tract 

 is the tractus olfacto-habenularis, first described by Edinger in 

 fishes and afterward recognized in all lower vertebrates (Acipenser, 

 Johnston). 



The comjnissura anterior (Fig. x) too, must be regarded as 

 belonging to the olfactory system. The greater part of its fibers 

 I have already mentioned in the description of the medial olfactory 

 tract, whose feebly medullated decussating fibers form its fore- 

 most part, while the decussating fibers of the more heavily medul- 

 lated medial olfactory bundle (some of whose fibers perhaps run 

 backward with the tr. olfacto-hypothalamicus medialis while the 

 greater part end more laterally) form the more medullated part 

 of the commissure in Gadus just as it has been described bv 



