36 



olIL 



nerve have been more fully worked out in Squalus than in any other 

 animal, and, although it has been observed in primitive forms like 

 Heptanchus and Hexanchus, which would form a more natural beginning 

 for the series, nevertheless, it will be first described in Squalus and 

 at somewhat greater length than in any other form. 



As an aid to appretiating the relations of the new nerve, it will 

 be well to review, very briefly, the main anatomical features of the 

 olfactory nerve in selachians. The olfactory cup in all selachians is 

 relatively large and in all there is a well developed tractus. It is 



essential to distinguish between 

 the olfactory tractus and the ol- 

 factory nerve. The latter con- 

 sists of nerve fibres (the fila ol- 

 factoria) which arise, at least in 

 selachians, much earlier than the 

 olfactory lobe^). The tractus is 

 a development from the olfactory 

 lobe, and when the latter arises 

 and becomes elongated, the al- 

 ready formed olfactory nerve is 

 carried forward on its tip. The 

 nerve proper is relatively short 

 in all selachians and the tractus 

 relatively long. 



Fig. 1 shows the brain of 

 adult Squalus acanthias as seen 

 from above. In front are the ol- 

 factory cups, two hollow bodies 

 in the interior of which the nasal 

 membrane is arranged in plate- 

 like folds. The fila olfactoria 

 spring from nerve-cells located 

 mainly within this membrane, and pass towards the brain, uniting into 

 small bundles. As the latter emerge from the olfactory cup, they are 

 combined into larger bundles, the fibers of which intermingle and cross, 

 and, finally, are gathered into two great divisions, one more lateral 

 {n. olf. I.) and one more median in position (w. olf. m.). A similar 

 division of the olfactory nerve seems to be general among the se- 

 lachians. The fissure separating the two divisions of the nerve is 



Fig. 1. Brain of an adult Squalus 

 acanthias, from above, nat. size, gn ganglion. 

 n.nov new nerve, n.olf.l lateral division of 

 the olfactory nerve. n.opt optic nerve. 

 ms'ence mesencephalon, cbl cerebellum. 



1) LocY, loc. cit. 



