42 A NEW CRANIAL NERVE IN SELACHIANS. 



pendent nerves. But the chief bundles (Edinger) recognized as belonging to the 

 olfactory system are present simultaneously with these nerves, and the anatomical 

 descriptions which follow will make it clear that their fibres have no connection with 

 the olfactory glomeruli, and throughout their course have a striking independence. 

 This, taken in connection with their embryonic history, is what to my mind justifies 

 calling them a pair of "new cranial nerves." It is my hope to be able to complete in 

 the near future observations on the bundles in the olfactory tract, and to publish 

 them in another connection, with a critical examination of the entire olfactory 

 system. 



II. DESCRIPTIONS OF THE NEW NERVE IN ADULT SELACHIANS. 



i. In Squalus acanthias. The brain of an adult Squalus acanthias is repre- 

 rented in Figure 1 (PI. V). A brief general description of the parts of the forebrain 

 will be given first in order to make clearer the course of the new nerve. In front are 

 the olfactory cups, two hollow bodies in the interior of which the nasal membrane is 

 thrown into plate-like folds. The fila olfactoria, or fibres of the olfactory nerves, arise 

 in cells of the nasal membrane and pass backward, uniting just below the capsules 

 into distinct bundles. The smaller bundles of the fila olfactoria are gathered into 

 two great divisions, one more lateral (n. olf. L) and one more median in position (n. 

 olf. m. in other figures). This partition of the olfactory fibres into two large complex 

 divisions seems to be general among the selachians. The complete separation of the 

 two divisions is well shown in Scoliodon terrse-nova? (Fig. 9) ; in S. acanthias the sepa- 

 sation is not so distinct, but it is nevertheless complete. The fibres composing each 

 division cross and mingle in a complicated manner and enter the enlarged end of the 

 olfactory tract (Fig. 1, trt. olf.). There is thus formed a rounded enlargement at the 

 base of the capsule called the bulbus olfactorius. The ends of the fila olfactoria 

 divide into brush-like tufts which come into communication with similar brush-like 

 endings of dendrites, belonging to a different series of cells, situated in the base of the 

 bulbus. The rounded masses formed by the union of the tufts of the filia olfactoria 

 and those of the dendrites are designated glomeruli olfactorii, and they mark the di- 

 vision between the neurons of the first and second order respectively. The neuraxons 

 of the neurons of the second order pass backward in the tractus, and, entering the 

 forebrain, they separate into the various tracts or bundles of the olfactory system. 



The front part of the forebrain in S. acanthias is divided by a median furrow into 

 two parts. This extends backward only about one-half the distance to the thalam- 



