44 A NEW CRANIAL NERVE IN SELACHIANS. 



furrow. In the latter position the two bundles of each nerve separate and penetrate 

 the brain-wall at different points. 



Having described the course of the nerve from the median furrow to the olfactory 

 cup, let us now consider its central connections. Near the bottom of the furrow the 

 divided bundles of the nerves penetrate the brain-wall. They enter this a little below 

 the level of the median horizontal plane, and, rising slightly (PI. VI, Fig. 18, n. nov.), 

 pass toward the inner wall of the brain- ventricle. Sections made in the horizontal 

 plane (PI. VI, Fig. 17, n. nov.) show that the nerves on each side pass backward in the 

 brain substance, dividing into two or three chief branches, which in turn are greatly 

 subdivided. The median branches of each side cross the median plane and spread 

 into a number of twig-like terminations, while the other branches spread out in 

 the brain substance of the same side as that on which they entered. The minuter 

 branches of these nerves terminate near the epithelial cells which form the lining 

 of the brain ventricle. 



The portion of the brain substance in which these fibres end apparently corre- 

 sponds to the epistriatum (Edinger) of the brains of reptiles, etc., although it is on 

 the median wall rather than the lateral wall of the brain ventricle. In the forebrain 

 of Squalus two thickened masses of cells meet in the median plane on the base of the 

 brain, and also unite vertically with an infolding of the cortex from the roof of the 

 brain. In cross-section this presents the appearance of a short pillar with an 

 enlargement above, the cortical infolding, and another below, the corpus striatum; 

 the fibres terminate in the region between the two (the epistriatum?). The fibres 

 can be traced very close to the brain ventricle, but I have not been able to make out 

 the nature of their final terminations. 



Figure 18 (PL VI) shows the course of the nerve within the brain substance as 

 seen in a sagittal section; Figure 17 shows a partly diagrammatic reconstruction of 

 sections made in the horizontal plane; and Figure 19 shows the position of the nerve 

 in a cross-section. 



As to peripheral connections, the study of serial sections shows that the fibres of 

 the new nerve are mainly distributed to the olfactory membrane in the antero-lateral 

 portions of the olfactory cup. There is in S. acanthias, as in other selachians, an 

 obvious division of the olfactory chamber into two parts, one median, the other 

 lateral. The olfactory membrane of these two parts is continuous and, in both, is 

 thrown into similar folds, but membranous flaps, or valves, developed from the firm 

 ring surrounding the entrance to the cavity, are so arranged as to make two openings 

 and separate the chamber into two parts. The separation of the fila olfactoria into 

 two divisions apparently has reference to these two parts. The fibres of the new 



