A NEW CRANIAL NERVE IN SELACHIANS. 43 



encephalon and, therefore, the hinder portion of the forebrain is undivided. The 

 epiphysis is not represented in the drawing. Connected with the base of the thalam- 

 encephalon are the optic nerves (n. opt.). Behind the thalamencephalon come the 

 midbrain (ms'ence.), the cerebellum (cbl), and the medulla oblongata. 



The course of the new nerve (Fig. 1, n. nov.) may now be described. It may, 

 for convenience, be spoken of as passing from the brain to the olfactory cups with- 

 out prejudicing the question of the source of origin of its fibres. Starting deep in 

 the median furrow, it passes forward across the anterior surface of the forebrain; 

 it then curves in the angle formed by the union of the olfactory tract and the fore- 

 brain, and finally passes along the inner margin of the tract to reach the median 

 division of the fila olfactoria. It crosses this obliquely and enters the fissure between 

 the two divisions of the fila. These are unequal in breadth the median one being 

 broader so that the point at which the nerve disappears in the furrow is about two- 

 thirds the space across. Just as the nerve reaches the fissure it branches, unequally, 

 into three stems (not shown in the drawing) ; a small one passing obliquely back- 

 ward and downward to unite with the lateral division near its base, another slender 

 one curving upward and inward to unite with the median division, and the chief 

 central stem which runs forward and laterally, uniting with the outer bundle of the 

 lateral division of the fila olfactoria (n. olf. 1). This chief stem breaks into anumber 

 of small branches that pass among the fibres constituting the lateral division. This 

 stem also dips downward into the furrow and reaches about the level of the median 

 horizontal plane of the olfactory cup. By separating the fila olfactoria it can be 

 traced close to the membranous covering of the olfactory capsule, and well out along 

 the lateral portion of the olfactory cup. Here it penetrates the connective-tissue 

 covering of the cup and enters (in connection with the fila olfactoria) the nasal epithe- 

 lium. All this, with the exception of the last item, can be made out by careful work 

 under a dissecting microscope, and has been many times confirmed by repeated obser- 

 vations. One point that forcibly strikes the observer is that this new nerve pre- 

 seTves its independence, often mingling with the fila olfactoria. Actual anastomosis 

 could not be made out. The two sets of fibres commingle very intimately, but in no 

 case have they been seen to unite with each other. 



There are two ganglionic enlargements on the nerves in Squalus acanthias that 

 vary in position in different specimens. The more prominent one (Fig. 1, gn.} is 

 near the base of the bulbus, and the other near the angle of union between forebrain 

 and tractus. The nerve is composed of two bundles united within the same enve- 

 lope of connective tissue. The distinction between the two bundles is well seen in 

 sections at the angle between tractus and forebrain, and, again, within the median 



