CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
737 
ventral surface, on which is a longitudinal furrow, the representative of the anterior 
median fissure. 
Apparent Origin of the Cranial Nerves. 
First pan-.—The olfactory nerves pass out from the anterior end of the bulbi olfac- 
torii, and passing through the dense connective tissue partitions, are distributed on ~ 
the pectinated mucous membrane of the olfactory organ; their course and terminations 
do not come within the scope of this work. 
Second pair.—The optic nerves present a well marked chiasma which projects 
beyond the anterior end of the lobus infundibuli or hypoarium. 
Third pair (figs. 1 and 2)—Oculomotorii—rise from the usual position on the 
inferior surface of the medulla oblongata. Their origins are covered here by the 
hypophysis cerebri, and there is a slight interval between them and the posterior end 
of the hypoaria. This fact seems to show that the posterior lobes of these bodies, 
which are well developed in the Teleostei, are not represented in the Plagiostomata. 
Fourth pair—Trochlea,res —have also a well-defined and constant superficial origin ; 
they invariably emerge from a furrow between the optic lobes and the cerebellum, 
marking the point of demarcation between the mid-brain and the hind-brain. 
Fifth pair (figs. 1, 2, and 3)— Trifacial — arise by two roots in the Rays, one the 
anterior from the side of the crura cerebelli, the other more dorsal from the folds of 
the restiform bodies ; these two join together and pass through the foramen into the 
orbit, and immediately join the Gasserian ganglion. 
The course of the ophthalmic branch is rather peculiar ; it is derived from the upper 
root in the foramen, and divides into two branches, one of which passes over and the 
other under the external rectus muscle of the eyeball; these, uniting again into one 
trunk, pass over the olfactory sac, and emerging through a passage in the car¬ 
tilage go on to supply the anterior pointed angle of the body. 
In Acanthias (fig. 3) the trifacial arises by one root only from the lower edge of the 
restiform bodies. It passes out of the skull through its own foramen, and immediately 
joins the Gasserian ganglion. The three usual divisions arise from this ganglion ; the 
superior of the three, the ophthalmic, seems here to consist almost entirely of 
the frontal branch, which passes horizontally through the orbit, crosses above the 
peduncle of the olfactory lobe, and is distributed to the skin as far as the anterior 
extremity of the snout. The next division, the largest of the three, emerges from 
the anterior end of the inferior side of the ganglion, having the origins of the recti 
muscles between itself and the last branch, and passes downward and forward through 
the deep part of the orbit to the ventral edge, where it divides into two branches ; of 
these the inferior supplies the region opposite to and below the foramen of the optic 
nerve; the other branch passes forward beneath the peduncle of the olfactory lobe, 
and supplies the inferior surface of the snout behind the distribution of the frontal. 
The third division, the smallest of the three, arises from the inferior angle of the 
MDCCCLXXXVI. 5 B 
