CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
743 
entering the wider chamber the narrow part of the fourth ventricle becomes triangular 
in shape, with the base of the triangle turned upwards. The segment of this ventricle, 
which forms the chamber beneath the cerebellum, has the same relations that the 
fourth ventricle has in the human subject. It is shaped like a cocked hat when seen 
from above ; the top of the hat being directed backwards, and the sides running 
out into pointed extremities ; below, there is a longitudinal furrow on the floor ; above, 
its roof at first flat afterwards becomes furrowed in the centre. There is a large 
triangular opening in its roof which places this ventricle in communication with 
the outside of the brain beneath the cerebellum ; anteriorly, a lozenge-shaped passage 
(fig. 5, aq.Sy.), the aquseductus Sylvii, forms a communication between this ventricle 
and the ventricle of the optic lobes. Before opening into the latter the aqueduct spreads 
out into a wide depressed fissure. In the roof a longitudinal furrow extends from the 
anterior part of the fourth ventricle, and in the floor a similar furrow extends from the 
middle of the narrow part of the ventricle as far forward as the entrance of the 
aqueduct into the ventricle of the optic lobe. 
With regard to this last space, Gottsche (34) considered that it corresponded to the 
third, combined with the lateral ventricles, and Fritsch (66) even is doubtful whether 
it does or does not homologise with the lateral ventricles ; but I think that after all 
it is merely an expansion of the aquseductus Sylvii. That it does not correspond to 
the third ventricle appears nearly certain, since the fissure which properly corresponds 
to that ventricle is in front, and communicates with the space in question by a small 
foramen, which opens behind the posterior commissure. The third ventricle (fig. 2, 
v.th.) is situated between the two bundles of fibres which pass from the cerebral lobes 
into the hypoaria, and also between the two optic tracts. In front, this fissure com- 
municates through a narrower passage with a small oblong chamber, placed close 
behind the anterior limits of the brain, between the bases of the two cerebral lobes, 
which seems to correspond to the lamina terminalis of the foetus. The above-men- 
tioned chamber and the narrower passage are lined by a stratum of nerve-cells ; 
inferiorly, the third ventricle contracts into a funnel-shaped passage, the infundi- 
bulum, which leads into a space surrounded by the hypophysis, and which receives 
on each side the openings of the passage from the ventricles of the hypoaria. 
Apparent Origin of the Cranial Nerves , 
In the Teleostei, all the cranial nerves, with the exception of the hypoglossal and 
spinal accessory, may be distinguished, if not as separate roots, at least as divisions of 
some of the others. Thus, the nerve which is usually considered to be the facial is a 
branch of the trifacial ; and the supposed glossopharyngeal is sometimes a branch of 
the acusticus. The four anterior nerves do not require much discussion ; they arise in 
the same manner in all the Teleostei that I have examined. 
The olfactory (fig. 1 , n. 1), perhaps, in those fishes in which the lobe is applied close 
to the anterior end of the cerebral lobes may be considered as a true nerve. 
MDCCCLXXVIII. 5 c 
