CRANIAL NERVES. 169 
olfactory nerve, is distributed to the olfactory epithelium. Apparently 
the same nerve occurs in-the human embryo and it may be looked for 
elsewhere. It is called the terminalis nerve and probably belongs to 
the general cutaneous system. 
II. The Optic Nerve arises from the floor of the diencephalon and 
extends to the eye where it spreads over the inner surface of the retina. 
Together with the olfactory nerve it is usually stated to differ from the 
other cranial nerves in being an outgrowth from the brain. In its 
history, which is closely connected with that of the eye, there is first 
formed the optic stalk with the optic vesicle at its tip (see eye for details). 
The stalk grows out from the recessus opticus and hence is clearly 
dorsal in position. Soon after the involution of the optic cup, nerve 
cells are proliferated from the distal surface of the retina, which pass 
through the chorioid fissure and along the groove on the ventral side of 
the optic stalk. These fibres and not the cells of the stalk form the 
definitive optic nerve of the adult, and the cells from which they arise 
form the optic ganglion, which, to a certain extent, is comparable to the 
ganglion of a dorsal root. This view also lessens the differences be- 
tween the optic and other cranial nerves, a view which was natural 
before the history of the nerve was known and when it was thought 
that the stalk itself was transformed into the nerve. 
The nerve fibres, in their centripetal growth, do not stop on reaching 
the diencephalon, but continue across its ventral surface and become 
connected with the opposite side of the brain. There is thus a crossing 
or chiasma of the optic nerves, that from the left eye going to the right 
side of the brain and vice versa. In most vertebrates the chiasma is 
plainly seen from the surface, but in cyclostomes and dipnoans it may 
occur in the substance of the brain itself. In the lower vertebrates the 
chiasma is complete and the nerves from the two sides may simply over- 
lap or they may interlace with varying degrees of complexity. In the 
mammals, on the other hand, the chiasma can be analyzed only by 
microscopic methods, so intimately are the fibres interwoven, while here 
some of the fibres (‘lateral fibres’), instead of crossing, enter the cor- 
responding side of the brain. The internal connections of the optic 
nerves are not with the ’twixt-brain, but the fibres, after passing the 
chiasma, grow dorsally and posteriorly and become connected with the 
dorsal part of the mid-brain, hence called the optic lobes. 
There has been described in the embryo elasmobranch, under the name thal- 
amic nerve a small strand arising between the di- and mesencephalon. It disap- 
