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- 



