184 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The terminal nerve (Nervus terminalis) is a very recent discovery 

 (first found in dipnoi and now known to occur in all classes of verte- 

 brates, including man). The terminal (or preoptic nerve, fig. 192), 

 leaves the brain near the base of the olfactory nerve, has its ganglion, 

 and terminates in the mucous membrane of the nose. Its cerebral 

 connexions (it has been traced into the hypothalamus) are uncertain 

 and nothing is known of its functions. 



II. The optic nerve ( Nervus opticus) arises from the floor of the 

 diencephalon and extends to the eye, where its branches are distrib- 

 uted over the inner surface of the retina, in the inner layer of which are 

 its gangHon cells. On the floor of the 'twixt-brain the fibres from the 

 right eye pass to the left side of the brain and those of the left eye to 

 the right side, forming a crossing or chiasma on the lower surface. 

 From thence the fibres run dorsally and backward and enter the 

 optic lobes. In most vertebrates the chiasma is plainly seen from the 

 outside, but in the cyclostomes it is imbedded in the brain. In the 

 lower vertebrates the chiasma is complete, and the nerves may simply 

 overlap, or the fibres may be variously interlaced. In the mammals 

 the chiasma can only be analyzed by microscopic methods, while in 

 this class the crossing is incomplete, there being crossed and uncrossed 

 fibres, a pecuHarity connected with binocular vision. 



Several peculiarities of the optic nerve can be understood only on the basis 

 of its development. The eye grows out from the dorsal zone of the fore-brain, 

 the details of the development being given in connexion with that of the eye 

 (p. 214). This outgrowth forms a cup, the distal surface of which is to form the 

 retina. The ' inner ' (distal) cells of the cup proliferate the nerve fibres which 

 grow from this point, through a gap in the wall of the cup (the chorioid fissure), 

 back along the stem of the cup to the brain. A continuation in this course 

 results in the chiasma and the connexion with the mid-brain, although the eye 

 starts from the 'twixt-brain. 



Both the optic and the olfactory nerves are frequently stated to be out- 

 growths from the brain, but in the broader sense, neither of them is. The older 

 view was that the stalk of the cup was directly transformed into the optic nerve, 

 a belief which led to the usual statement. 



A small thalamic nerve, arising between the di- and mesencephalon, has been 

 described in the embryo of some elasmobranchs. It disappears without leav- 

 ing a trace, unless it contributes nervous material to the ciliary ganglion. 



III, IV, VI. Nervi Oculomotorius, Trochlearis andAbducens. — 

 These three nerves supply the muscles which move the eye in its 

 socket and, as the eye -muscle nerves, may be treated here together 

 (figs. 144, 193). The oculomotor nerve arises from the ventral sur- 

 face of the mid-brain and supplies the superior, medial and inferior 



