OPTIC NERVE AUDITORY NERVE. 387 



lobe. 3. External, from a long fasciculus which is traced for a 

 considerable distance along the fissure of Sylvius, into the middle 

 lobe. The union of these roots forms a grayish white nerve, pris- 

 moid in form and soft in structure, which expands into a bulb (bulbus 

 olfactorius), and rests upon the cribriform plate. Its branches are 

 transmitted through the numerous foramina in the cribriform plate, 

 to be distributed to the mucous membrane of the nose. The inner- 

 most are reddish in colour and soft, and spread out upon the septum 

 narium ; the external branches are whiter and more firm, they pass 

 through bony canals in the outer wallbf the nose, and communicate 

 freely with each other previously to their distribution in the mucous 

 membrane of the superior and middle turbinated bones. 



2d pair, OPTIC. The optic nerve arises by two roots ; one from 

 the corpus geniculatum externum, the other from the anterior pair 

 (nates) of the corpora quadrigemina or optic lobes. It winds around 

 the crus cerebri as a flattened band, and unites with its fellow of the 

 opposite side, to form the commissure. The two nerves then diverge 

 from each other, to enter the orbit through the optic foramen, pierce 

 the sclerotic and choroid coat of the eyeball, and expand in the 

 retina. The optic commissure rests upon the processus olivaris of 

 the sphenoid bone, and its posterior border is closely connected with 

 the tuber cinereum, from which it receives fibres ; in its interior the 

 innermost fibres of the two nerves cross each other, while the exter- 

 nal proceed directly onwards in their course. On entering the orbit 

 the nerve obtains a firm sheath from the dura mater, which is con- 

 tinuous with the sclerotic coat of the eyeball. This sheath is formed 

 by the splitting of the dura mater at the foramen opticum into two 

 layers, the one surrounding the optic nerve, while the other is con- 

 tinuous with the periosteum of the orbit. Near to the globe of the 

 eyeball the optic nerve is pierced by the arteria centralis retinae, 

 which runs forwards in the centre of the nerve, and reaching the 

 retina distributes branches upon its internal surface, forming its 

 vascular layer. 



7th pair, AUDITORY (portio mollis). This nerve arises from the 

 anterior wall or floor of the fourth ventricle, by means of the white 

 fibres, HnecB transversce, of the calamus scriptorius. It winds around 

 the corpus restiforme, from w r hich it receives fibres, and emerges 

 upon the posterior border of the crus cerebelli : it then enters the 

 meatus auditorius internus, together with the facial nerve, which 

 lies in a groove on its superior and anterior surface, and at the 

 bottom of the meatus divides into two branches, cochlear and vestibu- 

 lar, which are distributed to the internal ear. It is soft and pulpy 

 in texture, and whilst situated in the meatus auditorius sends several 

 filaments to the facial nerve. 



NERVES OF MOTION. 



We have already seen that the corpora pyramidalia are the con- 



