OPTIC AUDITORY MOTOR OCULI. 537 



The commissure, somewhat quadrilateral in form, rests upon the olivary process 

 of the sphenoid bone, being bounded, in front, by .the lamina cinerea ; behind, by 

 the tuber cinereum ; on either side, by the anterior perforated space. Within 

 the commissure, the optic nerves of the two sides undergo a partial decussation. 

 The fibres which form the inner margin of each tract, 

 are continued across from one to the other side of the 

 brain, and have no connection with the optic nerves. 

 These may be regarded as commissural fibres (inter- 

 cerebral) between the thalami of opposite sides. 

 Some fibres are continued across the anterior border 

 of the chiasma, and connect the optic nerves of the 



two sides, having no relation with the optic tracts. ~V^ -fw 



They may be regarded as commissural fibres between the two retinas (inter-retinal 

 fibres). The outer fibres of each tract are continued into the optic nerve of the same 

 side. The central fibres of each tract are continued into the optic nerve of the 

 opposite side, decussating in the commissure with similar fibres of the opposite tract. 1 



The optic nerves arise from the fore part of the commissure, and, diverging from 

 one another, become rounded in form, firm in texture, and are inclosed in a sheath 

 derived from the arachnoid. As each nerve passes through the corresponding 

 optic foramen, it receives a sheath from the dura mater ; and as it enters thejorbit, 

 this sheath subdivides into two layers, one of which becomes continuous with the 

 periosteum of the orbit ; the other forms a sheath for the nerve, and surrounds it 

 as far as the sclerotic. The nerve passes through the cavity of the orbit, pierces 

 the sclerotic and choroid coats at the back part of the eyeball, a little to the nasal 

 side of its centre, and expands into the retina. A small artery, the arteria cen- 

 tralis retinae, perforates the optic nerve a little behind the globe, and runs along 

 its interior in a tubular canal of fibrous tissue. It supplies the inner surface of 

 the retina, and is accompanied by corresponding veins. 



AUDITORY NERVE. 



The AUDITORY NERVE, the portio mollis of the seventh pair, is the special nerve 

 of the sense of hearing, being distributed exclusively to the internal ear. The 

 portio dura of the seventh pair, the facial nerve, is the motor nerve of the muscles 

 of the face. It will be described with the motor cranial nerves. 



The auditory nerve arises from numerous white striae, the lineae transversae, 

 which emerge from the posterior median fissure in the anterior wall, or floor, of 

 the fourth ventricle. It is also connected with the gray matter of the medulla, 

 which corresponds to the locus caeruleus. According to Foville, the roots of this 

 nerve are connected, on the under surface of the middle peduncle, with the gray 

 substance of the cerebellum, with the flocculus, and with the gray matter at the 

 borders of the calamus scriptorius. The nerve winds round the restiform body, 

 from which it receives fibres, and passes forwards across the posterior border of 

 the crus cerebelli, in company with the facial nerve, from which it is partially 

 separated by a small artery. It then enters the meatus auditorius, in company 

 with the facial nerve, and, at the bottom of the meatus, divides into two branches, 

 cochlear and vestibular, which are distributed, the former to the cochlea, the 

 latter to the vestibule and semicircular canals. The auditory nerve is very soft 

 in texture (hence the name portio mollis), destitute of neurilemma, and, within the 

 meatus, receives one or two filaments from the facial. 



2. THE MOTOR CRANIAL NERVES. 

 THIRD OR MOTOR OCULI NERVE. 



The THIRD or MOTOR OCULI NERVE supplies all the muscles of the eyeball, except 

 the Superior oblique and External rectus ; it also sends motor filaments to the iris. 



1 A specimen of congenital absence of the optic commissure is to be found in the Museum of 

 the Westminster Hospital. 



