AUDITORY. COT 



The commissure, or chiasma, 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 perfo- 

 rated 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 Fig. 338. Course of the 

 one to the other side of the brain, and have no con- Fi . bres in the P tic Oom - 

 nection 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. They may be regarded as com- 

 missural 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 and firm in texture, and are inclosed 

 in a sheatli 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 the orbit, 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 centralis 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 (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, or facial nerve, is the motor nerve of the mus- 

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



The auditory nerve arises by numerous white striae, the linese transversse, 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, 

 corresponding 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. 



(The other nerves of special sense must be described with the glosso-pharyn- 

 geal and fifth nerves, of which they are parts.) 



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

 the Westminster Hospital. 



