722 THE NERVOUS SYSTEM. 



The great majority of the fibres of the optic nerve consist of the afferent axons 

 ofi nerve-cells in the retina. Some few, however, are efferent fibres, and grow out 

 from the brain. The afferent fibres end in arborizations around the cells in the 

 external geniculate body, pulvinar, and upper quadrigeminal body, which are some- 

 times termed the lower visual centres. From these nuclei other fibres are prolonged 

 to the cortical visual centre, which, according to most observers, is situated in the 

 cuneus, and probably also in the lingual lobule of the occipital lobe. 



It should be stated that some fibres are detached from the optic tract, and pass 

 through the crus cerebri to the nucleus of the third nerve. These fibres are small, 

 and may be regarded as afferent branches for the sphincter pupillas and ciliary 

 muscles. Other fibres pass to the cerebellum through its superior peduncles, while 

 others, again, are lost in the pons. 



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 enclosed in 

 a sheath derived from the pia mater and 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 con- 

 tinuous with the periosteum of the orbit ; the other forms the proper sheath of the 

 nerve and surrounds it as far as the sclerotic. The nerve passes forward and out- 

 ward through the cavity of the orbit, pierces the sclerotic and choroid coats at the 

 back part of the eyeball, about one-eighth of an inch to the nasal side of its centre, 

 and expands into the retina. A small artery, the arteria centralis retince, perfo- 

 rates 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. 



Surgical Anatomy. The optic nerve is peculiarly liable to become the seat of neuritis or 

 undergo atrophy in affections of the central nervous system, and, as a rule, the pathological 

 relationship between the two affections is exceedingly difficult to trace. There are, however, 

 certain points in connection with the anatomy of this nerve which tend to throw light upon the 

 frequent association of these affections with intracranial disease : (1) From jts mode of develop- 

 ment and from its structure the optic nerve must be regarded as a prolongation of the brain-sub- 

 stance, rather than as an ordinary cerebrospinal nerve. (2) As it passes from the brain it receives 

 sheaths from the three cerebral membranes a perineural sheath from the pia mater, an inter- 

 mediate sheath from the arachnoid, and an outer sheath from the dura mater, which is also con- 

 nected with the periosteum as it passes through the optic foramen. These sheaths are separated 

 from each other by spaces which communicate with the subdural and subarachnoid spaces 

 respectively. The innermost or perineural sheath sends a process around the arteria centralis 

 retinae into the interior of the nerve, and enters intimately into its structure. Thus inflamma- 

 tory affections of the meninges or of the brain may readily extend themselves along these spaces 

 or along the interstitial connective tissue in the nerve. 



The course of the fibres in the optic commissure has an important pathological bearing, and 

 has been the subject of much controversy. Microscopic examination, experiments, and pathology 

 all seem to point to the fact that there is a partial decussation of the fibres, each tract supplying 

 the corresponding half of each eye, so that the right tract supplies the right half of each eye, 

 and the left tract the left half of each eye. At the same time, Charcot believes and his view 

 has met with general acceptation that the fibres which do not decussate at the optic commis- 

 sure have already decussated in the corpora quadrigemina, so that lesion of the cerebral centre 

 of one side causes complete blindness of the opposite eye, because both sets of decussating fibres 

 are destroyed. Whereas should one tract say the right be destroyed by disease, there will be 

 blindness of the right half of both retinae. 



An antero-posterior section through the commissure would divide the decussating fibres, 

 and would therefore produce blindness of the inner half of each eye ; while a section at the 

 margin of the side of the optic commissure would produce blindness of the external half of the 

 retina of the same side. 



The optic nerve may also be affected in injuries or diseases involving the orbit, m fractures 

 of the anterior fossa of the base of the skull, in tumors of the orbit itself, or those invading this 

 cavity from neighboring parts. 



The Third Nerve (Figs. 391, 392, 393). 



The Third or Motor oculi nerve (n. oculo-motorius) supplies all the muscles 

 of the orbit except the Superior oblique and Externajl rectus; it also supplies, 

 through its connection with the ciliary ganglion, the S Miincter muscle of the ins 



