794 THE NERVOUS SYSTEM. 



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. Arnold describes a 

 communication between the optic nerve in the orbit and the ascending branches 

 of Meckel's ganglion. A small artery, the arteria centralis retince, 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. 



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 its mode of 

 development (see page 123) and from its structure the optic nerve must be regarded as a prolonga- 

 tion of the brain-substance, rather than as an ordinary cerebro-spinal nerve. (2) As it passes 

 from the brain it receives sheaths from the three cerebral membranes a perineural sheath from 

 the pia mater, an intermediate sheath from the arachnoid, and an outer sheath from the dura 

 mater, which is also connected 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 retinas into the interior of the nerve, and enters intimately into the struc- 

 ture of the nerve. Tims inflammatory 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, in 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. 



Third Nerve (Figs. 384, 479, 480, 481, 486). 



The Third or Motor Oculi Nerve supplies all the muscles of the orbit, except 

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

 and the ciliary muscle. It is a rather large nerve, of rounded form and firm 

 texture. Its apparent origin is from the inner surface of the crus cerebri, 

 immediately in front of the pons Varolii. 



The deep origin may be traced through the locus niger and tegmentum of the 

 crus to a nucleus situated on either side o the median line beneath the floor of 

 the aqueduct of Sylvius. On emerging from the brain the nerve is invested with 

 a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It 

 then pierces the dura mater in front of and external to the posterior clinoid pro- 

 cess, passing between the two processes from the free and attached borders of the 

 tentorium, which are prolonged forward to be connected with the anterior and 

 posterior clinoid processes of the sphenoid bone. It passes along the outer wall 

 of the cavernous sinus, above the other orbital nerves, receiving in its course one 

 or two filaments from the cavernous plexus of the sympathetic and from the oph- 

 thalmic division of the fifth nerve. It then divides into two branches, which 

 enter the orbit through the sphenoidal fissure between the two heads of the 

 External rectus muscle. On passing through the fissure the nerve is placed below 

 the fourth and the frontal and lachrymal branches of the ophthalmic nerve, and 

 has passing between its two divisions the nasal nerve. 



