978 



THE NERVE SYSTEM 



From the optic chiasm the optic tracts wind as flattened bands obliquely caudo- 

 laterad jiround the crura cerebri to subdivide, each into two bands, one (mesal 

 root) passing to the medial geniculate body and not a true continuation of the 

 optic pafti (see Gudden's commissure, p. 912), the other (lateral root) passing to* 

 the lateral geniculate body, the pulvinar, and the superior quadrigeminal body 

 (p. 912). 



The optic path has been described on page 911. 



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

 undergo atrophy in affections of the central nerve system, and, as a rule, the pathological rela- 

 tionship 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 

 and from its structure the optic nerve must be regarded as a prolongation of the brain substance. 

 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, an intermediate 

 sheath from the arachnoid, and an outer sheath from the dura, 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 retinae into the 

 interior of the nerve, and enters immediately into its structure. Thus, inflammatory infections 

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

 interstitial connective tissue in the nerve. 



To optic nerve 

 of same side. 



Of opposite 

 side. 



FIG. 727. The left optic nerve and optic tracts. FIG. 728. Course of the fibres in the optic chiasm. 



The course of the fibres in the optic chiasm 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 chiasm will 

 decussate 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. 



A sagittal section through the optic chiasm 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 chiasm 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. 



