i470 



HUMAN ANATOMY. 



Blood-vessel 



Bundles of 

 nerve-fibres 



Interfascicular 

 connective tissue 







a: 



^-"v 





)-, 



 -/J 



Transverse section of part of optic nerve, showing several fasciculi of 



nerve-fibres. X 125. 



membrane, the lamina cribrosa, which is formed by interlacing bundles from the 

 inner third of the sclera and from the pial sheath. As they penetrate the lamina 

 cribrosa they lose their medullary sheaths ; in consequence the optic nerve is 

 reduced one third in diameter. The intervaginal lymph-space ends abruptly, being 



separated from the choroid 

 Fig. 1227. by the fibres of the pia which 



arch outward to join the 

 sclera. The nerve projects 

 slightly into the eyeball on 

 account of the thickness of 

 the layer of arching nerve- 

 fibres and forms, therefore, 

 a circular elevation, known 

 as the optic papilla o; 

 optic disc, about 1.5 mm. 

 in diameter, the center of 

 which is occupied by a fun- 

 nel-shaped depression, the 

 so-called physiological exca- 

 vation. The axis of the nerve 

 is occupied by the central 

 artery of the retina, which 

 gives off minute branches for 

 the nutrition of the nerve, 

 that anastomose with the 

 pial vessels, and, through the circulus arteriosus Zinni, with branches of the posterior 

 ciliary arteries. When seen in transverse sections (Fig. 1227), the optic nerve 

 appears as a mosaic of irregular polygonal areas composed of bundles of meduUated 

 nerve-fibres surrounded by connective tissue envelopes. Although provided with 

 medullary sheaths, the optic fibres are devoid of a neurilemma, in this respect 

 agreeing with the nerve-fibres composing the central nervous system. The entire 

 nerve corresponds to a huge funiculus, the perineurium being represented by the 

 pial sheath, and the endoneurium by the interfascicular septa of connective tissue 

 prolonged from the pia between the bundles of fibres. Numerous connective 

 tissue cells occur along the strands of fibrous tissue. 



Practical Considerations. Any disturbance of the optic nerve-fibres passing 

 from the retina to the cortex of the brain (page 1225) will cause disturbance of 

 vision, and within certain limits the lesion may be localized by the character of 

 the symptoms produced. 



The most characteristic symptom from a lesion on one side behind the chiasm 

 is a homonymous lateral hemianopsia, — that is, the right or the left half of each eye will 

 be blind. This is explained by the fact that the optic tracts are made up of fibres 

 coming from the corresponding lateral halves of both retinae, — i.e.., the fibres from 

 the right half of each retina pass to and make up the right optic tract, and pass 

 therefore to the right half of the brain. It will thus be seen that anything com- 

 pressing the optic fibres of the right side behind the chiasm, for instance a hemorrhage, 

 would produce a blindness — more or less complete according to the extent of the 

 fibres involved — of the right half of each eye. 



Since most of the optic fibres enter the lateral geniculate bodies, a lesion there 

 always causes hemianopsia, or half-eye blindness. Lesions of the optic thalamus, 

 or of the superior quadrigeminal body, may also by compression of the adjacent 

 optic tract produce hemianopsia. 



In the optic radiation are other than optic fibres, so that hemianopsia may or 

 may not follow lesions in that tract, according to whether optic fibres are involved 

 or not. The exact course of the visual fibres in the optic radiation is uncertain. 

 If the visual area of the brain cortex is involved by the lesions, no other symptoms 

 will be present, but the hemianopsia will be complete and homonymous — that is, the 

 corresponding halves of the two eyes will be blind. 



