GENERAL PROPERTIES OF THE OPTIC NERVES. 769 



scope. The second case, reported by Dr. "W. Thomson, presented the same condition 

 following partial hemiplegia, the result of sunstroke. The peculiar affection of vision in 

 these cases, called hemiopsia, especially as illustrated in the first case, reported by Dr. 

 Keen, can be explained by assuming the following as the course of the decussating fibres 

 of the optic tracts : From the left side of the encephalon, visual fibres pass to the right 

 eye, supplying the inner mathematical half of the retina, from a vertical line passing 

 through the macula lutea. Visual fibres also pass to the left eye, supplying the outer 

 half of the retina, beginning at the macula lutea. The macula lutea, then, and not the 

 point of entrance of the optic nerve, is in the line of division of the visual field. The 

 outer half of the left and the inner half of the right retina are supplied by fibres from the 

 left side ; and the outer half of the right and the inner half of the left retina are sup- 

 plied from the right side. Although this anatomical arrangement has not been actually 

 demonstrated, it is rendered exceedingly probable by pathological cases like those just 

 cited. In the case reported by Dr. Keen, the left side of the brain was injured, as the 

 paralysis occurred in the right leg and arm. 



With the exception of the few filaments derived from what have been described as the 

 gray roots, the fibres of the optic tracts and the optic nerves are of the medullated variety, 

 and they present no differences in structure from the ordinary cerebro-spinal nerves. 



The optic commissure is covered with a fibrous membrane and is consequently more 

 resisting than the optic tracts. From its anterior and outer border, arise the optic nerves, 

 which take a curved direction to the eyes. The nerves are rounded and are enclosed in 

 a double fibrous sheath derived from the dura mater and the arachnoid. They pass into 

 the orbit upon the two sides by the optic foramina and penetrate the sclerotic at the 

 posterior, inferior, and internal portion of the globe. As the nerves enter the globe, they 

 lose their coverings from the dura mater and arachnoid. The sheath derived from the 

 dura mater is adherent to the periosteum of the orbit at the foramen opticum, and, when 

 it reaches the globe, it fuses with the sclerotic coat. Just before the nerves penetrate the 

 globe, they each present a well-marked constriction. At the point of penetration, there 

 is a thin but strong membrane, presenting numerous perforations for the passage of the 

 nervous filaments. This membrane, the lamina cribrosa, is in part derived from the 

 sclerotic, and in part, from the coverings of the individual nerve-fibres, which lose their 

 investing membranes at this point. In the interior of each eye, there is a little, mammil- 

 lated eminence, formed by the united fibres of the nerve. The retina, with which the 

 optic nerve is connected, will be described as one of the coats of the eye. 



In the centre of the optic nerve, is a minute canal, lined by fibrous tissue, in which are 

 lodged the central artery of the retina and its corresponding vein, with a delicate nervous 

 filament from the ophthalmic ganglion. The vessels penetrate the optic nerve a short 

 distance (from $ to of an inch) behind the globe. The central canal does not exist 

 behind these vessels. 



General Properties of the Optic Nerves. There is very little to be said regarding the 

 general properties of the optic nerves, except that they are undoubtedly the only nerves 

 capable of conveying to the cerebrum the special impressions of sight, and that they are 

 not endowed with general sensibility. 



That the optic nerves are the only nerves of sight, there can be no doubt. Their 

 division or injury always involves loss or impairment of vision, directly ooTBWpOBding 

 with the lesion. It is interesting, however, to note that they are absolutely insensible to 

 ordinary impressions. "We can, in a living'animal, pinch, cauterize, cut, destroy in any 

 way the optic nerve without giving rise to the slightest painful sensation ; whether it be 

 taken before or after its decussation, it seems completely insensible in its entire length.'" 

 (Longet.) 



Not only are the optic nerve and retina insensible to pain, but any irritation produce 

 the impression of light. This wa stated in the remarkable paper, Idea of a 'my 



49 



