OPTIC NERVES. 673 



With the exception of a few grayish filaments, the fibres of the optic 

 tracts and the optic nerves are of the ordinary, medullated variety, and they 

 present no differences in structure from the general cerebro-spinal nerves. 



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

 resisting than the optic tracts. The optic nerves are rounded and are enclosed 

 in a double sheath derived from the dura mater and the arachnoid. They 

 pass into the orbit upon either side and penetrate the sclerotic, at the pos- 

 terior, 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 sphenoidal fissure, 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 a number of 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, mammillated 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 to f of an inch (8*5 to 19-1 mm.) 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 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 

 corresponding with the extent of the lesion. It is important, 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 ic 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 their 

 stimulation produces luminous impressions. This was stated in the remark- 

 able paper, Idea of a Neio Anatomy of the Brain, printed by Charles Bell, 

 in 1811. A few years later, Magendie, in operating for cataract, passed the 

 needle to the bottom of the eye and irritated the retina, in two persons. The 

 patients experienced no pain but merely an impression of flashes of light. 

 The insensibility of the optic nerves has also been repeatedly noted in surgical 

 operations in which the nerves have been exposed. If an electric current be 

 passed through the optic nerves, a sensation of light is experienced. The 



