OPTIC NERVES. 43 



In the centre of the optic nerve, is a minute canal, lined 

 by fibrous tissue, in which is lodged the central artery of the 

 retina and its corresponding vein, with a delicate, nervous fila- 

 ment from the ophthalmic ganglion. The vessels penetrate 

 the optic nerve a little (from -J- 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 ca- 

 pable of conveying to the cerebrum the special impressions 

 of sight, and that they are not endowed with general sensi- 

 bility. 



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 

 lesion. 1 It is interesting, however, to note that they are ab- 

 solutely 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 sensa- 

 tion ; whether it be taken before or after its decussation, it 

 seems completely insensible in its entire length." 2 



Not only are the optic nerve and retina insensible to pain, 

 but any irritation produces the impression of light. This 

 was stated in the remarkable paper, Idea of a New Anatomy 

 of the Brain, printed by Charles Bell, in 1811. 3 A few 

 years later, Magendie, in operating for cataract, passed the 



1 In 1828, Magendie, influenced probably by his favorite theory that the 

 fifth pair of nerves was necessary to the perfect operation of all the special 

 senses, reported two cases of atrophy of the optic nerves, without complete loss 

 of sight, and assumed, as a possibility, that sight might be retained to a limited 

 degree, when the optic nerves had been gradually destroyed. This view does 

 not merit discussion, and is simply one of the curiosities of physiological litera- 

 ture. (MAGENDIE, La vue peut-elle etre conservee malgre de destruction des nerfs 

 optiques? Journal de physiologic, Paris, 1828, tome viii., p. 27.) 



2 LONGET, Traite de physiologic, Paris, 1869, tome iii., p. 479. 



8 BELL, Idea of a New Anatomy of the Brain, London, 1811. Documents 

 and Dates of Modern Discoveries in the Nervous System, London, 1839, p. 43. 



