240 



attribute to the aneurismal or varicose enlargements of those smal 

 sels, the spots which are seen in objects, in the disease to which Maitre 

 Jean gave the name of imaginations P In order to form the retina, the 

 optic nerve which penetrates into the globe of the eye, by piercing the 

 sclerotica, to which the covering given to that nerve by the dura mater 

 is connected; the optic nerve penetrates through a very thin membrane 

 perforated by a number of small holes, and closing the opening left by 

 the nerve, and which belongs as much to the choroid as to the sclerotic 

 coats; it then spreads out to furnish the expansion which lines the con- 

 cavity of the choroid, and covers over the convex surface of the vitreous 

 humour*. The whole extent of the retina, which is equally nervous and 

 sentient, may receive the impressions of the luminous rays, though this 

 faculty has, by several philosophers, been exclusively assigned to its cen- 

 tral part, called the optical axis or porus opticus. This central part is 

 easily recognized, in man, by a yellow spot discovered by Soemmering: 

 in the middle of this spot, situated at the. outer side of the entrance of 

 the optic nerve into the globe of the eye, there is seen a dark spot and a 

 slight depression, the use of which is not understood. This peculiar 

 structure, recently discovered, is met with only in the eye of man and 

 of monkeys. 



CXIX. Mechanism and phenomena of vision. The rays of light passing 

 from any point of an enlightened object, form a cone of which the apex 

 answers to the point of the object, and of which the base covers the an- 

 terior part of the cornea. All the rays, more diverging, which fall 

 without the area of the cornea on the t eye-brows, the eye-lids, and the 

 sclerotica are lost to vision. Those which strike the mirror of the eye 

 pass through it, under a refraction proportioned to the density of the 

 cornea, which much exceeds that of the atmosphere, and to the convexity 

 of that membrane, approaching the perpendicular, they now pass through 

 the aqueous humour, less dense, and fall upon the membrane called the 

 iris. All those that fall upon this membrane are reflected, and show its 

 colour, different in different persons, and apparently depending on the 

 organic texture, and on the particular and very diversified arrangement 

 of the nerves, of the vessels, and cellular tissue, which enter into its 

 structure. None but the most central traverse the pupil and serve to 

 sight. These will pass that opening, in greater or less number, as it is 



* The optic nerves differ very remarkably from the other cerebral pairs, both in their 

 thickness, and in the delicacy of their substance, which appears to be an immediate 

 continuation of medullary fibres of the brain, to which the meninges furnish one com- 

 mon envelope, and not a distant membranous canal for each fibre. 



The nerves of vision cross each other before the cella tunica, in a manner similar to 

 the primitive crossing, which their roots, as well as those of the other nerves, undergo 

 in the substance of the brain. These double decussations may be said to neutralize each 

 other, and consequently, each optic nerve may be considered to arise primarily from 

 the hemisphere corresponding to the eye which it supplies. In hemiplegia, the affected 

 eye is not on that side of the body struck with the paralysis it is on the opposite side 

 that the pupil is dilated, and this pathological phenomenon, which is easily proved in 

 persons who have experienced a paralytic seizure, seems to be one of the best proofs 

 which can be opposed to those who suppose that the optic nerves, where they approach 

 each other, experience an approximation of their fibres only. It may be farther argued, 

 in support of the actual decussation of these nerves, that the wasted optic nerve of an 

 eye that has been for some time in a state of atrophy can be traced towards the opposite 

 lobe of the brain ; and that an evident crossing of the optic nerve may be observed m 

 many of the classes of fishes. Copland. 



