176 ON THE SENSES. 



— form tlie different mediums tlirougli whicli are transmitted the rays of light 

 proceeding from external objects to the retina. This last is a sensitive mem- 

 brane which lines the background of the eye, and upon it the rays, after under- 

 going refraction in their passage through the mediums just mentioned, again 

 unite to form an image .of the object from which they issued. The convex 

 cornea and the crystalline lens together correspond to the glass lens in the front 

 opening of the camera obscura. The iris represents a blind or curtain which 

 intercepts all the rays which enter through the lateral parts of the cornea and 

 for ceitain reasons would disturb the formation of a distinct image, so that only 

 the rays transmitted through the central opening of the cornea — the pupil — are 

 permitted to penetrate to the background of the eye. The pupil, an incon- 

 spicuous opening, often scarcely a line in width, is the window through which 

 we observe the world, or, rather, through which the world penetrates to us, since, 

 through this aperture, everything emitting light, from the inconceivably remote 

 t.i the nearest and most minxTte, if it falls within the sphere of vision, throws a 

 part, if but a small part of its rays on the delicate organ whose office it is to 

 convert them into perceptions of light and color and of the locdl relations of 

 visible things. Hapless is he for whom this small window is closed, for whom 

 the transparent lens is clouded ; he is indeed despoiled of his most precious 

 heritage. 



The retina, an extremely delicate, almost transparent membrane, which, as 

 the figure shows, is concentrically outspread upon the inner surface of the cho- 

 roid nearly as far as the edge of the lens, and which thus, lines the cavity filled 

 with the vitreous body, is nothing else than the flattened extension of the end 

 of the optic nerve J but of how wonderfully compounded a structure, of how ad- 

 mirable a distribution and stratified arrangement of the separate and delicate 

 elements. It is impossible, in a rapid sketch, to convey to the novice a com- 

 plete idea of the mechanism of the retina; we shall give, therefoi'e,.but a few 

 outlines. The optic nerve O penetrates, with its vast number of closely com- 

 pressed filaments, the outer integuments of the eye at the point of entrance, and 

 advances to the cavity of the vitreous body ; having reached this, its collectivcf 

 fibres bend around on all sides at right angles, and spread themselves in all di- 

 rections into a beautiful network of fasciculi or bundles disposed in the plane of 

 the retiiia, and reaching as far as the anterior margin of that membrane. Every- 

 Vi'here in this course single fibres bend around from this innermost layer of th j 

 retina to the external layers, and terminate on the outer surface of the retina in 

 rod-like and flask-shaped or conical organizations, which, as the proper terminal 

 apparatus of the optic nerve, enable, as will hereafter be seen, the waves of light 

 to take effect on its fibres. The external layer of the retina consists of a del- 

 icate mosaic of these rods and cones, which are all disposed perpendicularly to 

 the surface ; it represents, therefore, a mosaic of the extremities of the optic 

 nerve, of which extremities those situated in the part of the retina directly op- 

 posite to the pupil are most closely aggregated with one another ; those, on the 

 contrary, situated laterally to the former, stand so much the more widely apart 

 the nearer their position to the anterior margin of the retina, a fact which we 

 shall make use of further on. The retina thus constituted corresponds, as is 

 apparent from the foregoing, to the smooth tablet or photographic collodion 

 plate in the camera obscura. 



To examine the texture of the rest of the organizations of the eye would 

 lead us too far ; one point, however, must not be passed without notice. Every 

 one has probably observed, or can at any moment satisfy himself, that the 

 pupil, which appears as a round black spot in the middle of the eye, is some- 

 times wider, sometimes narrower, according as the sight is exerted in obscurity 

 or in light. The iris owes this power of changing the diameter of the pupil to 

 certain muscular fibres, microscopically fine, which, at the excitation of the 

 nerves leading to them, become shorter in their longitudinal diameter, and are 



