426 ORGANS OF THE SENSES. 



The vitreous humor or hyaloid meinbrane is composed of 

 collagenous tissue, and resembles the gelatine of Wharton, 

 especially in young subjects ; it is contained in a very thin 

 and transparent sack, called the hyaloid membrane. 



Fig. 113. Development of the crystalline lens (Remak).* 



B. Refraction. This group of media forms, in a physical 

 point of view, a series of three lenses, differing greatly from 

 each other : the first, consisting of the cornea and the aqueous 

 humor, is a convexo-concave lens, and is extremely compli- 

 cated on account of the various layers of which the cornea 

 is composed. The second, or crystalline, is a bi-convex lens, 

 the anterior surface of which is more curved than the back ; 

 this, too, is very complicated, its concentric layers increas- 

 ing in density from the periphery to the centre. Finally, in 

 the third place, the vitreous body consists of a concavo-con- 

 vex lens, being hollowed out in front for the reception of the 

 crystalline lens. Immediately behind this latter lens is found 

 a membrane which is sensitive to light and which is called the 

 retina. 



Let us suppose for the sake of simplicity that, instead of 

 these three lenses, we have a single lens of the same total 

 converging power, and we shall then easily understand the 

 final result of the progress of the rays of light. The whole 

 system, in short, may be represented by a lens formed of a 

 substance whose index of refraction is from 1.39 to 1.40, 

 and whose focal distance is 17 mm., 0.48. The luminous rays, 

 from a given point outside, diverge as they fall upon the 



* A, B, C mark the different degrees of imagination or separation of the 

 pouch from which the crystalline lens is formed. 1, Epidermic fold. 2, Thick- 

 ening of this fold, pouch of the separated crystalline (at B). 3, Crystalline 

 fossette, which will afterwards appear as the centre of the lens. 4, Primitive 

 ocular vesicle (nerve pouch arising from the encephalic centre), the forepart of 

 which has a depression for the lens. 7, Cavity formed by the compression of the 

 ocular vesicle, and which is to be occupied by the vitreous body. 6, Point at 

 which the lens is detached from the epidermic fold. 



