8fi 



THE MODERN SYSTEM 



increased vision : and in the Horse, most es- 

 pecially, it, as a matter of course, follows, that 

 the rays strike the nerve twice, only at the 

 upper part, as it is there that the green pig- 

 m.ent is situated ; but these rays are obviously 

 the weakest, especially when the Horse's head 

 is near the ground, which is a natural position 

 from his propensity to grazing ; the black 

 pigment also receiving the strong superior 

 rays, and the green pigment being the 

 Aveakest : thus, by this combination the rays 

 are equalized, and sharp keen sight pro- 

 duced. 



THE CRYSTALLINE LENS. 



The crystalline lens is not exactly a per- 

 fect lens, the anterior side being rather the 

 flattest ; it is contained in a perfect capsule or 

 bag, a fluid separating the capsule from the 

 lens, which is called the liquor morgagn' 

 (from a person of that name, M. Morgagni, 

 who first discovered it). This capsule does 

 not adhere to the tunica vitrea, but is confined 

 in its situation by the tunica vitrea, being re- 

 flected over its edge, at which part also it is 

 surrounded by the choroid coat, formed into 

 folds, called the ciliary processes. It was a 

 doubt formerly how these parts grew, but 

 that mystery has since been solved, on dis- 

 covering that both arteries and veins are 

 possessed in these parts ; for as they grew, of 

 course there must be materials required. But 

 this was not in early times believed, in conse- 

 quence of the parts being so very transparent, 

 but the transparency depends upon the situa- 

 tion of its vessels and its organization ; but this 

 fine arrangement may be destroyed in a se- 

 cond, its organization lost and become opaque. 

 The lens are not only flatter anteriorly, but it 

 is not of the same structure throughout, the 



outer surface being like a mucus or jelly, 

 but within it becomes harder, even to the 

 centre ; thus you at once see how the rays of 

 light are bent from one degree to another, in 

 in proportion as they approach the centre. 

 From this cause, in conjunction with the iris 

 conveying to the eye that singular property of 

 acting both as a microscope and a telescoj^e, 

 which in no one instrument has been efiiected 

 by man. 



There have been many conjectures on this 

 point ; but my opinion is, it depends upon the 

 lens, and there is one circumstance that bears 

 me out ; and that is, after the operation for 

 cataract, the person so operated on, em- 

 ploys two sorts of glasses, concave and convex 

 ones. But the iris now comes here under 

 observation, as it alters under different effects 

 of light. This is an involuntary power ; but 

 there is another power independent of light, 

 for, if I vvill, to look at a minute object, 

 though the light be weak, and consequently 

 less stimulus, yet the pupil will become 

 smaller, by the iris expanding ; and looking 

 at a large object in a strong light, the pupil 

 will expand in defiance. The pupil being 

 small, the rays (as in the first instance) are 

 admitted and confined to those from the 

 object alone. Thus there is no confusion of 

 images on the retina, but the smallest object 

 is seen, and, as in the last instance, when the 

 object to be seen is large, the mind excites 

 the iris to contract, that the rays may be ad- 

 mitted from all parts through the lens, for a 

 perfect representation. 



THE CILIARY PROCtSSES. 



In examining the ciliary processes, which 

 appear to be a continuation of the choroid 

 coat, that surround the capsule of the crystal- 



