172 



EYE. 



sphere, and at a short distance behind the trans- 

 parent convex portion or cornea, is fixed a 

 double convex lens, called the crystalline lens 

 or crystalline humour; and between this cor- 

 nea and crystalline lens is interposed a parti- 

 tion or screen called the iris, with a circular 

 aperture in its centre called the pupil. The 

 inner surface of this hollow sphere, as well as 

 the back of the iris or screen, are covered or 

 stained with a black material. The space be- 

 tween the cornea and crystalline lens, in which 

 the iris is placed, is filled with a transparent 

 fluid, called the aqueous humour, and the 

 space between the crystalline lens and the bot- 

 tom of the sphere is filled with a similar fluid, 

 called the vitreous humour. The annexed figure 

 represents a section of this simple piece of opti- 

 cal mechanism, much larger than natural to 

 render the parts more distinct. 



Fig. 100. 



An acquaintance with the laws which regu- 

 late the transmission of the rays of light through 

 transparent bodies, and with the manner in 

 which the lenticular form changes the direction 

 of these rays, teaches that a correct image of ex- 

 ternal objects is formed in the bottom of the eye 

 in consequence of the above adjustment of its 

 parts. First, the rays of light acquire a con- 

 vergence in their passage through the cornea 

 and aqueous humour, then the central portion 

 of the pencil of rays is transmitted through the 

 pupil, and, finally, the rays in their passage 

 through the crystalline lens acquire such addi- 

 tional convergence, that they are brought to a 

 focus on the bottom as represented in the an- 

 nexed diagram. 



Such are the essential component parts of 

 the eye, considered as a piece of optical me- 

 chanism, but viewed as a piece of anatomical 

 mechanism, its construction is much more com- 

 plicated, and the materials of which it is com- 

 posed are necessarily totally different from those 

 of any human contrivance of a similar nature. 

 It lives in common with the body of which it 

 forms a part, it grows and is repaired ; conse- 

 quently, the animal organisation destined for 

 such functions must constitute an essential 

 part of its construction. 



The organ derives its permanent spherical 

 form, its external strength, and the support of 

 the delicate parts within it, from a strong opaque 

 membrane called the sclerotic coat; while 

 the convex portion, called cornea, in front, 

 equally strong, being transparent, allows the 

 rays of light to pass without interruption. The 

 interior of the portion of the sphere formed by 

 the sclerotic coat is lined throughout by a soft 

 membrane called the choroid, necessarily con- 

 stituting another hollow sphere, accurately 

 adapted and adhering to the inside of the for- 

 mer. This also has its circular aperture ante- 

 riorly, into which is fitted the screen called iris, 

 as the cornea is fitted into the aperture in the 

 sclerotic. While the external surface of this 

 choroid coat is comparatively rough and coarse 

 in its organization, as it adheres to the equally 

 coarse surface of the sclerotic, the interior is 

 exquisitely smooth and soft, being destined to 

 embrace the retina, another spherically dis- 

 posed membrane of extreme delicacy. The 

 screen called iris, which is fitted into the cir- 

 cular aperture anteriorly, is as different from 

 the choroid coat in its organization as the cor- 

 nea is from the sclerotic : it is perfectly plane, 

 and therefore forms with the concave surface 

 of the cornea a cavity of the shape of a plano- 

 convex lens, called the anterior chamber. In 

 or on the choroid coat the principal vessels and 

 nerves, destined to supply the interior of the 

 organ, are distributed, and in its texture and 

 upon its inner surface is deposited the black 

 material, which in this part of the chamber, as 

 well as on the back of the iris, is so essential a 

 provision. At the anterior margin the choroid 

 is more firmly united to the corresponding mar- 

 gin of the sclerotic by a circular band of pecu- 

 liar structure called the ciliary ligament, and on 

 its inner surface, in the same place, it is fur- 



Fig. 101. 



