SENSE OF SIGHT. 613 



" hyaloid membrane" (Fig. 189, 10). Its inner surface is in contact with 

 the vitreous body, its outer surface with the retina. It extends unin- 

 terruptedly over the posterior and middle portions of the vitreous body 

 until it reaches a point anteriorly corresponding with the ciliary body 

 of the choroid. Here it becomes thicker and divides into two layers. 

 The anterior layer, which is the stronger of the two, the zone of Zinn, 

 extends forward and inward, remaining adherent to the folds of the 

 ciliary body, and terminates in the capsule of the crystalline lens, just 

 in front of its lateral border. The posterior layer of the hyaloid mem- 

 brane, after separating from the anterior, passes inward and a little 

 backward, and terminates also in the capsule of the lens, but a little 

 behind its lateral border. The triangular canal left between the two 

 separated layers of the hyaloid membrane and the lateral border of the 

 lens is the canal of Petit (Fig. 189, n), and is filled with a little trans- 

 parent serosity. The lens is thus suspended on all sides by a double 

 layer derived from the hyaloid membrane. The anterior portion of 

 this double layer, or the zone of Zinn, being the stronger of the two, 

 and presenting a distinctly fibrillated texture, is regarded as more 

 especially fulfilling the part of a suspensory ligament of the crystalline 

 lens. 



Crystalline Lens The lens is a transparent, refractive body, of cir- 

 cular form, with convex anterior and posterior surfaces, placed directly 

 behind the pupil, and retained in its position by the counterbalancing 

 pressure of the aqueous humor and the vitreous body, and by the two 

 layers of the hyaloid membrane attached to its capsule round its circular 

 border. It is composed of flattened fibres, adherent to each other by 

 their adjacent surfaces and edges, and so arranged as to pass in a 

 curvilinear direction, parallel to the surface of the lens, from one of its 

 two opposite poles to the other. Notwithstanding the fibrous structure 

 of the lens, the ribbon-shaped elements of which it is composed being 

 united by simple juxtaposition, without the intervention of any different 

 material, the entire body is transparent, and allows the passage of the 

 light without perceptible absorption or irregular dispersion. 



As the refractive power of the substance of the crystalline is greater 

 than that of the cornea or the aqueous humor, it acts, by virtue of its 

 double-convex form, as a converging lens, to change the direction of 

 the luminous rays passing through it, and bring them to a focus at 

 some point situated behind its posterior surface. The amount of con- 

 vergence thus effected by a refractive lens depends both upon the index 

 of refraction of the substance of which it is composed and the greater 

 or less curvature of its surfaces. The stronger the curvatures, for 

 lenses composed of the same material, the greater the amount of con- 

 vergence impressed upon luminous rays passing through them. In the 

 case of the crystalline lens of the human eye, the two surfaces are dif- 

 ferent in curvature ; the anterior surface being comparatively flat, the 

 posterior much more convex. According to the estimates of Listing, 

 based upon a variety of measurements, and adopted by Helmholtz, the 





