THE SENSES. 523 



without affecting the external configuration of the cornea. The pos- 

 terior and larger portion of the cavity of the eyeball is filled by a semi- 

 fluid gelatinous substance, the vitreous body, so called from its trans- 

 parent and glassy appearance. Its refractive power, according to 

 Helmholtz, though slightly greater than that of the aqueous humor, 

 does not differ much from that of water. It distends the greater part 

 of the cavity of the sclerotic, supports the retina which lies upon its 

 surface, and preserves the spheroidal form of the eyeball. 



The vitreous body is enveloped by an exceedingly thin, colorless 

 membrane, for the most part without definite structure, and according 

 to Kolliker, not more than 4 mmm. in thickness. This is the " hyaloid 

 membrane " (Fig. 130, 10 ). It extends over the posterior and middle 

 portions of the vitreous body to a zone 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 Zmn, extends forward and inward, remaining adherent to the ciliary 

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

 of its lateral border. The posterior layer passes inward and a little 

 backward, terminating also in the capsule of the lens, but a little behind 

 its lateral border. The triangular canal between the two layers of the 

 hyaloid membrane and the lateral border of the lens is the canal of 

 Petit (Fig. 130, n ), and is filled with a transparent serosity. The lens 

 is thus suspended, from all sides, by a double layer derived from the 

 hyaloid membrane. 



Crystalline Lens. The lens is a transparent, refractive body, with 

 convex anterior and posterior surfaces, placed directly behind the pupil, 

 where it is retained in position by the counterbalancing pressure of the 

 aqueous humor and the vitreous body, and by the suspensory layers of 

 the hyaloid membrane. 



As its refractive power is greater than that of the cornea or the 

 aqueous humor, it acts, by virtue of its double-convex form, as a con- 

 verging lens, to change the direction of rays passing through it, and 

 bring them to a focus behind its posterior surface. The amount of con- 

 vergence thus effected by a refractive lens depends on the substance 

 of which it is composed and the curvature of its surfaces. The stronger 

 the curvatures, for lenses composed of the same material, the greater 

 their refractive power for luminous rays. In the crystalline lens of the 

 human eye, the two surfaces are different in curvature ; the anterior 

 being comparatively flat, the posterior more convex. According to the 

 estimates of Listing, based on a variety of measurements and adopted 

 by Helmholtz, the radius of curvature for the anterior surface is, on 

 the average, 10 millimetres, that for the posterior surface 6 millimetres. 



This makes the crystalline lens the strongest refracting body in the 

 eyeball, and by its aid parallel or diverging rays are brought to a focus 

 at the retina. This effect is not due entirely to the lens, since the con- 

 vex form of the cornea and the more or less spheroidal figure of the 

 whole eyeball have in some degree a similar action. According to 



