THE SENSE OF SIGHT. 649 



by the capillary blood-vessels of the ciliary body. From this origin it passes 

 through the pupil into the anterior chamber. It serves to keep the cornea 

 tense and smooth. The ocular tension depends partly on the presence of 

 this fluid in the eyeball. There is every reason for believing that there is a 

 constant stream of fluid from the blood-vessels into the eye and from the 

 eye through the spaces of Fontana at the base of the iris into the canal of 

 Schlemm, and so into the blood. Any interference with the exit of this fluid 

 rapidly increases the intra-ocular tension. 



The lens is the transparent biconvex body situated just behind the iris, 

 in the concavity of the vitreous. The thickness of the lens is 3.6 mm., 

 the diameter about 9 mm. It consists of a transparent capsule containing 

 elongated hexagonal fibers which, having their origin near the anterior 

 central portion of the lens, pass out toward the margin, where they bend 

 around to terminate in a triradiate figure on the opposite side. Chemically 

 the lens consists of water, a globulin body (crystallin) , and salts. 



The Suspensory Ligament. The lens is held in position by the suspensory 

 ligament, formed in part by the hyaloid membrane and in part by fibers 

 derived from the ciliary processes. The former becomes attached to the pos- 

 terior surface, the latter to the anterior surface of the lens near the equator. 

 The space between the two layers of the ligament is the canal of Petit. The 

 anterior surface of the ligament presents a series of plications conforming 

 to corresponding plications on the surface of the ciliary processes. 



The relations of all the parts entering into the structure of the eye are 

 shown in Fig. 307. 



THE PHYSIOLOGY OF VISION. 



The Retinal Image. The general function of the eye is the formation 

 of images of external objects on the free ends of the percipient elements of 

 the retina, the rods and cones. The existence of an image on the retina 

 can be readily seen in the excised eye of an albino rabbit, when placed 

 between a lighted candle and the eye of an observer. Its presence in the 

 human eye can be demonstrated with the ophthalmoscope. It is this image, 

 composed of focal points of luminous rays, that stimulate the rods and 

 cones, which is the basis of our sight-perceptions, and out of which the mind 

 constructs space relations of external objects. In only two essential re- 

 spects as far as space relations go, does the image on the retina differ from 

 the appearance of the object, aside from the fact that the object has usually 

 three, the image only two, dimensions viz., in size and position. What- 

 ever the distance, the image is generally smaller than the object; it is also 

 reversed, the upper part of the object becoming the lower part of the image, 

 and the right side of the object the left side of the image. 



The Dioptric Apparatus. The formation of an image is made pos- 

 sible by the introduction of a complex refracting apparatus consisting of the 

 cornea, aqueous humor, lens, and vitreous humor. Without these agencies 

 the ether vibrations would give rise only to a sensation of diffused luminosity. 

 Rays of light emanating from any one point that is, homocentric rays- 

 arriving at the eye must traverse successively the different refracting 

 media. In their passage from one to the other, they undergo at the surfaces 



