700 TEXT-BOOK OF PHYSIOLOGY 



From the relation of the ganglion cells, in which the optic nerve-fibers 

 take their origin, to the visual cells and the bipolar cells, the former may 

 be regarded as the terminal visual organ, the intermediary between the 

 ether vibrations and the ganglion cell. The visual cells are directed toward 

 the chorioid, away from the entering light, dipping into the pigment cells. 

 They, with the pigment-layer, are the elements by which the ether vibrations 

 are transformed into nerve energy. 



In the fovea most of the retinal elements are wanting or are reduced in 

 thickness. The cones alone are present. The cone-fibers with their nuclei 

 are directed obliquely upward and outward along the slope of the fovea, 

 to end in tufts which come into physiologic relation with the dendrites of 

 the ganglion cells, which at the top of the fovea are generally increased in 

 number (Fig. 287). 



It is estimated that the optic nerve contains about 500,000 nerve-fibers, 

 and that for each fiber there are about 7 cones, 100 rods, and 7 pigment 

 cells. In accordance with this estimate there would be about 3,500,000 

 cones, 50,000,000 rods, and 3,500,000 pigment cells. The distance be- 

 tween the centers of two adjacent cones in the fovea is 4 micromillimeters. 



The Refracting Media. The refracting media enclosed by the fore- 

 going membranes are the aqueous humor, the lens and the vitreous humor. 



The Aqueous Humor. The aqueous humor is small in amount in compari- 

 son with the vitreous and is found in the space bounded by the cornea, the 

 ciliary body, the suspensory ligament, and the lens. The projection of the 

 iris into this space partially divides it into an anterior and a posterior portion 

 or chamber. The aqueous humor is a clear, watery, alkaline fluid derived 

 from or secreted 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 believ- 

 ing 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. Inasmuch as 

 the aqueous humor has the same refracting power as the cornea the two may 

 be regarded as a single body. 



The Lens. 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 contain- 

 ing elongated hexagonal fibers which, having then* 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 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 pro- 

 cesses. The former becomes attached to the posterior 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 plica- 

 tions on the surface of the ciliary processes. 



The Vitreous Humor. The vitreous humor is the largest of the refracting 



