THE SENSE OF SIGHT 685 



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

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

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

 lether 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. 290). 



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

 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>5> 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 

 this origin it passes through the pupil into the anterior chamber, 

 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 

 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^ 

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

 ing elongated hexagonal fibers which, having their ori.m near the anterior 



" 



the anterior surface of the lens near the equator. The space between he 

 two kyers of "he ligament is the canal of Petit. The anterior surface of the 

 Sament presents a series of plications conforming to correspondmg &<* 



largest of the refracting 



