648 TEXT-BOOK OF PHYSIOLOGY. 



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. 306). 



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. 



Media. The vitreous humor is the largest of the refracting media and 

 occupies by far the largest portion of the interior of the eyeball. From its 

 position it gives support to the retina. Anteriorly it presents a concavity, 

 in which the crystalline lens is lodged. The vitreous humor consists of 



FIG. 307. HORIZONTAL SECTION OF THE EYEBALL, i. Sclera. 2. Cornea. 3. Chorioid. 

 4. Iris. 5. Ciliary muscle. 6. Retina. 7. Lens. 8. Suspensory ligament. 9. Canal of Schlemm. 

 10. Canal of Petit, u. Optic nerve. (Deaver). 



water (97 per cent.), organic matter and salts, enclosed in a transparent 

 membrane, the tunica hyaloidea. The mass of the vitreous humor is pene- 

 trated by a species of connective tissue. 



The aqueous humor is small in amount in comparison with the vitreous 

 and is found in the space bounded by the cornea, the ciliary body, the sus- 

 pensory ligament, and the lens. The projection of the iris into this space 

 partially divides into an anterior and posterior portion or chamber. The 

 aqueous humor is a clear, watery, alkaline fluid derived from or secreted 



