THE SENSE OF SIGHT. 



605 



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

 transparent membrane, the tunica hyaloidea. The mass of the vitre- 

 ous humor is penetrated by a species of connective tissue. 



The aqueous humor is small in amount in comparison with the 

 vitreous and is found in the space between the cornea and the lens 

 (the anterior and posterior chambers). It is a clear, watery, alkaline 



FIG. 281. HORIZONTAL SECTION THROUGH THE MACULA AND FOVEA OF A MAN 

 SIXTY YEARS OLD. The section is not through the exact center of the fovea, 

 for there are only cone visual cells and no remnants of the confluence of the inner 

 granule and ganglion cell layers are present, i. Cones. 2. External limiting 

 membrane. 3. Outer nuclear layer. 4. Henle's fiber layer. 5. Outer molec- 

 ular or reticular layer. 6. Inner nuclear layer. 7. Inner molecular or reticular 

 layer. 8. Layer of ganglion cells. 9. Nerve-fiber layer. (After Schaper, 

 Stohr's" Histology.") 



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 partly depends 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 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 



