THE CORNEA, IRIS AND AQUEOUS HUMOR 809 



These channels are lined with columnar epithelium which becomes 

 ciliated in places. Their walls are strengthened by muscle tissue which 

 on contraction tends to enlarge their lumen. This is especially true 

 of Homer's muscle which envelops the posterior wall of the lacrimal 

 sac and which, during the closure of the lids,- widens this passage and 

 aspirates the tears through the dilated punctum. Conversely, the 

 opening of the lids tends to compress the lacrimal sac so that its con- 

 tents are forced onward into the nasal duct. 1 At this time the 

 sphincter-like punctum is closed, while the valve of Hasner guarding 

 the orifice of the nasal duct, is opened. It should also be mentioned 

 that the tears are ordinarily prevented from escaping across the edges 

 of the eyelids by the oily deposits furnished by the Meibomian glands. 

 The latter are sebaceous in character and are arranged in rows along the 

 inner margin of each lid. The tears themselves are alkaline in reaction 

 and are chiefly composed of water (98.1 per cent.). They contain 

 albumin (0.1 per cent.) mucin, epithelial cells (0.1 per cent.) and salts, 

 principally sodium chlorid (0.4 to 0.8 per cent.). 



CHAPTER LXIX 

 THE CORNEA, IRIS AND AQUEOUS HUMOR 



The Refractive Power of the Cornea. The cornea of the mam- 

 malian eye is a perfectly stationary structure possessing a certain 

 curvature and refractive power. In the birds, on the other hand, it is 

 set in a cartilaginous ring and its convexity may be altered by muscular 

 activity. This fact indicates that in these animals it is made to serve 

 as a powerful adjunct to the lens and thus, is in large part responsible 

 for the keen sense of vision possessed by them. In the higher mam- 

 mals, its importance is relatively slight, because its radius of curvature 

 is only 7.8 mm., 2 but this measurement pertains only to its central 

 area situated directly in front of the pupil. Its marginal zone is of 

 practically no optical importance even when the pupillar orifice is 

 enlarged. It may be concluded, therefore, that the cornea, by virtue 

 of its convexity, renders the entering rays of light slightly more con- 

 vergent. In addition, it collects many of the otherwise too divergent 

 rays, and directs them through the pupillar opening so that they may 

 still be subjected to the refraction of the lens. 



The Aqueous Humor. It has been stated above that the anterior 

 cavity of the eyeball consists of the anterior and posterior chambers, 

 the former being situated in front of the iris and the latter, between 

 the posterior surface of this partition and the anterior aspect of the 



1 Scimeni, Archiv fur Physiol., 1892, Suppl. 291. 



2 Helmholtz, Physiolog. Optik, Berlin, 1896. 



