270 



HUMAN PHYSIOLOGY 



CHAP. 



the nose. Sometimes this duct closes up, and then 

 tears constantly gather at the inner angle of the eye. 

 It should be opened by an oculist so that the tears may 

 drain off as they should. 



1. With a small mirror examine carefully the orbits of your own 

 eyes. Notice that a ruler placed on the bones of the orbit does not 

 touch the eyeball at all. 



2. Study the eyebrows and notice their position, and the direction 

 and color of the hair. 



3. Study the eyelids. Notice a rudimentary lid at the inner 

 corner of each eye. Do you find an opening on an elevation at the 

 inner corner of the lower lid? This is the opening into the lachrymal 

 canal. 



186. The Inside of the Eyeball. The eyeball is a 

 large hollow globe filled with liquids. The space just 

 back of the cornea is filled with aqueous humor, in which 

 the iris floats. Beyond this we find the crystalline lens, 

 which is about one third of an inch long and composed of 

 a number of concentric layers like the coats of an onion. 

 The lens curves out both in front and behind ; we say it 



is double convex. The 

 main part of the eyeball 

 is filled with the vitreous 

 (glassy] humor, that looks 

 like the white of an egg 

 and serves to keep the deli- 

 cate retina in position. 



FlG. 145. Showing changes in the 

 lens in accommodation (Zuppke). 



The two humors and the crystalline lens bend the 

 rays of light that enter the pupil of the eye so that 

 they are brought to a point on the sensitive retina. 

 The lens is important in focusing the light because it 



