112 



CONSCIOUS NKK'Ynrs n]>K|{ ATK'NS 



edges, in little grooves, are minute glands from which an 

 oily secretion flows to the free edges of the lids and 



prevents their adhesion 

 when closed. Above 

 the orbits the thick 

 ridges set with hairs, 

 called eyebrows, also 

 serve to shield and 

 shade the eye. 



In a depression in the 

 upper and outer part of 

 the orbit lies the lach- 

 rymal gland, with ducts 

 opening on the inner 

 surface of the upper 

 lid. It secretes a wa- 

 tery fluid designed to 

 lubricate the surface of 

 the eyeball. When stimulated by the irritation of the 

 mucous membrane of the eye, the nose, or the mouth, or 

 by strong mental emotion, the lachrymal fluid becomes 

 excessive, and is called tears. Canals or ducts placed at 

 the inner angle of the eye carry off the ordinary supply 

 of lachrymal fluid to the nasal passages. Branches from 

 the fifth (trigeminal) cranial nerve supply this gland and 

 send fibers also to the eyelids and to inner portions of the 

 eyeball. 



151. The Eye as an Optical Instrument. As has been 

 said, the eye is like a photographer's camera obscura, the 

 various parts of which all have to do with the produc- 

 tion of distinct images of external objects upon the back 

 portion of the box ; that is, in the camera upon the ground 

 glass screen, in the eye upon the nervous membrane called 



Fig. 67. Front view of the right eye, 

 showing the position of the lachrymal 

 apparatus. 



G lachrymal gland. 



CC upper and lower lachrymal ducts. 



B naso-lachrymal duct. 



