7l6 THE SENSES 



The Eyelids and Lachrymal Apparatus. The eyeball is kept moist 

 over its free surface and protected from external injury by the eyelids, 

 by the glands that secrete the lachrymal fluid to moisten the surface of the 

 cornea, and by the oil glands that secrete oil on the margins of the lids. 



The conjunctiva, or lining membrane of the lids, which is reflected on to 

 the free surface of the eyeball, protects the eye from injury by its extraor- 

 dinary sensitiveness to irritation by dust or other external substance. Its 

 stimulation produces reflex secretion of the lachrymal fluid that flows over 

 the surface of the eye, and tends to wash away the stimulating substance. 



FIG. 441. Section of the Eyeball. 



The Eyeball and its Parts. The detail of the structure of the eye- 

 ball is too complex to be given here except in so far as seems necessary for a 

 clearer presentation of the physiological facts. A gross dissection reveals 

 the tough, white, outer coat, the sdera; the intermediate thin, vascular 

 pigmented coat, the choroid; and the inner nervous coat, the retina. 



The section also shows that the eyeball is specialized in structure in its 

 anterior region and that its contained cavity is divided into two parts, viz., 

 the anterior and posterior chambers. The anterior chamber is filled with the 

 transparent aqueous fluid. This fluid is like lymph in its composition. 

 The posterior chamber between the lens and the retina is filled with the 

 clear jelly-like vitreous substance. 



The Cornea. The sclera is continuous with the cornea in front of the 

 eyeball, but the cornea is transparent and its radius of curvature is less than 

 that of the main portion of the eye. It is composed of stratified epithelial 

 cells, and is richly supplied with sensory nerves that form an intraepithelial 



