% IN HUMAN PHYSIOLOGY. 195 



and glaring light. The hairs of the eyebrows overlap 

 each other and are set obliquely outward. 



25. What is the use of winking? 



It serves to wash the eyeballs, and thus keep the " win- 

 dows of the soul " clean. The necessity for winking is 

 shown by the great effort required to restrain it even for 

 a short time. First discomfort, then congestion of the 

 mucous membrane, and then a profuse watering of the 

 eye follow any attempt at stopping this necessary act. It 

 is an obscure sense of discomfort, not usually noticed by 

 the consciousness, that excites this movement, the objects 

 of which are periodically to cleanse the exposed part of 

 the eyeball, to moisten and lubricate it with the secretions 

 from the neighboring glands, and probably in this way to 

 aid in the preservation of the polish and translucency of 

 the epithelial layer on the transparent portion of the 

 globe. At the same time it carries towards the inner 

 corner all foreign bodies, and directs the residual secre- 

 tions towards the lachrymal ducts. Finally it allows a 

 brief but periodical rest to the levator muscle of the upper 

 eyelid. MARSHALL. 



26. When you wink, do the eyelids touch at once along 

 their whole length ? Why ? 



In winking, both lids move, but the upper one much 

 the more extensively. Moreover, they do not come in 

 contact all along their margins at the same instant of 

 time, but meet first at the outer corner and then rapidly 

 inwards as far as the lachrymal papillae on which the lach- 

 rymal ducts are situated. By this sweeping movement, all 

 foreign bodies are carried to the lachrymal lake. MAR- 

 SHALL. 



