SENSE OF SIGHT. 219 



sleepy person. No sooner does he begin to grow drowsy, 

 than his eyes begin to close ; in other words, " they grow 

 heavy." If he wishes to keep awake he makes a desperate ef- 

 fort to open his lids ; and in the drunkard, whose involuntary 

 muscles overpower the voluntary ones, it is quite ludicrous to 

 watch the expression of countenance, and see how quick the 

 disobedient, rebel lids close, upon the oft-repeated efforts to 

 keep them open. 



46. The opening and closure of the lids ordinarily occu- 

 pies about the eighth part of a second, so that there is no 

 interruption to the continuance of vision. Besides serving 

 as a curtain to protect the eye, the lids serve, by their mo- 

 tions to diffuse the tears and mucous secretions, thus keep- 

 ing the surface of the cornea moist, and wiping off any 

 foreign matter, as dust, that may be lodged on it. The eye- 

 lids when closed, do not entirely prevent the transmission of 

 light ; for when we have been refreshed by sleep, the stimulus 

 of the light falling upon the lids, is sufficient to arouse sen- 

 sibility and awake us. 



47. It is a curious fact, that while the upper eye-lid falls, 

 the lower eye-lid is moved towards the nose, and thus aids 

 in collecting all offensive particles in the corner of the eye. 

 This can readily be shown by marking the edges of the lids 

 with a black spot ; when the lids are opened and closed, the 

 spot on the upper eye-lid will rise and fall perpendicularly, 

 while that on the lower one will play horizontally like a 

 weaver's shuttle. When any thing gets into the eye, there 

 is this continual play of the lids maintained, till the offend- 

 ing particles have been driven into the inner corner of the 

 eye, when relief is at once obtained. When the object is 

 very small, or not sufficiently irritating to excite these 

 muscles of the lids to action, it is common to p-lace an eye- 

 stone, which is a smooth piece of sulphate of lime in the 

 eye. This not only excites an abundant secretion of tears, 

 but also causes the necessary motions of the lids, by which 

 the object is soon washed into the angle of the ye. 



