OF SIGHT. 179 



NOTES. 



(A) A delicate transparent membrane has been discovered by 

 Dr. Jacob of Dublin, between the retina and chorioid, and 

 adhering to both.* 



(B) I am not satisfied with any account which I have hitherto 

 seen, of the function of the eyelids with respect to the tears. 

 Perhaps the tears pass over the ball of the eye as low as the edge 

 of the superior tarsus, which is so applied to the ball as not 

 ordinarily to allow of their ready escape under it.f As the lids 

 (the under has but little motion) cover the eye during sleep and 

 their fine inner edges meet, the whole of the ball is at this time 

 readily preserved moist. But when the eyes are open, the front 

 of the eye between the lids would not be moistened unless the 

 upper tarsus occasionally descended with the fluid contained 

 behind it. The fluid thus brought upon the front of the eye, 

 trickles down, after winking, by its gravity as far as the inferior 

 tarsus, which also occasionally ascending a little, raises it some- 

 what. Winking thus preserves the front of the eye constantly 

 moist during the waking state. 



It may be also observed that when the tarsi approximate, as 

 they drive before them the moisture of the front of the eye-ball, 

 they quite inundate the puncta lachrymalia, by which circum- 

 stance the puncta are of course enabled to carry off a large 

 quantity of the secretion, and ordinarily to prevent its overflow, 

 which would occur at the centre of the lower tarsus. During 

 sleep the puncta are not so copiously supplied, as they have only 

 the same share of tears as the eye in general j and there is less 

 occasion for it, because the removal of the stimulus of air and 

 light by the closure of the eyelids, lessens the secretion. 



* Phil. Trans. 1819. 



t The object of this firm application of the tarsi to the eye must be th« 

 exclusion of foreign matters from the orbit. 



N 2 



