THE SENSE-ORGANS 505 



Aside from these conjunctival glands there appear in mam- 

 mals certain glands associated with the eyelashes. These are 



1 i ) the tarsal [meibomian], which are modified sebaceous, and 



(2) the ciliary, modified perspiratory glands. These open 

 along the edges of the lids and produce narrow lines of oil 

 which repel the lacrimal fluid and assist in retaining it within 

 the peripheral folds. As the eyelashes are modified hairs, the 

 tarsal glands may be looked upon as the associated sebaceous 

 glands, considerably hypertrophied, and changed somewhat in 

 their relation to the hairs. 



This entire lacrimal apparatus, including the glands and the 

 nasolacrimal duct, becomes much reduced in such aquatic mam- 

 mals as the hippopotamus, seal and otter, and in the pelagic 

 whales and porpoises is entirely rudimentary. In snakes there 

 occurs a singular adaptation, which protects their eyes from 

 the danger of the thick grasses and twigs by fusing the two 

 eyelids together over the eyeball and then rendering them ab- 

 solutely transparent. There is thus formed a plate, in shape 

 like a watch glass, and serving as a second cornea. This is 

 shed with each successive skin and forms a conspicuous feature 

 of the exuviae, or " snake-skins," objects commonly met with 

 in fields frequented by snakes. A lacrimal apparatus is 

 wholly wanting. 



As special protective organs to the eye may be mentioned the 

 long superciliary bristles, which in cats and a few mammals 

 project over the eye and when touched cause the automatic 

 closing of the lids ; also the eyebrows of the higher anthropoids, 

 especially Man, the hairs of which point outwards and curve 

 downwards at the outer end to receive the perspiration of the 

 forehead and convey it away from the eyes. 



