220 PHYSIOLOGY. 



48. Many animals have what may be called a third eye- 

 lid, called the nicitating membrane. This slides from one 

 angle of the eye to the opposite side, under the lids, whether 

 open or shut. Its use is to clear away all matter which 

 might be irritating to the eye. Birds that see best by night, 

 such as owls, defend their eyes against the light of the sun, 

 by drawing this curtain over them. 



49. The eye-lashes, or cilia, also aid in the protection of 

 the eye. They ward off insects, protect the eye from par- 

 ticles floating in the air, and also break the intensity of light 

 Indeed, when moist, as they often are with perspiration, the 

 little drops serve to decompose the rays of light, causing the 

 appearance of a luminous zone around the flame of a candle. 

 {The eye-brows are composed of loose cellular substance, 

 covered with skin, from which spring short bristly hairs 

 projecting outwards. Like all hairs, they penetrate little 

 follicles, and become besmeared with an oily secretion, which 

 preserves their glossiness^.so that the drops of sweat which 

 may accumulate on the brow are prevented from trickling 

 over the eye-lids, where they might interfere with vision/ 



50. Lachrymal Apparatus. rlThe tears are secreted by the 

 lachrymal gland, which is a small spongy body of a flatten- 

 ed form, seated in the hollow of the bone, in the upper and 

 outer part of the orbit, just beneath the outer end of the 

 bone4 These serve as fountains, as well as laboratories or 

 distilleries, separating, as they do, a pure water from the red 

 blood, and discharging it by means of seven or eight little 

 tubes, upon the inner surface of the upper eye-lid, from 

 whence it is spread along with the mucus from the con- 

 junctiva over the eye, by the movements of the lids. But 

 what becomes of the tears ? ^In the inner corner of the eye 

 are two very minute apertures, one in each lid, which are 

 the orifices of two canals, which communicate with a pipe, 

 by which the superfluous moisture is drained off into the 

 nostrils. This process is constantly going on without notice 

 But when there is much irritation, then the natural chan 



