192 COMPARATIVE ANATOMY. 



of development. They are lined on the surface looking towards 

 the eyeball by the conjunctiva, and in the Ichthyopsida and 

 Sauropsida are usually not sharply marked off from the rest of the 

 skin, being capable of no, or of only very slight movement. 1 



The case is quite different in Mammals, in which the eyelids, 

 more particularly the upper one, are extremely moveable, and 

 are provided with hairs (eyelashes) on their free margin. They 

 are closed by a circular muscle which surrounds the whole slit 

 between the lids ; a levator is also present in the upper eyelid. 

 In Sauropsida and many Mammalia (e.g. Ungulates) there is also 

 a depressor of the lower lid. 



The want of or slight development of upper and lower eyelids 

 in all Vertebrates below the Mammalia is compensated for, at 

 any rate to a certain extent, by the presence of a nictitating 

 membrane. This "third eyelid" differs from the others in 

 having nothing to do with the true outer skin, consisting simply 

 of a reduplicature of the conjunctiva, and being regulated by 

 special muscles (p. 191). 



The nictitating membrane, which is present in rudiment in many 

 Elasmobranchs, and which encloses a cartilage, is situated beneath 

 the lower eyelid, or it may lie more towards the anterior angle of 

 the eye. The former condition is seen in Anura and Reptilia, for 

 instance, in which a third eyelid is so largely developed as to be 

 capable of covering the whole freely exposed portion of the eyeball. 

 In Birds and Mammals it always lies in the anterior angle of the 

 eye ; in Primates it becomes reduced to a small half-moon-shaped 

 fold (plica semilunaris), and so comes into the category of rudi- 

 mentary organs. 2 * 



(c) GLANDS. 



The glands in connection with the eye may be divided into 

 three sections : (1) the lacrymal, (2) the Harderian, or gland 

 of the nictitating membrane, and (3) the Meibomian glands. 



The secretions of all these three serve to keep the free surface 

 of the eyeball moist, and to wash away foreign bodies. 



In Fishes, the outer medium appears to suffice for this purpose, 

 but the first attempt of a Vertebrate to exchange an aquatic for an 

 aerial existence necessitates the development of a secretory apparatus 

 in connection with the eye. 



Thus in Urodeles a glandular organ is developed from the 

 conjunctival epithelium along the whole length of the lower eyelid ; 



1 In many Reptiles and Birds the upper eyelid is supported by a membrane-bone 

 or fibre-cartilage, and large lymphatic and cavernous spaces are developed within the 

 tissue of the lid. In Geckos and Snakes the two eyelids grow together to form a 

 transparent membrane overlying the eye, and this comes away with the rest of the 

 outer skin when the latter is shed. 



2 In the Caucasian race the plica semilunaris is only 1| to 2 millimetres broad, 

 while in the Malayan Orang-Sakai race it reaches a breadth of 5 to 5J millimetres. 



