GLANDS 217 



(6) EYELIDS 



In Fishes and other lower aquatic forms the upper and lower 

 eyelids are usually very rudimentary, having at most (e.g., Elasmo- 

 branchs) the form of stiff folds of the skin ; and in all Verte- 

 brates below the Mammalia they never reach a very high stage of 

 development. They are lined on the surface looking towards the 

 eyeball by a continuation of the epidermis, the conjunctiva (p. 210), 

 and in the Ichthyopsida and Sauropsida are usually not sharply 

 marked off from the rest of the skin, being capable of no, or only 

 of very slight, movement. 1 



In Mammals, the eyelids, more particularly the upper one, are 

 extremely movable, and are provided with hairs (eyelashes) on 

 their free margin. In their interior a hard body, the so-called 

 " lid-cartilage " is developed, and 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 a depressor of the lower lid. 



The want, or comparatively slight development of upper and 

 lower eyelids in Vertebrates below the Mammalia is compen- 

 sated for in certain forms, 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 outer skin proper, 

 consisting simply of a reduplicature of the conjunctiva, and being 

 regulated by special muscles (see p. 216). 



The nictitating membrane, which is represented in certain 

 Elasmobranchs (e.g., Carcharias, Galeus, Zygsena, Mustelus, comp. 

 p. 143) and which often encloses a cartilage, is situated within 

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

 of the eye. The former condition is seen, e.g., in Anurans, and 

 the latter in Birds, in which a third eyelid is so largely developed 

 as to be capable of covering the whole freely exposed portion of 

 the eyeball. In Reptiles 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), but in Monkeys 

 and certain races of Mankind traces of the cartilage are present. 



(c) GLANDS. 



The glands in connection with the eye are : (1) the lachrymal, 

 (2) the Harderian, or gland of the nictitating membrane, and (3) 

 the Meibomian glands. 



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

 eyeball moist, and to wash away foreign bodies. In Fishes and 



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

 bone or fibre-cartilage. In Geckos, Amphibsenians 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 part of the skin when the latter is shed. 



