EYELIDS 287 



(&) EYELIDS 



In Fishes and other lower aquatic forms the upper and lower 

 eyelids are usually very rudimentary, having at most the form of 

 stiff folds of the skin, and in all other Vertebrates below the 

 Mammalia they never reach a very high stage of development : 

 even in those Mammals in which the facial muscles are not much 

 differentiated they are not greatly specialised. They are lined 

 on the inner surface by a continuation of the epiderm, the con- 

 junctiva, which is continued over the cornea (Fig. 203 ), 1 and in 

 the Ichthyupsida are usually not sharply marked off from the 

 rest of the skin, and are capable at most of very slight movement : 

 the eyelids in these forms therefore only protect the eye to a 

 slight extent, and allow it comparatively little free movement. 2 



In the Sauropsida they are in certain respects much more highly- 

 differentiated. Occasionally (Chameleon) they are circular and are 

 movable by muscles, while in Geckos, Amphisbsenians, and Snakes 

 the two e} T elids 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 it is shed. 



A levator of the upper lid, which latter is usually the better 

 differentiated of the two and which in many Reptiles and Birds 

 may be supported by a membrane-bone or fibro-cartilage, occurs in 

 Chelonians, Crocodiles, Birds, and Mammals. Lizards, Chelonians, 

 Birds, and many Mammals (e.g. Ungulates) possess a depressor of 

 the lower lid. 



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

 extremely movable, and may be provided with hairs (eyelashes) 

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

 " lid-cartilage," enclosing the Meibornian glands, is developed 

 (Fig. 210, B), and they are closed by a circular muscle (orbicufaris 

 or sphincter oculi) which surrounds the whole slit between the lids, 

 and is a derivative of the mimetic musculature. 



The want or comparatively slight development of upper and 

 lower eyelids in Vertebrates below the Mammalia is compensated 

 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 



1 In Fishes and Amphibian larva; the conjunctiva retains essentially the 

 same structure as the epiderm from which it is derived, while in higher forms it 

 undergoes modifications, and the stratum corneuni becomes less horny. 



2 Accessory lid-folds occur, e.g. , in the Herring and Salmonida;. 



:: The structure known as a " nictitating membrane " in many Elasmobranchs 

 is not exactly comparable to the nictitating membrane of higher Vertebrates. It 

 is secondarily derived from the lower lid-fold, and is provided with several 

 muscles, supplied by the trigeminal (or facial) nerve, and derived from integu- 

 mentary muscles in the region of the spiracle. 



