532 COMPAEATIVE ANATOMY. 



iutegument. The inner lamella of these folds is a continuation of 

 the conjunctiva which extends on to the bulb, and which is 

 continuous with the integument at the margin of the lid. Eyelids 

 of this kind are found even in Fishes. In the Selachii there are 

 two slightly projecting and movable folds, which appear to be 

 indications of an upper and lower eyelid ; in many Sharks there is 

 also a third fold at the anterior angle of the eye, which can be 

 drawn over the outer surface of the bulb (nictitating membrane). 

 In the Ganoidei and Teleostei the immovable folds are alone present, 

 or there may be merely indications of them ; they are ordinarily 

 distinguished as the anterior and posterior eyelids. Most commonly 

 the integument passes at once into the cornea. In the Perenni- 

 branchiata and Derotremata there is a connection of this kind. 

 Many Salamanders, and the majority of the anourous Amphibia, 

 are provided with horizontal eyelids, of which the lower, and more 

 movable one, functions as a nictitating membrane. 



In the Eeptilia and Aves there is an upper and a lower movable 

 eyelid in addition to the nictitating membrane. In some Saurii 

 (Ascalabotfe) and in the Ophidii, the eyelids are developed as an 

 annular fold, which continues to grow until at last it forms a pellucid 

 membrane which lies in front of the eye, and which completely sepa- 

 rates the cornea from the external medium. The circular rudiment of 

 this structure corresponds to the circular eyelid of the Chamaeleons. 

 There is a muscular apparatus for the horizontal eyelids, as well as 

 for the nictitating membrane. Whilst the two horizontal eyelids 

 persist in the Mammalia, the nictitating membrane undergoes 

 degeneration. It is supported, as are the two other eyelids, by a 

 cartilaginous lamella. It is generally reduced to a fold, which is 

 placed at the anterior (inner) angle of the eye ; in the Primates it 

 has lost its primitive significance, and forms the plica semilunaris. 



A glandular apparatus for the eyelids is first differentiated 

 in the Amphibia and Reptilia. In Eeptiles and Birds, and also in 

 Mammals, there is a gland which opens below the nictitating 

 membrane (Harderian gland, or gland of the nictitating membrane), 

 which is placed at the inner angle of the orbit ; it is not present in 

 the Primates. Its secretion is different from that of the lachrymal 

 gland. 



The Lachrymal glands, which are placed at the outer angle 

 of the eye, are first seen in Reptiles, where they are smaller than the 

 Harderian gland ; they have the same characters in Birds. They 

 are larger in the Chelonii and Mammalia (except the Cetacea), 

 where the lachrymal gland consists of a complex of separate glands, 

 which are generally united into larger masses. 



A special efferent duct into the nasal cavity is formed for the 

 secretion of these glands, which is passed out below the upper eye- 

 lid. A canal of this kind, formed by an epithelial thickening on 

 the surface of the head, is present even in the Amphibia. In the 

 Amniota the development of the lachrymal duct is connected 

 with that of the face. The groove, which is formed between the 



