506 THE IRIS. 



The Iris and Ciliary processes. The walls of the edge of the 

 optic cup become very much thinner than those of the true retinal part. In 

 many Vertebrates (Mammalia, Aves, Reptilia, Elasmobranchii, etc.) the 

 thinner part, together with the mesoblast covering it, becomes divided into 

 two regions, viz. that of the iris, and that of the ciliary processes. In the 

 Newt and Lamprey this differentiation does not take place, but the part in 

 question simply becomes the iris. 



Accessory Organs connected with the Eye. 



Eyelids. The most important accessory structures connected with 

 the eye are the eyelids. They are developed as simple folds of the integu- 

 ment with a mesoblastic prolongation between their two laminae. They 

 may be three in number, viz. an upper and lower, and a lateral one the 

 nictitating membrane springing from the inner or anterior border of the 

 eye. Their inner face is lined by a prolongation of conjunctiva, which is 

 the modified epiblast covering the cornea and part of the sclerotic. 



In Teleostei and Ganoidei eyelids are either not present or at most 

 very rudimentary. In Elasmobranchii they are better developed, and the 

 nictitating membrane is frequently present. The latter is also usually found 

 in Amphibia. In the Sauropsida all three eyelids are usually present, but in 

 Mammalia the nictitating membrane is rudimentary. 



In many Mammalia the two eyelids meet together during a period of 

 embryonic life, and unite in front of the eye. A similar arrangement 

 is permanent through life in Ophidia and some Lacertilia ; and there is a 

 chamber formed between the coalesced eyelids and the surface of the cornea, 

 into which the lacrymal ducts open. 



Lacrymal glands. Lacrymal glands are found in the Sauropsida 

 and Mammalia. They arise (Remak, Kolliker) as solid ingrowths of the 

 conjunctival epithelium. They appear in the chick on the eighth day. 



Lacrymal duct. The lacrymal duct first appears in Amphibia, and 

 is present in all the higher Vertebrates. Its mode of development in the 

 Amphibia, Lacertilia and Aves has recently been very thoroughly worked 

 out by Born (Nos. 380 and 381). 



In Amphibia he finds that the lacrymal duct arises as a solid ridge of 

 the mucous layer of the epidermis, continued from the external opening 

 of the nasal cavity backwards towards the eye. It usually appears at 

 about the time when the nasal capsule is beginning to be chondrified. As 

 this ridge is gradually prolonged backwards towards the eye its anterior 

 end becomes separated from the epidermis, and grows inwards in the 

 mesoblast to become continuous with the posterior part of the nasal sack. 

 The posterior end which joins the eye becomes divided into the two 

 collecting branches of the adult. Finally the whole structure becomes 

 separated from the skin except at the external opening, and develops a 

 lumen. 



