144 EYE 



the cornea, as above described, and a posterior which gives rise to the iris and the 

 mesenchyme of the pupillary membrane (fig.185). The separation begins just in front 

 of the ciliary region. Here the posterior lamella is composed of loosely arranged 

 mesenchyme, applied to the pars iridis of the optic cup. It gives origin to the 

 stroma of the iris. The cleavage proceeds from the periphery over the front of 

 the lens, but here only an excessively thin lamella is separated off, in which the 

 vessels of the pupillary membrane develop. It is at first directly continuous 

 with the iris (fig. 185), but when the vessels disappear it is dissociated from that 

 structure, and if any trace of it remains it appears to be incorporated with the 

 lens-capsule. 



Accessory structures. The eyelids make their appearance as folds 

 of integument, subsequently to the formation of the eyeball. About the third 

 month of foetal life the two folds, one forming the upper and the other the 

 lower lid, meet and unite by a growth together of the epithelium at the margins 

 of the folds, so as to cut off the conjunctival sac from the exterior. Shortly 

 before birth they again become disunited. 



A third fold (of the conjunctiva) appears at the inner canthus, and in many 

 vertebrates develops into a well-marked third eyelid, the membrana nictitans. 

 In man it remains rudimentary, forming the plica semilunaris. 



The glands, hairs, and other structures belonging to the eyelids are developed 

 in the same way as the corresponding structures in the rest of the integument. 



The lacrymal gland is developed in the third month as a number of out- 

 growths from the deeper layer of the epithelium, at the upper and outer part of 

 the conjunctival sac. The outgrowths are at first solid, and branch into the 

 surrounding connective tissue as in the case of racemose glands, subsequently 

 becoming hollowed out and differentiated into ducts and acini. 



The lacrymal canals and ducts were formerly described as being directly 

 developed by the enclosure of the fissure which separates the lateral nasal 

 process from the maxillary process (see Development of Face, p. 86), and 

 which passes in the early embryo from the eye to the upper part of the naso- 

 buccal cavity (lacrymal fissure). But it has been shown, chiefly by the researches 

 of Born, that in most animals the canal is at first formed as a thickening of the 

 rete mucosum of the epidermis, which sinks into the corium along the line of that 

 fissure. The thickening subsequently becomes separated from the rest of the 

 epidermis, and hollowed out to form an epithelial tube, which leads from the 

 conjunctiva into the nasal cavity. 



The bifurcation of the duct where it opens on the conjunctiva was formerly believed to be 

 produced (Ewetsky) by a broadening out of the epithelial cord at the inner canthus, and its 

 subsequent separation into two parts by an ingrowth of connective tissue in its middle, the 

 two parts developing into the upper and lower lacrymal canals. It has been shown, however, 

 (Matys, Fleischer, and Ask ' ), that, although the epithelial cord remains a long time in continuity 

 with the conjunctiva, both the lacrymal canaliculi and nasal duct are produced by proliferation 

 of the epithelial blastema in the connective tissue, and come secondarily and simultaneously 

 into contact with conjunctival and nasal epithelium respectively. The lumen appears first 

 in the inferior canaliculus, and then at different points in the cord. The upper canal lags 

 behind the lower in development : it opens at first into the conjunctiva close to the inner 

 canthus; while the lower canal opens considerably further out along the edge of the lid. 

 Space is thus left in the lower lid, between the punctum 1 aery male and the inner canthus, for 

 the development of Meibomian follicles. As the lower canaliculus enlarges, these are compressed, 

 and therefore atrophy, and the tissue in which they lie becomes the caruncula lacrymalis. 



1 Matys, Zeitschr. f . Augenheilk. xiv ; Fleischer, Archiv f. Ophthal. Ixii. ; Ask, Anat. Anzeiger, 

 xxx. 1907. 



