416 THE IRIS. 



and the other the anterior part of the vascular sheath of the lens with its 

 niembrana pupillaris. Between the two appears the aqueous humour. 



The membrana capsulo-pupillaris is simply a provisional embryonic 

 structure, subserving the nutrition of the lens. The time of its dis- 

 appearance varies somewhat for the different Mammalia in which this 

 point has been investigated. In the human embryo it lasts from the 

 second to the seventh month and sometimes longer. As a rule it is com- 

 pletely absorbed at the time of birth. The absorption of the anterior part 

 commences in the centre and proceeds outwards. 



In addition to the vessels of the vascular capsule round the lens, there 

 arise from the arteria centralis retinae, jiist after its exit from the optic 

 nerve, in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional 

 vascular branches which extend themselves in the posterior part of the 

 vitreous humour. Near the ciliary end of the vitreous humour they 

 anastomose with the vessels of the membrana capsulo-pupillaris. 



In Mammals the choroid slit closes very early, and is not perforated 

 by any structure homologous with the pecten. The only part of the slit 

 which remains open is that perforated by the optic nerve ; and in the centre 

 of the latter is situated the arteria centralis retina} as explained above. 

 From this artery there grow out the vessels to supply the retina, which 

 have however nothing to do with the provisional vessels of the vitreous 

 humour just described (Kessler). On the atrophy of the provisional 

 vessels the whole of the blood of the arteria centralis passes into the 

 retina. 



It is interesting to notice (Kessler, No. 372, p. 78) that there seems to be 

 a blood-vessel supplying the vitreous humour in the embryos of nearly all 

 vertebrate types, which is homologous throughout the Vertebrata. This 

 vessel often exhibits a persisting and a provisional part. The latter in 

 Mammalia is the membrana capsulo-pupillaris and other vessels of the 

 vitreous humour; i'.i Birds and Lizards it is the part of the original 

 vascular loop, not included in the pecten, and in Osseous Fishes that 

 part (?) not involved in the processus falciformis. The permanent part 

 is formed by the retinal vessels of Mammalia, by the vessels of the pecten 

 in Birds and Lizards, and by those of the processus falciformis in Fishes. 



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, Elasmobraiichii, etc.) 

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

 into two regions, \iz. 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 tlte 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 cpiblast covering the cornea and part of the sclerotic. 



