XII.J MEMBEAJ^A CAPSULO-PUPILLARIS. 389 



branches to the retina, and becomes known as the 

 arteria centralis retince. It is homologous with the 

 arterial limb of the vascular loop projecting into the 

 vitreous humour in Birds. 



Before becoming enveloped in the optic nerve this 

 artery is continued through the vitreous humour (Fig. 

 128), and when it comes in close proximity to the lens 

 it divides into a number of radiating branches, which 

 pass round the edge of the lens, and form a vascular 

 sheath which is prolonged so as to cover the anterior 

 wall of the lens. In front of the lens they anastomose 

 with vessels, coming from the iris, many of which are 

 venous, and the whole of the blood from the arteria 

 centralis is carried away by these veins. The vascular 

 sheath surrounding the lens is the memhrana capsulo- 

 pupillaris. The posterior part of it is either formed 

 simply by branches of the arteria centralis, or out 

 of the mesoblast cells involuted with the lens. The 

 anterior part of the vascular sheath is however enclosed 

 in a very delicate membrane, the membrana pupillaris, 

 continuous at the sides with the membrane of Descemet. 



The membrana capsulo-pupillaris is simply a pro- 

 visional embryonic structure, subserving the nutrition 

 of the lens. 



In many forms, in addition to the vessels of the 

 vascular capsule round the lens, there arise from the 

 arteria centralis retinse, just after its exit from the optic 

 nerve, provisional vascular branches which extend them- 

 selves in the posterior part of the vitreous humour. 

 Near the ciliary end of the vitreous humour they anas- 

 tomose with the vessels of the membrana capsulo-pu- 

 pillaris. 



