474 EMBRYOLOGY. 



special apparatus for nutrition. This is furnished in Mammals by 

 the tunica vasculosa lentis (figs. 266, 267 tv}. By this is understood 

 a highly vascular connective -tissue membrane, which envelops the 

 outer surface of the capsule of the lens on all sides. In Man it is 

 already distinctly developed as early as the second month. Its 

 vessels are derived from those of the vitreous body. Consequently 

 on the posterior wall of the lens they are large. These, resolved 

 into numerous fine branches, bend around the equator of the lens, 

 and run toward the middle of the anterior surface, where they form 

 terminal loops, and also unite with blood-vessels of the choroid 

 membrane (fig. 267 x). 



Separate parts of the nourishing membrane of the lens, having 

 been discovered at different times by various investigators, have 

 received special names, as membrana pupillaris, m. capsulo-pupillaris, 

 m. capsularis. The first to be observed was the membrana pupillaris, 

 the part of the vascular membrane which is situated behind the 

 pupil on the anterior surface of the lens. It was the most easily 

 found, because occasionally it persists even after birth as a fine 

 membrane closing the pupil, and producing atresia pupillw congenita. 

 Later it was found that the membrana pupillaris is also continued 

 laterally from the pupil on the anterior face of the lens, and this 

 part was called membrana capsulo-pupillaris. Finally it was dis- 

 covered that the blood-vessels are spread out on the posterior wall of 

 the lens the membrana capsularis. It is superfluous to retain all 

 these names, and most suitable to speak of a nutritive membrane of 

 tlie lens, or a membrana vasculosa lentis. 



This vascular membrane attains its greatest development in the 

 seventh month, after which it begins to degenerate. Ordinarily it 

 has entirely disappeared before birth ; only in exceptional cases do 

 some parts of it persist. Toward the end of embryonic life, more- 

 over, the chief growth of the lens itself has ceased. For according 

 to weighings carried on by the anatomist HUSCHKE, it has a weight 

 of 123 milligrammes in the new-born child, and 190 milligrammes 

 in the adult, so that the total increase which the organ undergoes 

 during life amounts to only 67 milligrammes. 



(b) The Development of the Vitreous Body. 



The question of the development of the vascular membrane of the 

 lens leads to that of the vitreous body. As was previously men- 

 tioned, there grows out from the embryonic connective tissue a 



