HYALOID CIRCULATION; VITREOUS 113 



more and more outlying layers of fibers until the dead, firm centrum of 

 the lens has grown in size (by middle age) to the point where little 

 accommodatory change of shape of the lens is any longer possible (see 

 Fig. 15, p. 35). The lens is thus unique among the organs of the body 

 in that its development never ceases, while its senescence commences 

 even before birth. 



The Hyaloid Circulation — In mammals, though not in any other 

 class, the developing lens is nourished by an elaborate temporary net- 

 work of blood vessels. The first signs of their development are seen while 

 the optic cup is just being formed. From a plexus of embryonic capil- 

 laries lying beneath the vesicle, one especially plump vessel is taken up 

 into the groove of the optic stalk (Fig. 42a) so that when the lips of this 

 groove finally close, the little vessel lies along the axis of the future optic 

 nerve and forms the 'hyaloid artery'. At the optic-cup end of the groove 

 of the optic stalk, it emerges into the cup cavity. The healing of the 

 embryonic fissure of the optic cup fixes the point of emergence of the 

 hyaloid artery at the site of the apex of the fissure. As it traverses the 

 vitreous cavity it branches around the lens to form a vascular tunic on 

 the latter, and some of these branches make connections at the rim of the 

 optic cup with other vessels clinging to the outer surface of the cup, the 

 beginnings of the chorioidal circulation. A ring-shaped 'annular vessel' 

 is formed at the cup margin, and from it capillary loops are thrown over 

 the anterior face of the lens, budding through the mesodermal tissue 

 which has squeezed in between the lens and the surface ectoderm, and 

 thus laying down the circulation of the embryonic iris (Fig. 42b). 



The whole vascular net around the lens, the other branches of the 

 hyaloid artery which run along the inner surface of the retina, and 

 the hyaloid artery itself eventually (before birth) atrophy back to the 

 head of the optic nerve. Here the hyaloid (now called the central 

 retinal artery) gives off new branches into the retinal tissue, accom- 

 panied by branches of the central retinal vein, to give the retina its 

 definitive circulation. 



The Vitreous — In among these temporary vessels in the cavity of the 

 young optic cup there is a gelatinous tissue, the 'primary vitreous', whose 

 few fibers are of dual origin, some being produced by mesodermal cells 

 which invaded the cup with the hyaloid vessel, others coming from the 

 foot-plates of the Miiller fibers of the developing retina, and even from 

 the cells of the lens until the formation of the capsule shuts off further 



