VISUAL ORGAN. 357 



(7) Blood-vessels of the Eyeball. 



There are to be distinguished in the eyeball two systems 

 of blood-vessels, the retinal system, and the ciliary system 

 (Fig. 267). These systems are marked off sharply from one 

 another, and anastomose only at the place of entry. 



The retinal system of vessels is formed by the vasa centralia 

 retince. The central retinal artery (Fig. 267, a) runs in the axis 

 of the optic nerve until it reaches the papilla, where it divides 

 into two main branches. One of these runs forward and the other 

 posteriorly. Each breaks up in the nerve-fibre layer of the retina 

 into numerous small branches, which in turn form capillary 

 networks. These supply the cerebral layer of the whole pars 

 optica retinaa as far as the ora serrata. The neuro-epithelial 

 layer and the fovea centralis are non-vascular. The branches 

 of the retinal artery form so-called end arteries, for they anasto- 

 mose with one another only by means of their larger twigs. 



The veins arising from the capillaries run parallel with the 

 arteries, and join finally, giving origin to two main trunks, 

 which form the central retinal vein, a, in the axis of the optic 

 nerve. The arteries give off on the way small twigs between 

 the fibre bundles of the optic nerve. Some of these anastomose 

 with the vessels of the outer coats of the eye, while others join 

 with branches of the arterise ciliares posticse breves (y). Also 

 the branches of the central retinal vessels form a connection 

 with the smaller vessels and capillaries of the chorioid (<5). 



In the eyes of embryos we meet with a vessel, the arteria 

 hyaloidea, which is really a branch of the central retinal 

 artery. This hyaloid artery runs through the vitreous body 

 up to the posterior surface of the lens. It supplies the capsule 

 of the lens and sends branches into the vitreous body. This 

 vessel begins to degenerate before birth, and remains only as 

 the so-called Cloquet's canal (canalis hyaloideus), which is 

 filled with fluid. 



The ciliary system of vessels is formed of : 



(a) The arterias ciliares posticse breves ; 



(b) The arterire ciliares posticse longre; and 



(c) The arteriae ciliares anticaa. 



