320 THE BLOODVESSELS OF THE EYE, BY TH. LEBER. 



to the intergranule layer ; the external granule layer and the 

 bacillar layer are, like the fovea centralis, destitute of vessels. 



In the foetus the central artery gives off the arteria hyaloidea, which 

 runs forwards from the papilla, through a canal in the vitreous, to the 

 posterior surface of the lens which it covers with vessels. In the new- 

 born child it is already completely atrophied, the artery being very rarely 

 visible after birth, and even then being generally in an obliterated 

 state. 



In many animals the retinal vessels are absent, or are only distributed 

 to a definite portion of the retina. 



In Birds, many Amphibia, and Fishes, they are altogether absent ; 

 but are here for the most part, though not always, replaced by the 

 vessels of the hyaloidea, which are distributed to the inner surface of 

 the retina (Huschke, Hyrtl, H. Muller). Amongst Mammals, the 

 Rabbit possesses vessels only in that portion of the retina characterized 

 by the presence of medullated nerve-fibres. In the Horse there are 

 only very small vessels, which break up to form a circle of capillary 

 loops not more than from three to six millimeters in diameter.* In 

 the Guinea-pig, very fine vessels are occasionally seen on the papilla of 

 the nerve, which, however, cannot be followed upon the retina. 



2. CILIARY OR CHOROIDAL VASCULAR SYSTEM. 



The entire choroidal tract, the sclerotic with the margin of 

 the cornea, and the immediately adjoining parts of the scleral 

 conjunctiva, are supplied by the so-called ciliary vessels. They 

 are the following : 



a Arteries. 



1. The short posterior ciliary arteries (arterice ciliares pos- 

 teriores breves, figs. 367 and 368, a,) form from four to six small 

 trunklets, that spring from the ophthalmic artery or its first 

 branches. Pursuing the same course as the optic nerve, they 

 divide into a great number of branches (about twenty), which 

 perforate the posterior segment of the sclerotic in a nearly 

 straight direction from without inwards. The most numerous 

 as well as the largest branches occur in the vicinity of the pos- 

 terior pole of the eye, internal to the entrance of the optic nerve 



* H. Muller, Notiz uber die Netzliautgefasse beimanchem Thieren (Ob- 

 servations on the retinal vascular system in various animals), Wiirzburg 

 Naturwissenscliaft. Zeitschrift, Band ii., p. 64. 



