THE EYE. 



737 



Fipr. 301. 



b^o^vnish-yello^v, brown, and black eyes, it is owing to a special iris- 

 2^i(/mcnt, wbich is very unequally tlistributeil, and thus produces the 

 peculiar markings of the anterior surface. This pigment is seated, in 

 the first place, in the stroma itself, and in fact, chiefly in its fusiform 

 cells, but also, as it appears to me, occurs free among the fibres and 

 vessels, and in the fibre-cells of the sphincter iJupilliV ; lastly, in the 

 anterior epithelial layer, it consists of larger and smaller cells, gold- 

 yellow or brownish irrcgular-sizcd granules, aggregations of granules and 

 streaks, never of the regular pigment-granules of the true ocular pigment. 

 The vessels of the tunica vasculosa are extremely numerous, and arc 

 variously disposed in its different parts. The choroid receives its blood 

 from the short posterior ciliary arteries, about twenty small vessels, 

 penetrating the sclerotic, in the posterior part of the eyeball, at a 

 greater or less distance from the optic nerve, and which, dividing in a 

 dichotomous manner in the middle or vascular layer of the choroid, run 

 anteriorly, and subdivide into three sets of branches : 1, external, which, 

 having attained a certain fineness by continued division, pass directly 

 into the vence vorticosa ; 2, internaJ, which pass into a capillary plexus 

 immediately beneath the pigment, in the so-termed mcmhrana chorioca- 

 pillaris, or Ruyschiana ; and 3, anterior, which are continued into the 

 ciliary body and iris. The above-mentioned capil- 

 lary plexus of the innermost layer of the choroid 

 — which in animals having a tapetum lies upon its 

 internal aspect, and may easily be demonstrated as 

 a special membrane, as may also occasionally be 

 done in Man— -is one of the most elegant and 

 closest that exists, inasmuch as its meshes, formed 

 by vessels of 0'004 of a line, do not measure more 

 than 0'002-0*005 of a line, the capillaries arising 

 from the larger vessels, as it were in a stelliform 

 manner. It extends, as has been already said, only 

 as far as the ora serrata, where it gives place to 

 somewhat coarser convolutions of vessels, O'OO-l of 

 a line in diameter, which, proceeding from the an- 

 terior branches of the short posterior ciliary arte- 

 ries, constitute the ciliary processes, and are so 

 closely approximated that, besides the vessels and 

 a homogeneous sheath supporting the processes, the 

 latter seem to contain no other tissue. From these 

 various points, and from the ciliary muscle, which 



Fig. 301. — Vessels of tbe choroid and iris of a Child, after Arnold ; viewed from within; 

 magnified 10 diameters : a, capillary plexus of the posterior segment of the choroid, terminating 

 at the ora serrata, b ; c, arteries of the corona ciliaris, supplying the ciliary processes, (/, and 

 in part passing upon the iris, e ; f, capillary plexus on the inner surface of the pupillary 

 margin of the iris. 



47 



