THE MIDDLE COAT THE VASCULAB TUNIC 



637 



because it contains the ramifications of the ciliary arteries and veins, 

 is by far the thickest of the three layers of the choroid. It may he ar- 

 bitrarily separated into an outer stratum, consisting chiefly of dense 

 interlacing bundles of connective tissue fibers which inclose only the 

 larger blood-vessels, and an inner stratum of similar structure, but every- 

 where permeated by a close network of small vascular twigs. So dense 

 is this network near the posterior pole of the eye, as to give the layer 

 the appearance of an almost continuous sheath of small blood-vessels. 



The Lamina Capillaris. Within the vascular layer is the capillary 

 membrane (lamina capillaris, lamina choriocapillaris, tunica Ruyschi- 



i\OI A7. tt>. 



FIG. 542. FROM A MERIDIONAL SECTION OP THE CHOROID COAT. 



a, membrane of Bruch; b, the inner margin of the vascular layer. Between a 

 and b is the capillary layer or choriocapillaris; c, venule containing blood cor- 

 puscles; d, fibrous layer of the choroid or lamina suprachoroidea. Highly magni- 

 fied. (After Cadiat.) 



ana), which contains an exceedingly close-meshed capillary network. 

 This network is specially dense near the macula lutea at the posterior pole 

 of the eyeball. Its inner surface forms a very thin homogeneous mem- 

 brane, the lamina basalis, lamina vitrea or membrane of Bruch, which 

 increases somewhat in thickness as age advances. The inner surface 

 of the lamina basalis is indented by the bases of the adjacent pigment 

 cells of the retina. Anteriorly the vessels of the choriocapillaris, like 

 those of the vascular layer, become continuous with the vessels of the 

 ciliary body and iris. 



Between the pars vascularis and the choriocapillaris may be distin- 

 guished a narrow dense fibre-elastic boundary zone, free of pigment. 

 In ruminants this layer becomes pronounced, due to the presence of ro- 

 bust connective tissue fibers, and is known as the tapetum fibrosum. 

 This layer gives to the eyes of ruminants their characteristic metallic 

 luster. In the eyes of carnivora and certain fishes the tapetum is com- 

 posed of rectangular epithelioid cells, filled with peculiar glistening 



