THE EYE AND ITS APPENDAGES. 



343 



are so intimately united that they constitute a layer of considerable 

 consistence. The largest vessels occupy the most superficial 

 part of the stratum, those next in size the middle, while the smallest 

 approach the capillary zone. The most conspicuous of the large 

 superficial blood-channels are the four venae vorticosae, with their 

 whorls of tributaries. These veins occupy positions around the 

 equator at points about equidistant, towards which the smaller vessels 

 converge from all directions, returning the blood not only from the 

 choroid but also from the ciliary body and the iris. The veins of 

 the choroid are often surrounded by perivascular lymph-sheaths. 

 Many of the larger arteries, in addition to the well-marked circu- 

 larly-disposed muscle with which they are provided, are accompanied 

 by external longitudinal bundles of involuntary muscle. 



The innermost part of the stroma-layer, next the choriocapil- 

 laris, forms a narrow stratum (10 t>. in width) which is devoid, or 

 nearly so, of pigment, and constitutes the boundary zone. In 

 the eyes of many animals (horse, cow, sheep) this layer possesses 

 wavy bundles of connective tissue, to whose peculiar arrangement 



Human choroid seen from its inner surface, exhibiting surface view of cap- 

 illary net-work, or choriocapillaris ic, c) ; b, b, large blood-vessels of stroma- 

 layer beneath ; a, a, intervening stroma-tissue. 



is due the metallic reflex sometimes seen from such eyes ; this 

 shining structure is known as the tapetum fibrosum, as distinguished 

 from the iridescent tapetum cellulosum of the carnivora which is 

 dependent upon the presence of several layers of plate-like cells 

 containing innumerable small crystals. 



The capillary layer, or choriocapillaris, consists of a narrow 



