THE VASCULAR TUNIC OF THE EYE. 453 



In the inner portion of this layer is found a narrow zone, in 

 the human eye only about 10 fj. in thickness, consisting largely 

 of elastic fibers and free from pigment cells, known as the boundary 

 zone. This zone is somewhat thicker in many mammals, and in 

 some of these presents a characteristic structure. In the eyes of 

 ruminants and horses this zone consists of several layers of con- 

 nective-tissue bundles, and is known as the tapetum fibrosum. It 

 gives the peculiar luster often seen in the eyes of these animals. In 

 the eyes of carnivora this zone consists of several layers of endothe- 

 lioid cells, containing in their protoplasm numerous small crystals 

 and forming the iridescent layer known as the tapetum cellulosum. 



The lamina choriocapillaris contains no pigment and consists 

 principally of capillary vessels, which form an especially dense net- 

 work in the neighborhood of the macula lutea. As the venous cap- 

 illaries become confluent and form smaller veins, the latter arrange 

 themselves in long, radially directed networks, and form in this way 

 the more or less pronounced stellulce vasculosce (Winslowii). 



The vitreous or glassy membrane is a very thin (2 p) homo- 

 geneous membrane which shows on its outer surface the impressions 

 of the vessels composing the lamina choriocapillaris, and on its 

 inner surface those of the pigment epithelium of the retina. 



At the ora serrata the choroid changes in character ; from this 

 region forward, the choroidal tissue assumes more the appearance of 

 ordinary connective tissue, and the choriocapillary layer is wanting. 



The region of the vascular coat extending from the ora serrata 

 to the base of the iris is known as the ciliary body. Its posterior 

 portion, about 4 mm. broad, the orbicnlus ciliaris, is slightly thicker 

 than- the choroid, and presents on its inner surface numerous small 

 folds, meridionally placed, consisting of connective tissue and blood- 

 vessels. Anterior to the orbiculus ciliaris the ciliary body is thick- 

 ened by a development of nonstriated muscle the ciliary muscle 

 (see below) ; and on the inner surface of this annular thickening are 

 placed about seventy triangular folds, meridionally arranged the 

 ciliary processes. The attached border of these processes measures 

 from 2 to 3 mm. The anterior border attains a height of about 

 I mm. On and between these folds are found numerous small 

 secondary folds or processes of irregular shape. The ciliary pro- 

 cesses consist of fibrous connective tissue and numerous smaller 

 and larger vessels, which have in the main a meridional arrange- 

 ment. The vitreous membrane extends over the ciliary body, attain- 

 ing in the region of the ciliary processes a thickness of 3 fj. or 4 fj.. 

 Internal to the vitreous membrane, the ciliary body is covered by 

 a double layer of epithelial cells, the continuation forward of the 

 retina (pars ciliaris retinae). Of these, the outer layer is composed 

 of cells, which are deeply pigmented, and are of cubic or short 

 columnar shape, and derived from the outer layer of the secondary 

 optic vesicle, while the cells of the inner layer are nonpigmented 

 and of columnar shape, and are developed from the inner layer of 

 the secondary optic vesicle. In the region of the ciliary processes 



