THE CHOROID, CILIARY BODY, AND IRIS 



1095 



to the circumference of the iris. The iris is the circular septum, which hangs 

 vertically behind the cornea, presenting in its centre a large rounded aperture, 

 the pupil. 



The Choroid (chorioided) is a thin, highly vascular membrane, of a dark-brown 

 or chocolate color, which invests the posterior five-sixths of the globe, and is 

 pierced behind by the optic nerve, and in this situation is firmly adherent to the 

 sclera. It is thicker behind than in front. Externally, it is loosely connected 

 by the lamina fusca with the inner 

 surface of the sclera (p. 1092). Its 

 inner surface is attached to the 

 retina. 



Structure. The choroid consists of a 

 dense capillary plexus and of small arte- 

 ries and veins, carrying the blood to and 

 returning it from this plexus (Fig. 802), 

 and of branched and pigmented cells 

 which lie in connective tissue. There are 

 three layers in the choroid. Named from 

 without inward, they are the lamina 

 suprachoroidea, the choroid proper, and 

 lamina basalis. 



1. The lamina suprachoroidea is on 

 the external surface, that is, the surface 

 next to the sclera. It resembles the 

 lamina fusca of the sclera. It is com- 

 posed of delicate nonvascular lamellae, 

 each lamella consisting of a network of 

 fine elastic fibres, among which are 

 branched ' pigment cells. The spaces 

 between the lamellae are lined by endo- 

 theliurn, and open freely into the peri- 

 choroidal lymph space, which, in its turn, 

 communicates with the periscleral space 

 by the perforations in the sclera through 

 which the vessels and nerves are trans- 

 mitted. 



2. The choroid proper is internal to the lamiria suprachoroidea. In consequence of the 

 small arteries and veins of the choroid proper being arranged on the outer surface of the capillary 

 network, it is customary to describe this as consisting of two layers the outer (lamina vasculosa), 

 composed of small arteries and veins, with pigment cells interspersed between them, and the 

 inner (lamina choriocapillaris), consisting of a capillary plexus. The external layer of the 

 choroid proper or the lamina vasculosa consists, in part, of the larger branches of the short 

 posterior ciliary arteries (Figs. 801 and 804), which run forward between the veins, before they 

 bend inward to terminate in the capillaries; but this layer is formed principally of veins, which 

 have a whorl-like arrangement and empty into four or five large equidistant trunks, the venae 

 vorticosae (Figs. 801 and 802), which pierce the sclera midway between the margin of the cornea 

 and the attachment of the optic nerve. Interspersed between the vessels are dark star-shaped 

 pigment-cells, the processes of which, communicating with similar branchings from neighboring 

 cells, form a delicate network of stroma, which toward the inner surface of the choroid loses its 

 pigmentary character. The inner layer of the choroid proper consists of an exceedingly fine 

 capillary plexus, formed by the short ciliary vessels (Fig. 802), and is known as the membrane 

 of Ruysch (lamina choriocapillaris). The network is close, and finer at the hinder part of the 

 choroid than in front. About 1.2 cm. behind the cornea its meshes become larger, and are con- 

 tinuous with those of the ciliary processes. These two laminae are connected by an interme- 

 diate stratum, which is destitute of pigment cells and consists of fine elastic fibres. On the 

 inner surface of the lamina choriocapillaris is a very thin, structureless, or faintly fibrous .mem- 

 brane, called the lamina basalis (membrane of Bruch) ; it is closely connected with the stroma 

 of the choroid, and separates it from the pigmentary layer of the retina; the cells of the latter are 

 found attached to this membrane. 



Dissection. The ciliary body should now be examined. It may be exposed, either by de- 

 taching the iris from its connection with the Ciliary muscle, or by making a transverse section of 

 the globe, and examining it from behind. 



FIG. 800. The choroid and iris. (Enlarged.) 



