1010 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



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

 the sclera. It is thicker behind than in front. Its outer surface is loosely connected 

 by the lamina suprachorioidea with the sclera; its inner surface is attached to the 

 pigmented layer of the retina. 



Structure. The choroid consists mainly of a dense capillary plexus, and of small arteries 

 and veins carrying blood to and returning it from this plexus. On its external surface is a thin 

 membrane, the lamina suprachorioidea, composed of delicate non-vascular lamellae each lamella 

 consisting of a net-work of fine elastic fibers among which are branched pigment cells. The spacer 

 between the lamellae are lined by endothelium, and open freely into the perichoroidal lymph 

 space, which, in its turn, communicates with the periscleral space by the perforations in the 

 sclera through which the vessels and nerves are transmitted. 



Internal to this lamina is the choroid proper, consisting of two layers: an outer, composed 

 of small arteries and veins, with pigment cells interspersed between them; and an inner, consist- 

 ing of a capillary plexus. The outer layer (lamina vasculosa) consists, in part, of the larger branches 

 of the short ciliary arteries which run forward between the veins, before they bend inward to end 

 in the capillaries, but is formed principally of veins, named, from their arrangement, the venae 



vorticosae. They converge to four or 

 five equidistant trunks, which pierce the 

 sclera about midway between the sclero- 

 corneal junction and the entrance of the 

 optic nerve. Interspersed between the 

 vessels are dark star-shaped pigment 

 cells, the processes of which, communicat- 

 ing with those of neighboring cells, form 

 a delicate net-work or stroma, which 

 toward the inner surface of the choroid 

 loses its pigmentary character. The 

 inner layer (lamina choriocapillaris) con- 

 sists of an exceedingly fine capillary 

 plexus, formed by the short ciliary vessels; 

 the net-work is closer and finer in the pos- 

 terior than in the anterior part of the 

 choroid. About 1.25 cm. behind the 

 cornea its meshes become larger, and are 

 continuous with those of the ciliary 

 processes. These two laminae are con- 

 nected by a stratum intermedium con- 

 sisting of fine elastic fibers. On the inner 



FIQ. 874. The veins of the choroid. (Enlarged.) ,. , . ...... 



surtace ot the lamina chonocapillaris is 

 a very thin, structureless, or faintly 



fibrous membrane, called the lamina basalis; it is closely connected with the stroma of the 

 choroid, and separates it from the pigmentary layer of the retina. 



One of the functions of the choroid is to provide nutrition for the retina, and to convey vessels 

 and nerves to the ciliary body and iris. 



Tapetum. This name is applied to the outer and posterior part of the choroid, which in many 

 animals presents an iridescent appearance. 



The Ciliary Body (corpus ciliare}. The ciliary body comprises the orbiculus 

 ciliaris, the ciliary processes, and the Ciliaris muscle. 



The orbiculus ciliaris is a zone of about 4 mm. in width, directly continuous 

 with the anterior part of the choroid; it presents numerous ridges arranged in a 

 radial manner (Fig. 875). 



The ciliary processes (processus ciliares) are formed by the inward folding of the 

 various layers of the choroid, i. e., the choroid proper and the lamina basalis, and 

 are received between corresponding foldings of the suspensory ligament of the lens. 

 They are arranged in a circle, and form a sort of frill behind the iris, around the 

 margin of the lens (Fig. 875) . They vary from sixty to eighty in number, lie side 

 by side, and may be divided into large and small; the former are about 2.5 mm. 

 in length, and the latter, consisting of about one-third of the entire number, are 

 situated in spaces between them, but without regular arrangement. They are 

 attached by their periphery to three or four of the ridges of the orbiculus ciliaris, 

 and are continuous with the layers of the choroid: their opposite extremities are 

 free and rounded, and are directed toward the posterior chamber of the eyeball and 





