THE EYE. 361 



a great amount of pigmentary matter, and divisible into a 

 larger posterior portion — the choroid , and a less extensive 

 anterior segment — the iris. 



The choroid is an easily lacerable membrane, -^ — ~" thick, 

 extending from the entrance of the optic nerve, where it is 

 perforated by a rounded opening, nearly to the anterior border 

 of the sclerotic, where it presents a thicker part — the corpus 

 ciliare, and is then continuous with the iris. Its external surface 

 is attached not only by larger vessels and nerves, but also other- 

 wise, tolerably intimately, to the sclerotic, so that in exposing the 

 choroid a portion of the membrane always remains more or less 

 adherent to the sclerotic, in the form of a brown tissue. This 

 is the so- termed lamina fusca of authors, which there is no 

 ground for separating from the vascular tunic and regarding 

 as a distinct membrane, although in many instances scattered 

 pigment- cells, such as exist in it, are found to extend even into 

 the connective tissue of the sclerotic. The inner surface of the 

 choroid is smooth and, at the or a serrata, very closely connected 

 with the retina, elsewhere more loosely; whilst anteriorly to 

 the or a serrata, and particularly in the processus ciliares, it is 

 very intimately united with the hyaloid membrane [zonula 

 Zinnh), so that the two are never completely separable. 



The choroid consists essentially of two portions, a vascular, 

 external thicker layer — the proper choroid, and an inner dis- 

 tinctly coloured lamina — the pigmentum nigrum; the former, 

 however, may be again subdivided into three, but by no means 

 sharply defined layers, viz. : 1. an external, brown, soft lamella, 

 supporting the ciliary nerves and long ciliary vessels, and, in 

 front, containing the ciliary muscle — the outer pigment-layer ; 

 2. the less deeply coloured, proper vascular layer, with the 

 larger arteries and veins ; and 3. a colourless, delicate, internal 

 layer, containing an extremely abundant capillary plexus — the 

 membrana choriocapillaris, which, however, does not extend 

 further in front than the ora serrata. The tissue of which 

 the proper choroid is constituted, except the vessels and nerves, 

 which indeed make up a considerable part of it, and the 

 ciliary muscle, is of a peculiar kind, and cannot conveniently 

 be described under any particular head, but like the fibres of 

 the lig. pectinatum of the iris, though in somewhat different 

 respects, is intermediate between the connective and elastic 



