THE EYE. 733 



53), appear still to demand confirmation ; and Arnold's " serous ves- 

 sels," in the normal condition of the membrane of Descemet ("Anat." 

 I. Tab. II. Fig. 5, II. p. 1015), are nothing more than the anastomo- 

 sing fibres of the ligamsntum pectinatum of the iris. The cornea, 

 although vascular only at its margin, is nevertheless not unfavorably 

 circumstanced as regards its nutritive conditions. Wounds in it rapidly 

 unite, portions of the epithelium or even of the fibrous layer when re- 

 moved, are restored, and ulcers are filled up from the bottom with new 

 corneal substance. Fatty deposits in its tissue, particularly in its cel- 

 lular elements (especially above and below, or even all round), produce 

 a yellow zone the so-termed arcus senilis (gerontoxori).* 



226. The vascular tunic (tunica vasculosd) or uvea. The second 

 tunic of the eyeball is a highly vascular membrane, containing a great 

 amount of pigmentary matter, and divisible into a larger posterior por- 

 tion the choroid, and a less extensive anterior segment the iris. 



The choroid is an easily lacerable membrane, 1-15-1-30 of a line 

 thick, extending from the entrance of the optic nerve, where it is per- 

 forated 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 otherwise, tolerably intimately, to 

 the sclerotic, so that in exposing the clwroid 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 regard- 

 ing as a distinct membrane, although in many instances scattered pig- 

 ment-cells, such as exist in it, are found to extend even into the con- 

 nective tissue of the sclerotic. The inner surface of the clwroid is 

 smooth and, at the ora serrata, very closely connected with the retina, 

 elsewhere more loosely ; whilst anteriorly to the ora serrata, and par- 

 ticularly in the processus ciliares, it is very intimately united with the 

 hyaloid membrane (zonula Zinnii), so that the two are never com- 

 pletely separable. 



The choroid consists essentially of two portions, a vascular, external 

 thicker layer the proper choroid, and an inner distinctly colored 

 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 colored, proper vascular layer, with 

 the larger arteries and veins ; and 3, a colorless, delicate, internal laytr, 



* [As shown by Mr. Edwin Canton' in his " Observations on the Arcus senilis, or fatty 

 degeneration of the Cornea." " Lancet," vol. i. 1850, p. 560. TRS.] 



