EYE. 



I 251 ] 



EYE. 



with it. It consists of areolar tissue, the 

 bundles of which are mostly straight, in- 

 timately united as in the tendons, forming 

 alternating, longitudinal and transverse 

 layers of various breadth and thickness. 

 Mingled with the areolar tissue are nume- 

 rous fine elastic fibres, in the form of a 

 network, with thickenings which indicate 

 the remains of the nuclei of the formative 

 cells; these, in the inner portions, contain 

 pigment. During life, the elements of this 

 network, in parts, appear to involve canals 

 with liquid contents; so that when dried, 

 they contain air. 



The cornea may be regarded as consisting 

 of three-layers : 1, the corneal conjunctiva ; 

 2, the true cornea ; and 3, the membrane of 

 the aqueous humour. 



The true cornea (fig. 213 c), which forms 

 the principal part of the membrane, consists 

 of a substance nearly allied to areolar tissue. 

 Its elements are pale bundles, from 1-6000 to 

 1-3000" in diameter, with still finer fibrillse, 

 united to form larger flat bundles, the sur- 

 faces of which are parallel to that of the 

 cornea ; these are connected with the bun- 

 dles before and behind, so as to form a coarse 

 reticular tissue. Between the bundles are a 

 large number of anastomosing, fusiform, and 

 stellate nucleated cells of imperfectly-deve- 

 loped elastic tissue. The cells undergo fatty 

 degeneration, partly forming the arcus seni- 

 lis ; and they sometimes contain pigment. 



The corneal conjunctiva (fig. 213 a b] con- 

 sists of laminated soft epithelium ; the under 

 layer of cells elongated and placed perpen- 

 dicularly to the surface, the middle cells 

 rounded, those in the upper layer forming 

 softer nucleated plates. Many of the latter 

 are furnished with larger or smaller depres- 

 sions, arising from mutual pressure, so as to 

 appear stellate in the side view. Beneath 

 the epithelium is a structureless layer, the 

 anterior elastic membrane, consisting of the 

 remains of the formerly vascular layer of the 

 corneal conjunctiva. 



The membrane of the aqueous humour (fig. 

 213 d) consists of an elastic, perfectly struc- 

 tureless membrane, somewhat loosely con- 

 nected with the cornea, and an inner epithe- 

 lial lining. Towards the circumference of 

 the cornea, the membrane of the aqueous 

 humour merges into a peculiar system of 

 fibres, which commence near the margin of 

 the cornea, at the anterior surface of the 

 aqueous membrane (fig. 213 g) as an extended 

 network of fine fibres, resembling elastic 

 fibrillse; this increases in thickness, and at 



the very margin of the cornea the aqueous 

 membrane becomes lost in a tolerably dense 

 network of these coarse fibres, which curve 

 around the margin of the iris (fig. 213 ), 

 some passing through the anterior chamber, 

 and become fused with the anterior surface 

 of this membrane and the ciliary ligament 

 (or muscle). These fibres form foe. pectinate 

 ligament of the iris, which is much more 

 distinct in some animals (as the dog) than in 

 man. 



The epithelium of the aqueous membrane 

 consists of a single layer of polygonal cells. 

 These become smaller near the margin of 

 the cornea, where the membrane terminates 

 as a continuous layer ; but isolated portions 

 of elongated or spindle-shaped cells are con- 

 tinued over the pectinate ligament to the 

 anterior surface of the iris. 



The cornea yields chondrine on boiling, 

 and not gelatine. 



The choroid membrane contains a large 

 number of blood-vessels, and abounds in 

 pigment. Its anterior, smaller, and trans- 

 verse portion forms the iris. 



The posterior portion, or proper choroid 

 membrane, is from 1-360 to 1-180" in thick- 

 ness, and extends from the entrance of the 

 optic nerve to near the anterior margin of 

 the sclerotica, where it becomes thicker, 

 forming the ciliary body, whence it is con- 

 tinued into the iris. It is connected with 

 the sclerotica by vessels and nerves, and by 

 some of the pigment-cells of its outer layer 

 being continued into the areolar tissue of the 

 sclerotica. The lamina fusca of authors is 

 constituted by a portion of the membrane 

 thus left adherent, when attempts are made 

 to separate it from the sclerotica. 



The choroid consists essentially of two 

 parts, an outer vascular and thicker layer, 

 the proper choroid; and an inner deeply 

 coloured layer, the pigmentum nigrum* 

 The former may again be separated into 

 three parts, although these are not really 

 distinct : 1, an outer, brown, soft layer, 

 which supports the ciliary nerves and long 

 ciliary vessels, and contains anteriorly the 

 ciliary ligament, the outer pigment layer ; 



2, a less highly coloured proper vascular 

 layer, with the larger arteries and veins ; and 



3, a colourless delicate inner layer, contain- 

 ing an extremely copious capillary network, 

 the choro- capillary membrane, which does 

 not extend anteriorly beyond the margin of 

 the retina. The stroma of the choroid pro- 

 per consists of elastic tissue, in the form of 

 very irregular spindle-shaped or stellate 



