EYE. 



[ 310 ] 



EYE. 



under layer of cells elongated and placed per- 

 pendicularly to the surface, the middle cells 

 rounded, those in the upper layer forming 

 softer nucleated plates. Many of the latter 

 are furuished 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 (b), 

 the anterior elastic or Bowman's luembrane, 

 consisting of the remains of the formerly 

 vascular layer of the corneal conjunctiva. 



The tnte cornea (tig. 207 c), w4iich forms 

 the principal part of the membrane, consists 

 of a substance nearly allied to connective 

 tissue. Its elements are pale bundles ft-nm 

 1-GOOO to 1-3000" in diameter, with still 

 finer fibrilL-B, united to form larger lamellar 

 bundles, the surfaces of which are parallel 

 to that of the cornea ; these are connected 

 with the bundles before and behind, so as 

 to form a coarse reticular tissue. Between 

 the bundles are a large number of anasto- 

 mosing, fusiform, and stellate lacimje, with- 

 in which are contained cell-structures form- 

 ing the corneal corpuscles. The cells under- 

 go fatty degeneration, partly _ forming the 

 arcus senilis ; and they sometimes contain 

 pigment. 



The cornea contains numerous fine nerves, 

 which ramify nearly to the surface, between 

 the epithelial cells. 



The sclerotica (fig. 207, Scl.) or tunica 

 alhuginea covers the posterior four fifths of 

 the ball of the eye ; it is a milk-white, very 

 firm, fibrous membrane, continuous poste- 

 riorly with the sheath of the optic nerve, 

 becoming gradually thinner in front, except 

 at its termination, 'where the tendons of the 

 recti muscles become fused with it. It con- 

 sists of connective tissue, the bundles of 

 which are mostly straight, intimately united 

 as in the tendons, forming alternating lon- 

 gitudinal and transverse layers of various 

 breadth and thickness. ]\Iingled with the 

 connective tissue are numerous fine elastic 

 fibres, in the form of a network, with thick- 

 enings 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 con- 

 tents ; so that, when dried, they contain air. 



The membrane of the aqueous Jnimour 

 (fig. 207 d) consists of an elastic, perfectly 

 structureless (Descemet's) membrane, some- 

 what loosely connected with the cornea, and 

 an inner epithelial fining. Towards the cir- 

 cumlereuce of the cornea, the membrane of 



the aqueous humour merges into a peculiar 

 system of fibres, which Ci.mmence ni.'ar tlie 

 margin of the cornea, at the anterior surface 

 of the aqueous membrane (fig. 207//) a-; an 

 extended network of fine fibres, resembling 

 elastic fibrilloe ; this increases in thickness, 

 and at the very margin of the cornea the 

 aqueous membrane becomes lost in a tole- 

 rably dense network of these fibres, which 

 curve around the margin of the iris (fig. 

 207■^), some passing through the anterior 

 chamber, and become fused -^-ith the ante- 

 rior surtiice of this membrane and the ciliaiy 

 ligament (or muscle). These fibres form 

 the pectinate lic/ament of the iris, which is 

 much more distinct in some animals (as the 

 dog) than in man. 



The epithelium {e) of the aqueous mem- 

 brane consists of a single layer of polygonal 

 cells. These become smaller near the mar- 

 gin of the cornea, where the membrane ter- 

 minates as a continuous layer ; but isolated 

 portions of elongated or spindle-shaped cells 

 are continued over the pectinate ligament 

 to the anterior sm'face of the iris. 



The cornea yields choudriue on boiling, 

 and not gelatine. 



The cJioruid 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 tliick- 

 uess, and extends from the entrance of the 

 optic nerve to near the anterior margin of 

 the sclerotica, where it becomes thicker, 

 forming the ciHary body, whence it is con- 

 tinued into the iris. 



The choroid consists essentially of two 

 parts : — an outer vascular and thicker layer, 

 the proper choroid ; and an inner deeply 

 coloured layer, the j)i(imentum nif/rnm. 

 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 colom-ed proper vascular 

 layer, with the larger arteries and veins ; 

 and 3, a colourless delicate inner layer, 

 containing an extremely copious capillary 

 network — the choro-capillary membrane, 

 which docs not extend anteriorly beyond 

 tlu! margin of the retina. The stroma of 

 tlie clioroid proper consists of very irregular 

 spindle-shaped or stellate cells, from l-loOO 

 to 1-GOOO" in length, either paler, or con- 



