EYE. 



[ 311 ] 



EYE. 



Fig. 208. 



taming a large quantity of pigment, and 

 anastomosing by numerous long and very 

 slender processes (fig. 208). These cells are 

 most distinct in the 

 outer layer ; whilst 

 more internally, and 

 especially in the cho- 

 ro - capillary mem- 

 brane, they gradually 

 pass into a homoge- 

 neous or slightly stri- 

 ated nucleated tissue, 

 containing but little 

 and ultimately no 

 pigment. 



In some animals 

 the choroid mem- 

 brane contains mus- 

 cular fibres. Cells from the stroma of 



Between the stro the choroid ;' containing 

 -*' pigment; b, fusiform cells 



ma and the pigmen- without pigment; c, ana- 

 turn nigrum is a very 8t niosi8 of the former. 

 , , . , , . , J Magn. 3oO diams. 



thin elastic layer ; 



this is either structureless, granular, or finely 

 reticulated, and is comparable to a base- 

 ment membrane. 



The ciliary muscle tensor choroidees (fig. 

 207 &) is composed of a tolerably thick 

 layer of radiating unstriated muscular 

 fibres; these, intermixed with pigment- 

 cells of the choroid, pass from the anterior 

 margin of the sclerotica to the ciliary body, 

 and lose themselves in its anterior half, 

 opposite the base of the ciliary processes. 

 The fibre-cells are 1-600" in length, broader 

 than most fibre- cells, and not easily isolated 

 in man. 



The ciliary processes consist of the same 

 stroma as the choroid; but the stellate 

 cells are more delicate and fewer, and, with 

 the exception of those at their base, do not 

 contain pigment; nor are they furnished 

 with the elastic lamina. 



The piymentum nigrum (fig. 207 in) lines 

 the inner surface of the choroid, and as far 



Fig. 209. 



Cells of the human pigmentnm nigrum a, surface 

 dew ; b, side view ; c, pigment-granules. 



Magnified 350 diameters. 



as the termination of the retina, consists of 

 a single layer of beautiful, regularly six- 

 sided cells (fig. 209 a, b), from 1-2000 to 

 1-1500" in diameter ; they contain abund- 

 ance of pigment. Beyond the margin of 

 the retina, the cells f orm mostly two layers, 

 and become rounded and more loaded with 

 pigment. The granules of pigment are 

 very minute, rounded, from 1-20,000 to 

 1-30,000" in diameter, and exhibit mole- 

 cular motion. In the eyes of albinos, and 

 in the region of the tapetum of animals, the 

 cells contain no pigment. 



The iris (fig. 207, I) consists of three 

 layers : an anterior epithelial layer ; a pos- 

 terior layer of pigment, called the uvea, and 

 continued from the inner pigment layer of 

 the choroid ; and a middle, the thickest or 

 fibrous layer. 



The fibrous layer differs from the choroid, 

 in containing connective tissue forming deli- 

 cate loose bundles, some of which pursue a 

 radiating, others a circular course, and in- 

 terlacing variously; in this tissue are a 

 number of spindle-shaped or stellate cells, 

 containing pigment, corresponding to those 

 of the choroid, and in addition to nume- 

 rous blood-vessels and nerves, two sets of 

 muscular fibres : the latter in some animals 

 are transversely striated ; but in man they 

 resemble the ordinary unstriped fibre-cells, 

 and are 1-600 to 1-400" in length. One set 

 forms a sphincter for closing the pupil, its 

 fibres taking a circular direction ; the other 

 set consists of bundles of radiating fibre- 

 cells, traversing the stroma of the iris. The 

 pigment layer or uvea consists of the same 

 elements as those of the corresponding laver 



A . i t i * D v 



of the choroid. 



The anterior coat consists of a single layer 

 of rounded, flattened epithelial cells. 



The blood-vessels of the choroid mem- 

 brane and ciliary processes are easily injected 

 (e. g. in the sheep or ox) from the ciliary 

 arteries, and form a magnificent object. 



The retina is the terminal membranous 

 expansion of the optic nerve within the 

 globe of the eye. It consists of nerve-cells 

 and fibres, imbedded in a spongy supporting 

 connective tissue. 



Nine layers are distinguishable in a 

 transverse section of the retina (fig. 210), 

 commencing from within: viz. 1, the inter- 

 nal limiting membrane (b) 2, the layer of 

 optic nerve-fibres (d) 3, a layer of nerve- 

 or ganglion-cells (e) ; 4, a molecular layer j 

 5, an inner granular layer (/) ; 6, an inter- 

 mediate granular layer (g) 7, an outer 



