EYE. 



[ 252 ] 



EYE. 



cells, from 1-1500 to 1-6000" in length, 

 either paler,oreontain- Yig. 214. 



ing a large quantity of 

 pigment, and anasto- 

 mosing by numerous 

 long and very slen- 

 der proeesses(fig.214). 

 These cells are most 

 distinct in the outer 

 layer ; whilst more 

 internally, and espe- 

 cially in the choro- 

 capillary membrane, 

 they gradually pass 

 into a homogeneous 



or slightly striated Cells from the stroma of the 



nucleated tissue, either choroid: «, containing pig- 



. . T 1 J ment ; o, fusiform cells with- 



COntammg little and out pigment ; c, anastomosis 



ultimately no pig- of the former. 



•Qjpjj^^ Magn. 350 diams. 



In some animals the choroid membrane 

 contains muscular fibres. Between the stro- 

 ma and the pigmentum nigrum, is a very 

 thin elastic layer ; this is either structure- 

 less, granular or finely reticulated, and is 

 comparable to a basement membrane. 



The ciliary ligament, or, properly, ciliary 

 muscle, — tensor choroids (fig. 213 k), — 

 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 sclero- 

 tica 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 pigmentum nigrum (fig. 213, m) lines 



Fig. 215. 



Cells of the human pigmentum nigrum: a, surface 

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



Magnified 350 diameters. 



the inner surface of the choroid, and as far 



as the termination of the retiaa consists of a 

 single layer of beautiful, regularly six-sided 

 cells (fig. 215, a, 5), from 1-2000 to 1-1500" 

 in diameter ; they contain abundance of pig- 

 ment. Beyond the margin of the retina, the 

 cells form 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 molecular motion. In the 

 e3^es of albinos, and in the region of the 

 tapetum of animals, the cells contain no pig- 

 ment. 



The iris (fig. 213, 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 areolar tissue, forming delicate 

 loose bundles, some of which pursue a radia- 

 ting, others a circular course, and interlacing 

 variously; in this tissue are a number of 

 spindle-shaped or stellate cells, containing 

 pigments, corresponding to those of the 

 choroid; and in addition to numerous blood- 

 vessels and nerves, two sets of muscular 

 fibres ; the latter in some animals are trans- 

 versely 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 pig- 

 ment layer or uvea consists of the same 

 elements as those of the corresponding layer 

 of the choroid. 



The anterior coat consists of a single layer 

 of rounded, flattened, epithelial cells. 



The blood-vessels of the choroid membrane 

 and ciliary processes are easily injected (e. g. 

 in the sheep or ox) from the ciliary arteries, 

 and form a magnificent object. 



Retina. — The structure of the retina is so 

 extremely complicated, that we have not 

 space to give more than a sketch of its com- 

 ponent elements. 



Eight layers are apparently present in a 

 transverse section of the retina (fig. 216), 

 excluding the hyaloid membrane, a ; viz. 1, 

 the layer of bacilli and cones (fig. 216, k, i) ; 

 2, an outer {h) ; 3, an intermediate [g), and 

 4, an inner (/) granular layer ; 5, a layer of 

 nerve-cells (e); 6, the expansion of the optic 

 nerve (rf) ; 7, the inner ends of the radial 

 fibres (c) ; and 8, the limiting membrane (6). 



