848 ORGAXS OF SPECIAL SEXSE 



thickening of the uveal tract in this region, muscle and folds and processes together, 

 is named the ciliary body. 



The iris projects into the interior of the front half of the eye in the form of a 

 circular disc perft)rated in the middle. The appearance of its anterior surface has 

 already been described. Its posterior surface exhiy)its numerous radial folds run- 

 ning from the ciliary ])rocesses to near the pupillary border; a thick layer of black 

 pigment covers it and curls round its inner edge, so as to conie into view all round 

 the pupil as seen from in front. The peripheral or ciliary border of the iris is 

 continuous with the front of the ciliary body, Avhere it also receives fibres from the 

 ligamentum pectinatum iridis; in other respects the iris is quite free, merely resting 

 on the front of the lens-capsule near the pupil. Its stroma is s})ongy in character, 

 being made up of vessels covered by a thick adventitia, running from the periphery 

 to the pupillary border, with interspaces filled by branching pigment cells, Avhich 

 are particularly abundant near the front surface. Deep in the stroma, running 

 round near the pupillary border, we find a broad flat band of smooth muscle fibres, 

 constituting the sphincter iridis. Immediately behind the vascular tissue lies a 

 thin membrane, consisting of fine, straight filjres running radially from the ciliary 

 border to just l)ehind the sphincter. The nature of these fibres has long been in 

 dispute, but they are now accepted as being undoubtedl}' smooth muscular, a dila- 

 tor iridis. 



The sphincter iridis and the ciliary muscle are supplied by the third nerve, and 

 the dilator iridis by the sympathetic, all by Avay of the ciliary ganglion. 



Quite posteriorly is the pigment already' mentioned, really consisting of two 

 layers of pigmented cells, each layer representing the extension forwards of one 

 subdivision of the retina. The front of the iris is covered by a delicate epithelial 

 layer, a continuation of that lining Descemet's membrane. The colour of the iris 

 in different individuals dej^ends upon the amount of stromal pigment. 



3. The innermost or nervous coat. — The inner surface of the uveal tract 

 is everywhere hned by a layer of i^igment of corresponding extent, which usually 

 adheres to it closely on dissection. Developmentally, however, this general pigment 

 lining is quite distinct from the uveal coat, and represents the outer wall of the 

 secondary optic vesicle or embryonic retina: it consists of a single layer of pig- 

 mented epithelial cells. The amount of pigment is greatest anteriorly, over the 

 ciliary region and iris, and there is again a small local increase posteriorly, corre- 

 sponding to the macula lutea and to the edge of the optic foramen. In the ciliary 

 region these cells have recently been described as lining numerous narrow tubular 

 depressions in the inner part of the uveal tract, and- they seem here to have a 

 special function, viz. that of secreting the intraocular fluids. 



From the manner in which the secondary optic vesicle, or optic cup, is formed, 

 its two walls are necessarily continuous in front, at what may be termed the lip of 

 the cup; we have just observed that the outer wall lines the uveal coat everywhere 

 and corresponds in extent; consequently, the li]) nntst be looked for at the edge of 

 the pupil, i.e. at the termination of this coat anteriorly. The inner icalloi the cu]), 

 consequently, reaches from the lip, or pupillary edge, in front, to the optic stalk or 

 nerve behind, and is in close apposition to the pigment-epithelium; unlike the outer, 

 however, this wall is represented in the developed eye by tissues very dissimilar in 

 structure in different parts of its extent. Tracing it backwards from the pupillary 

 edge, we find that over the whole posterior surface of the iris it exists as a single 

 layer of pigmented epithelimn, the developmental changes having here produced a 

 result similar to what we have found throughout in the outer wall: here, accord- 

 ingly, we have a double layer of pigment cells. At the root of the iris the single 

 inner layer of cells still exists; but now they become destitute of pigment, and this 

 condition obtains over the entire ciliary region, constituting what is known as the 

 pars ciliaris retinae. At the line of the ora serrata the tissue derived from the 

 imu-r wall a1)ruptly increases in thickness, and rapidly acquires that com])lexity of 

 structure characteristic of the retina proper, Avhich ext<nids from here to the optic 

 nerve. It consists of several layers — nerve-fibres, nerve-cells, and nerve-epithelium — 

 held together by a supporting framcAvork of delicate connective tissue. The nerve- 

 epithelium is on the outer surface, immediately applied to the pigment-epithelium; 

 at the posterior pole of the eye a small spot (fovea centralis; exists, where this is 



