1028 ORGANS OF SPECIAL SENSE 



consists of two layers of pigmented cells, each layer representing the extension for- 

 wards 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 depends upon the amount of stromal pigment. 



?>. The innermost or nervous coat. The inner surface of the uveal tract 

 is everywhere lined by a layer of pigment of corresponding extent, which usually 

 adheres to it closely on dissection. Development ally, 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 nerve-entrance. 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 are said to have here 

 a special function, viz., that of secreting the intraocular fluid. 



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 lip must be looked for at the edge of 

 the pupil, i. e., at the termination of this coat anteriorly. The inner wall of the cup, 

 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 epithelium, 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 

 inner wall abruptly increases in thickness, and rapidly acquires that complexity of 

 structure characteristic of the retina proper, which extends from here to the optic 

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

 held together by a supporting framework 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 

 the only retinal layer represented, and where consequently the retina is extremely 

 thin. The nerve-fibres run on the inner surface of the retina and are continuous 

 with those of the optic nerve; they constitute the only retinal layer that is con- 

 tinued into the intraocular end of the nerve. The nerve-cells are found between 

 these surface layers. The larger blood-vessels of the retina run in the inner layers, 

 and none encroach on the layer of nerve-epithelium. 



Within the coats mentioned, the interior of the eyeball is fully occupied by con- 

 tents, which are divided into three parts, and differently named according to their 

 consistence and anatomical form. They are all transparent, as through them the 

 light has to pass so as to gain the retina. Of these, the only one that is sharply and 

 independently outlined is the lens, which is situated in the anterior half of the globe 

 at the level of the ciliary processes, where it is suspended between the other con- 

 tents, which fill respectively the space in front of it and the space behind it. The 

 space in front is called the anterior or aqueous chamber; that behind the lens is 

 the vitreous chamber. 



The lens is a biconvex body, with its surfaces directed anteriorly and posteriorly; 

 these surfaces meet at its rounded-off edge or equator, which is near (but does not 

 touch) the ciliary processes all around. The posterior is considerably more convex 

 than the anterior surface; the central part of each surface is called its pole. The 

 lens is closely encased in a hyaline elastic capsule, thicker over the anterior than 

 over the posterior surface. Thus enclosed, it is held in position in the globe by a 

 suspensory ligament, attached to its capsule near the equator all around, and swung 

 from the ciliary region. Posteriorly, the lens rests in a cup formed by the front part 

 of the vitreous, while its anterior capsule is in contact with the aqueous fluid and 

 lies close against the back of the pupillary border of the iris. When in position 



