1062 



SPECIAL SENSE ORGANS 



Developmentally this general pigment lining is quite distinct from the vascular coat, and 

 represents the outer wall of the secondary optic vesicle or embryonic retina; it consists of a single 

 layer of pigmented epithelial cells. It is* known as the stratum pigmenti. The amount of 

 pigment is greatest anteriorly, over the ciliary region and iris, and there is again a small local 

 increase posteriorly, corresponding to the macula lutea and to the edge of the optic nerve en- 

 trance. In the ciliary region these cells have recently been described as lining numerous nar- 

 row tubular depressions in the inner part of the vascular 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 vascular coat every- 

 where 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 Hp, 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 backward 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 two layers of the cup having here 

 produced 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 con- 

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

 ciliaris retinae. At the Vne 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 and is termed the pars optica retinoe. It consists of several layers — nerve- 

 fibres, nerve-cells, and nerve-epithelium — held together by a supporting frame- 

 work of delicate connective tissue. 



The nerve-epithelium is on the outer surface, immediately applied to the pigment-epithe- 

 lium; 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 continued 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. 



Fig. 805. — The Lens. (Side view; enlarged.) 



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

 cents, which are divided into three parts, which are 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 of the lens called the aqueous chamber; that behind the lens is the 

 vitreous chamljer. 



The lens [lens crystallina] is a biconvex disc, with its surfaces directed ante- 

 riorly and posteriorly; these surfaces meet at its rounded-off edge or equator 

 [sequator lentis] which is near. (but docs not touch) the adjacent ciliary processes. 

 The posterior is considerably more convex than the anterior surface; the central 

 part of each surface is called its pole [polus anterior; polus posterior]. The lens 

 is clo.sely encased in a hyaline clastic capsule [capsula lentis] thicker over the an- 

 terior than over the posterior surface. Thus enclosed, it is held in position in the 

 globe by a suspensory ligament, attached to the lens capsule near the equator of 

 the eye, and swung from the ciliary region. Posteriorly, the lens rests in a cup 



