THE RETINA. 425 



Stretched across the interior of the eyeball, and attached by its 

 circumference, to the choroid, ciliary ligament, and cornea, is the thin 

 membranous curtain, called the iris, Fig. 83, i, Fig. 81, i i, perforated 



Fig. 81. 



Fig. 81. The ins or perforated colored diaphragm, removed from the eyeball, i i, its 6uter attached 

 border, p, the pupillary opening in its middle. 



a little to the inner side of its centre by a circular opening, the pupil, 

 p. The contraction and dilatation of this aperture, regulate the amount 

 of light which passes into the eye. In health, the size of the pupil 

 varies from 3*3 th to Jd of an inch. After dea-th, its average diameter 

 is nearly Jth of an inch. The anterior surface of the iris, which is 

 flat, contains pigment cells; it is brilliantly reflective, and gives the 

 eye its special color. The iris arid pupil appear to be larger and 

 nearer to the cornea than they really are; placing the eye under 

 water, removes this deceptive appearance. The iris is composed of 

 unstriped muscular fibres, a fibrous stroma, bloodvessels, nerves, and 

 a quantity of pigment cells. The muscular fibres consist of circular 

 and radiating fibres ; the circular fibres, placed at the back of the iri^, 

 opposite the ring named the annulus minor, form a narrow band, the 

 sphincter pupillce ; the radiating fibres pass from the circumference 

 towards the pupil, near the margin of which they blend with the circu- 

 lar fibres, which here lose their parallel arrangement. The fibrous 

 stroma is made up of delicate bundles of fibrous tissue, the greater 

 number of which radiate towards the pupil; others are arranged in a 

 circular manner. The bloodvessels form loops. The pigment cells in 

 the substance of the iris are ramified, and are of a yellow or brown 

 color, according to the color of the eye; on the posterior surface, the 

 pigment cells are of a dark-brown or black hue, and consist of several 

 layers, forming what is called the uvea. These cells are, as a rule, 

 darker in children than in adults; in the former, the delicate pale 

 blue tint of the white of the eye, is due to the sclerotic coat being very 

 thin, so that the pigment within, can be partly seen through it ; the 

 pigment cells are also darker in dark persons, and in the swarthy races 

 of mankind. 



Within the choroid, is the retina, or the sensitive coat of the eyeball, 

 Fig. 83, r. This structure is a delicate nervous membrane formed by 

 the expansion of the optic nerve. It is so supported as to present a 

 concave surface to the light ; it does not extend so far forward as the 

 choroid, but ends, at a short distance from the ciliary ligament, in a 

 jagged edge, called the ora serrata, from which an exceedingly fine 

 membrane, not nervous, extends forwards to the ciliary processes. By 

 its outer surface, the retina is slightly, though organically, connected 

 with the choroid ; its inner surface is bounded by a very delicate 



