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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
cleft, clearly due in a great number of cases to an arrest in development. In these cases the cleft is found at the lower aspect, extending directly downward from the pupil, and the gap frequently extends through the choroid to the porus opticus. In some rarer cases the gap is found in other parts of the iris, and is not then associated with any deficiency of the choroid.

Vessels and Nerves.—The arteries of the iris are derived from the long and anterior ciliary arteries, and from the vessèls of the ciliary processes (see p. 571). Each of the two long ciliary arteries, having reached the attached margin of the iris, divides into an upper and lower branch; these anastomose with corresponding branches from the opposite side and thus encircle the iris; into this vascular circle (circulus arteriosus major) the anterior ciliary arteries pour their blood, and from it vessels converge to the free margin of the iris, and there communicate and form a second circle (circulus arteriosus minor) (Figs. 877 and 878).
  The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion. They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the bloodvessels of the choroid. After reaching the iris they form a plexus around its attached margin; from this are derived non-medullated fibers which end in the Sphincter and Dilatator pupillæ their exact mode of termination has not been ascertained. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.

Membrana Pupillaris.—In the fetus, the pupil is closed by a delicate vascular membrane, the membrana pupillaris, which divides the space in which the iris is suspended into two distinct chambers. The vessels of this membrane are partly derived from those of the margin of the iris and partly from those of the capsule of the lens; they have a looped arrangement, and converge toward each other without anastomosing. About the sixth month the membrane begins to disappear by absorption from the center toward the circumference, and at birth only a few fragments are present; in exceptional cases it persists.


FIG. 879– Interior of posterior half of bulb of left eye. The veins are darker in appearance than the arteries. (See enlarged image)


The Retina (tunica interna).—The retina is a delicate nervous membrane, upon which the images of external objects are received. Its outer surface is in contact with the choroid; its inner with the hyaloid membrane of the vitreous body. Behind, it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward, and extends nearly as far as the ciliary body, where it appears to end in a jagged margin, the ora serrata. Here the nervous tissues of the retina end, but a thin prolongation of the membrane extends forward over the back of

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