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From without inward the three tunics are: (1) A fibrous tunic, consisting of the sclera behind and the cornea in front; (2) a vascular pigmented tunic, comprising, from behind forward, the choroid, ciliary body, and iris; and (3) a nervous tunic, the retina.
The sclera and cornea form the fibrous tunic (tunica fibrosa oculi) of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.
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The arteries of the iris are derived from the long and anterior ciliary arteries, and from the vessèls of the ciliary processes. 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).
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.
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1. Gray, Henry; Anatomy of the Human Body; Philadelphia; Lea & Febiger; 1918. [ISBN: 1-58734-102-6]
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