THE EYE 385 



It consists of circular and radiating fibers. The latter arise 

 near the union of the sclerotic and cornea, and are inserted 

 into the choroid opposite the ciliary processes; the former 

 surround the insertion of the iris. 



The iris gives to the eye its color. It is a thin, contractile, 

 circular membrane presenting, at about its centre, a circular 

 aperture, the pupil. It is suspended in the aqueous humor 

 behind the cornea and in front of the lens. Its circumference 

 is continuous with the choroid, and, through the lig amentum 

 pectinatum, with the cornea. Its posterior surface is covered 

 by dark pigment resembling that of a ripe grape; hence the 

 term "uvea. " The edges of the pupillary orifice are in con- 

 tact with the lens, the size of the pupil varying from TO to J 

 inch across. 



The muscle fibers are radiating and circular. The latter 

 form a sphincter for the pupil; the former constitute the dilator 

 muscle. 



The arteries are supplied from the long and anterior ciliary. 

 The nerves are branches of the lenticular ganglion and the 

 long ciliary from the nasal branch of the ophthalmic. They 

 form a plexus around the circumference of the iris, and end 

 in the muscular fibers and in a network on the front of the* 

 iris. The nerves to the circular fibers come from the motor 

 oculi; those to the radiating, from the sympathetic. 



The retina is a delicate nervous membrane on which the 

 image of perceived objects is formed. It lies between the 

 choroid and the hyaloid membrane of the vitreous, and is 

 composed of ten layers. Behind, the optic nerve expands into 

 it, and in front it terminates in a dentated margin, the ora 

 serrata, at the outer edge of the ciliary processes. It then 

 sends off a thin, non-nervous membrane, the pars ciliaris 

 retinae, to the tips of the ciliary processes. The inner surface 

 of the retina presents at its centre an elliptical spot about 

 YO- inch across, the macula lutea. In the centre of this spot 

 is a depression, the fovea centralis, which, on account of the 

 extreme thinness of the retina, shows the pigmentary layer of 

 the choroid, and hence presents the appearance of a foramen. 

 About T V inch to the inner side of the yellow spot is the poms 

 options, at which point the ' optic nerve enters, the nervous 

 matter being heaped up here so as to form the colliculus. Pass- 

 ing through nearly the entire thickness of the retina, supporting 

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