THE EYE. 



897 



of fine delicate bundles of fibrous tissue, of which some few fibres have a circular 

 direction at the circumference of the iris, but the chief mass consists of fibres 

 radiating toward the pupil. They form, by their interlacement, a delicate mesh, 

 in which the vessels and nerves are contained. Interspersed between the bundles 

 of connective tissue are numerous branched cells with fine processes. Many of 

 them in dark eyes contain pigment-granules, but in blue eyes and the pink eyes of 

 albinos they are unpigniented. 



3. The muscular fibre is involuntary, and consists of circular and radiating 

 fibres. The circular fibres (sphincter of the pupil) surround the margin of the 

 pupil on the posterior surface of the iris, like a sphincter, forming a narrow band 



Circ. fibres 

 of sclerotic. 



^_ f Pars ciliaris 

 ~\ retime. 



jssy. 



Circ. fibres / \ 

 of Ciliary muscle 



FIG. 531. Section of the eye, showing the relations of the cornea, sclerotic, and iris, together with the 

 Ciliary muscle and the cavernous spaces near the angle of the anterior chamber. (Waldeyer.) 



about one-thirtieth of an inch in width, those near the free margin being closely 

 aggregated ; those more external somewhat separated, and forming less complete 

 circles. The radiating fibres (dilator of the pupil) converge from the circumfer- 

 ence toward the centre, and blend with the circular fibres near the margin of 

 the pupil. 



4. Pigment. The situation of the pigment-cells differs in different irides. In 

 the various shades of blue eyes the only pigment-cells are several layers of small 

 round or polyhedral cells filled with dark pigment, situated on the posterior surface 

 of the iris and continuous with the pigmentary lining of the ciliary processes. The 

 color of the eye in these individuals is due to this coloring matter showing more 

 or less through the texture of the iris. In the albino even this pigment is absent. 

 In the gray, brown, and black eye there are, as mentioned above, pigment-granules 

 to be found in the cells of the stroma and in the epithelial layer on the front of the 

 iris, to which the color of the eye is due. 



The arteries of the iris are derived from the long and anterior ciliary and from 

 the vessels of the ciliary processes (see page 570). 



The nerves of the iris are derived from the ciliary branches of the lenticular 

 ganglion and the long ciliary from the nasal branch of the ophthalmic division of 

 the fifth. After reaching the iris in the manner described above (page 797) they form 

 a plexus around the attached margin of the iris ; from this are derived non- 

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