832 THE ORGANS OF SPECIAL SENSE. 



gated ; those more external somewhat separated, and forming less complete circles. 

 The radiating fibres (dilator pupillse) converge from the circumference toward the 

 certtre, and blend with the circular fibres near the margin of the pupil. These 

 fibres are regarded by some as elastic, not muscular. 



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 these the dark 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 509). The long ciliary arteries, 

 two in number, having reached the attached margin of the iris, divide into an 

 upper and a lower branch, and, encircling the iris, anastomose with corresponding 

 branches from the opposite side; into this vascular zone (circulus major) the ante- 

 rior ciliary pour their blood. From this zone vessels converge to the free margin 

 of the iris, and these communicate by branches from one to another and thus form 

 a second zone (circulus minor) in this situation. 



The nerves of the choroid and 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. They pierce the sclerotic around the entrance 

 of the optic nerve, and run forward in the perichoroidal space, and supply 

 the blood-vessels of the choroid. After reaching the iris they form a plexus 

 around its attached margin ; from this are derived non-medullated fibres 

 which terminate in the circular and radiating muscular fibres. Their exact 

 mode of termination has not been ascertained. Other fibres from the plexus 

 terminate in a network on the anterior surface of the iris. The fibres derived 

 from the motor root of the lenticular ganglion (third nerve) supply the circular 

 fibres, while those derived from the sympathetic supply the radiating fibres. 



Membrana Pupillaris. In the foetus the pupil is closed by a delicate transparent 

 vascular membrane, the membrana pupillaris, which divides the space into which 

 the iris is suspended into two distinct chambers. This membrane contains numerous 

 minute vessels, continued from the margin of the iris to those on the front part of 

 the capsule of the lens. These vessels have a looped arrangement, and converge 

 toward each other without anastomosing. Between the seventh and eighth months 

 the membrane begins to disappear, by its gradual absorption from the centre 

 toward the circumference, and at birth only a few fragments remain. It is said 

 sometimes to remain permanent and produce blindness. 



III. The Retina. 



The Retina is a delicate nervous membrane, upon the surface of which the 

 images of external objects are received. Its outer surface is in contact with the 

 choroid ; its inner with the vitreous body. Behind, it is continuous with the optic 

 nerve; it gradually diminishes in thickness from behind forward; and, in front, 

 extends nearly as far as the ciliary body, where it appears to terminate 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 the ciliary proc- 

 esses and iris, forming the pars ciliaris retince and pars iridica retinae, already 

 referred to. This forward prolongation consists of the pigmentary layer of the 

 retina together with a stratum of columnar epithelium. The retina is soft, semi- 

 transparent, and of a purple tint in the fresh state, owing to the presence of a 

 coloring-material named rhodopsin or visual purple ; but it soon becomes clouded, 

 opaque, and bleached when exposed to sunlight. Exactly in the centre of the 



