1014 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 





cleft, clearly due in a great number of cases to an arrest in development. In these cases tho 

 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 in 

 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 vessels 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 pupilla?; 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 with- 

 out 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. 



Optic disc 



Macula littea, 



Retina 

 FIG. 879. Interior of posterior half of bulb of left eye. The veins are darker in appearance than the arteries. 



The Retina (tunica internalThe 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 Be- 

 hind 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 o 



