830 THE ORGANS OF SPECIAL SENSE. 



between the former, but without regular alternation. The larger processes are 

 each about one-tenth of an inch in length, and are attached by their periphery to 

 three or four of the ridges of the orbiculus ciliaris, and are continuous with the 

 layers of the choroid : the opposite margin is free, and rests upon the circumfer- 

 ence of the lens. Their anterior surface is turned toward the back of the iris, 

 with the circumference of which they are continuous. The posterior surface is 

 connected with the suspensory ligament of the lens. 



Structure. The ciliary processes are similar in structure to the choroid, but 

 the vessels are larger, and have chiefly a longitudinal direction. They are covered 

 on their inner surface by two strata of black pigment-cells, which are continued 

 forward from the retina, and are named the pars ciliaris refines. In the stroma of 

 the ciliary processes there are also stellate pigment-cells, which, however, are not 

 so numerous as in the choroid itself. 



Anterior ciliary artery. 



Short ciliary tirterii'n. ^ 



Anterior ciliary artery. 



FIG. 4-43. The arteries of the choroid and iris. The sclerotic has been mostly removed. (Enlarged.) 



The Ciliary muscle (Bowman) consists of unstriped fibres : it forms a grayish, 

 semitransparent, circular band, about one-eighth of an inch broad, on the outer 

 surface of the fore part of the choroid. It is thickest in front, and gradually 

 becomes thinner behind. It consists of two sets of fibres, radiating and circular. 

 The former, much the more numerous, arise at the point of junction of the cornea 

 and sclerotic, and partly also from the ligamentum pectinatum iridis, and, passing 

 backward, are attached to the choroid opposite to the ciliary processes. One 

 bundle, according to Waldever, is continued backward to be inserted into the 



%/ 



sclerotic. The circular fibres are internal to the radiating ones and to some extent 

 unconnected with them, and have a circular course around the attachment of the 

 iris. They are sometimes called the " ring muscle " of Muller, and were formerly 

 described as the ciliary ligament. They are well developed in hypermetropic, but 

 are rudimentary or absent in myopic eyes. The Ciliary muscle is admitted to 

 be the chief agent in accommodation, i. e., in adjusting the eye to the vision of 

 near objects. Bowman believed that this was effected by its compressing the 

 vitreous body, and so causing the lens to advance ; but the view which now pre- 

 vails is that the contraction of the muscle, by drawing on the ciliary processes, 

 relaxes the suspensory ligament of the lens, thus allowing the anterior surface of 

 the lens to become more convex. The pupil is at the same time slightly con- 

 tracted. 1 



The Iris (iris, a rainbow) has received its name from its various colors in dif- 

 ferent individuals. It is a thin, circular-shaped, contractile curtain, suspended in 



1 See explanation and diagram in Power's Illustrations of some of the Principal Diseases of the Eye, 

 p. 590. 



