MEMBRANES OF THE EYE. 



599 



5. The iris is a process of the choroid, and continuous behind 

 with the ciliary processes; though modified in structure. For it 



Fig, 428. 



Fig. 429. 



Fig. 428. Diagram to show the position and action of the ciliary muscle, a. Sclerotic, b. Cornea. 

 c. Choroid, separated a little from the sclerotic, d. Situation of the ciliary ligament, and point from 

 which the ciliary muscle radiates, e. Iris. n. Lens connected with the ciliary processes of the 

 anterior wall of the canal of Petit, the situation of which is marked by the *. (Magnified 3 dia- 

 meters.) 



Fig. 429. Capillary network in choroid coat of the eye. 



has, 1, a stroma, mostly of collagenous tis- 

 sue ; 2, smooth muscular fibres ; and 8, a 

 layer of cells on both its anterior and its 

 posterior surface. The muscular fibres 

 form (1,) a distinct occlusor of the pupil 

 (sphincter pupillce) in the form of a smooth 

 ring ^ of an inch wide, close to the edge 

 of the iris. There is, besides, another very 

 narrow ring 7 |^ of a line wide. (2.) They 

 also form numerous slender fasciculi, but 

 not a distinct layer, extending from the 

 outer margin of the iris to the sphincter 

 pupillcz, into the border of which they are 

 inserted, constituting the dilator pupillce,. 



The layer of cells on the posterior sur- 

 face of the iris, constitutes the uvea; they 

 being closely filled pigment-cells. The 

 anterior layer of cells is a simple scaly 

 epithelium without pigment-granules. 

 The color of the iris in blue eyes depends 

 merely upon the pigment in the uvea, seen 

 through the substance of the iris; in hazel 



Fig. 430. 



Vessels of the choroid, ciliary 

 processes, and iris ; inner surface. 

 a. Portion of the capillary net- 

 work, or tunica Ruyschiana. b. 

 Ciliary processes, c. Portion of 

 the iris. From an infant. (After 

 Arnold. Magnified 14 diameters.) 



