498 



IRIS. CILIARY PROCESSES. 



The Iris (iris, a rainbow,) is so named from its variety of colour in 

 different individuals: it forms a septum between the anterior and 

 posterior chambers of the eye, and is pierced somewhat to the nasal 

 side of its centre by a circular opening, which is called the pupil. By 

 its periphery it is connected with the ciliary ligament, and by its 

 inner circumference forms the margin of the pupil ; its anterior surface 

 looks towards the cornea, and the posterior towards the ciliary pro- 

 cesses and lens. 



The iris is composed of two layers, an anterior or muscular, consist- 

 ing of radiating fibres which converge from the circumference towards 

 the centre, and have the power of dilating the pupil ; and circular, 

 which surround the pupil like a sphincter, and by their action produce 

 contraction of its area. The posterior layer is of a deep purple tint, 

 and is thence named uvea, from its resemblance in colour to a ripe 

 grape. 



The Ciliary processes may be seen in two ways, either by removing 

 the iris from its attachment to the ciliary ligament, when a front view 

 of the processes will be obtained, or by making a transverse section 

 through the globe of the eye, when they may be examined from 

 behind, as in fig. 154. 



The ciliary processes consist of a number of triangular folds, formed 

 apparently by the plaiting of the middle and internal layer of the 

 choroid. According to Zinn, they are about sixty in number, and 



* The anterior segment of a transverse section of the globe of the eye, seen 

 from within. 1. The divided edge of the three tunics ; sclerotic, choroid (the 

 dark layer), and retina. 2. The pupil. 3. The iris, the surface presented to 

 view in this section being the uvea. 4. The ciliary processes. 5. The scallop- 

 ed anterior border of the retina. 



