442 IRIS CILIARY PROCESSES. 



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

 inner circumference forms the margin of the pupil : its anterior sur- 

 face looks towards the cornea, and the posterior towards the ciliary 

 processes and lens. 



It is composed of two layers, an anterior or muscular, consisting 

 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 

 Fig. 137. sphincter, and by their action pro- 



duce 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 re- 

 moving the iris from its attachment 

 to the ciliary ligament, when a front 

 view of the processes \vill be ob- 

 tained, or by making a transverse 

 section through the globe of the eye, 

 when they may be examined from 

 behind, as in fig. 137. 

 The ciliary processes consist of a number of triangular folds, 

 formed apparently by the plaiting of the internal layer of the cho- 

 roid. They are, according to Zinn, about sixty in number, and may 

 be divided into large and small, the latter being situated in the 

 spaces between the former. The periphery is connected with the 

 ciliary ligament, and is continuous with the internal layer of the 

 choroid. The central border is free, and rests against the circum- 

 ference of the lens. The anterior surface corresponds with the 

 uvea ; the posterior receives the folds of the zonula ciliaris between 

 its processes, and thus establishes a connexion between the choroid 

 and the third tunic of the eye. > The ciliary processes are covered 

 with a thick layer of pigmentum nigrum, which is more abundant 

 upon them, and upon the anterior part of the choroid, than upon 

 the posterior. When the pigment is washed off, the processes are 

 of a whitish colour. 



3. Third tunic. The third tunic of the eye is the retina, which 

 is prolonged forwards to the lens by the zonula ciliaris. 



Dissection. If after the preceding dissection the choroid mem- 

 brane be carefully raised and removed, the eye being kept under 

 water, the retina may be seen very distinctly. 

 The Retina is composed of three layers : 



* 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 scalloped anterior border of the 

 retina. 



