THE IRIS. 31 



The suspensory ligament of the lens is thus relaxed and the lens becomes more 

 convex. 



The nerves of the choroid will be described with those of the iris. 



The iris is the contractile and coloured membrane which is seen behind the 

 transparent cornea, and gives the tint to the eye. In its centre it is perforated by 

 an aperture the pif/i/. 



At its circumferential border, which is nearly circular, the iris is continuous 

 with the choroid, and by the ligamentum pectinatum, with the cornea : the free 

 inner edge is the boundary of the pupil. The iris measures about 11 mm. across, 

 and, in a state of rest, about 5 mm. from the circumference to the pupil (slightly 

 less on the nasal side) : its thickness is about 0'4 mm. The anterior surface, 

 variously coloured in different eyes, is marked by waved lines converging towards the 

 pupil, near which they join in a series of irregular elevations ; and, internal to these, 

 other finer lines pass to the pupil. The appearance is produced by the sub- 

 jacent blood-vessels. The posterior surface is covered with dark pigment cells (pars 

 retinalis), and is marked by a number of fine converging folds or thickenings 

 prolonged from the ciliary processes. 



Fig. 38. CILIARY PROCESSES AS SEEN FROM BEHIND. 

 (Twice the natural size.) 



1 , posterior surface of the iris, with the sphincter muscle 

 of the pupil ; 2, anterior part of the choroid coat ; 3, ciliary 

 processes. 



The pupil is nearly circular in form, and is 

 usually placed a little to the inner side of the 

 centre of the iris. It is constantly varying in 

 size during life, the variation ranging from 1 mm. 

 to 8 mm. It is habitually wider in young than 

 in old persons. 



Structure of the iris. A delicate connective tissue forms the framework or 

 stroma of the iris (fig. 39). It contains also very numerous vessels and nerves. 

 The endothelial layer of the membrane of Descemet (fig. 36) is continued from 

 the margin of the cornea over the front of the iris ; its cells are smaller and more 

 granular than those which cover the membrane of Descemet, but are otherwise 

 similar. Depressions of some size have been described in this anterior surface of 

 the iris (Fuchs, Nue'l and Cornil): these have been termed stomata, but it is 

 doubtful if they are analogous to the openings of the same name which occur in 

 serous membranes and communicate with subjacent lymphatics. The posterior 

 surface of the iris is covered by a layer of pigmented epithelium two or three cells 

 deep. This is continuous with the (retinal) pigmentary layer covering the ciliary 

 processes, and ends abruptly at the margin of the pupil ; it forms what is known as 

 the pars retinalis iridis (pars iridicaretince), as distinguished from the pars choroidalis 

 or stroma. 



The stroma consists of cells and fibres of connective tissue, the latter directed 

 for the most part radially towards the pupil. In the substance of the iris anteriorly 

 and throughout its thickness are variously-shaped ramified pigment cells like those 

 in the choroid. The pigment contained in them is yellow, or of lighter or darker 

 shades of brown, according to the colour of the eye. The colour of the iris depends 

 partly on the pigment in the cells of the pars retinalis, partly on that in the stroma 

 cells ; in the eye of the infant and in the different shades of blue eye it arises 

 from the black pigment of the posterior surface appearing more or less through the 



