THE PUPIL 17 



the retinal pigment, hence have pink irides owing to the easy visibihty 

 of the numerous blood vessels of the iris. 



The Pupil — The function of the iris is to 'stop down' the lens (Fig. 2) 

 — to prevent the light coming in through the peripheral zone of the 

 cornea from passing through the edge of the lens and reaching the retina. 

 Only the central part of the lens is optically good, and within certain 

 limits the image on the retina will be sharper, the smaller the aperture 

 in the iris. At the same time, the image will be less bright with a smaller 

 pupil, and in a given illumination might not be intense enough to affect 

 the retina unless the pupil could be opened more widely. A wide dila- 

 tation of the pupil affects the illumination of the image more than its 

 area or the size of the visual field it subtends; but this increase in image 

 brightness entails a sacrifice of the clarity of the picture, owing to the 

 optical imperfection of the lens periphery which is brought into play. 

 The regulation of the size of the pupil, in sympathy with the vari- 

 ations in the sensitivity of the retina and the external illumination, is 

 accomplished by contractile elements in the iris. Some of these are full- 

 fledged involuntary muscle cells, indistinguishable from those of the 

 abdominal organs, and are organized into a ring-shaped 'sphincter 

 pupillje' embedded in the iris stroma and closely surrounding the pupil 

 {sm in Figs. 5, 7a, 7g). Contraction of this muscle reduces the diameter 

 of the pupillary circle, though of course there is an obvious minimum 

 below which it cannot be further reduced; so, a circular pupil like that of 

 man cannot be closed entirely. The antagonist of the sphincter is a com- 

 plex consisting of the elasticity of the tissue and the radial blood vessels 

 (Fig. 7f) of the iris (which are straightened out when the sphincter con- 

 tracts and which tend to return to an undulant resting shape) together 

 with the active contractility of the 'dilatator pupillae'. This latter (Fig. 

 7a, pbl; Fig. 7b-e) is not a true muscle, but a myoid lamina developed 

 from the anterior face of the pars iridica retinae : the sparsely pigmented 

 cells of the anterior layer of this epithelium have each a long spindle- 

 shaped portion containing contractile fibrillae and lying with its long axis 

 at right angles to that of the cell body, which is attached to the middle of 

 the spindle (Fig. 7b-e) . These spindles thus form a layer which appar- 

 ently lies alongside the anterior layer of epithelium (Fig. 7a, pbl) in the 

 pars iridica retinae, but is really a part of that epithelium (Fig. 7g) . The 

 spindles may even be fused with each other in a syncitial fashion, though 

 this point is uncertain. Their myofibrillae run radially in the iris, so that 



