CH. LVL] 



THE IRIS AND LENS 



769 



They are lined by a 

 The ciliary processes 



FIG. 568. Ciliary processes, as seen from 

 behind. 1, Posterior surface of the iris, 

 with the sphincter muscle of tho pupil ; 

 2, anterior part of the choroid coat ; 3, 

 one of the ciliary processes, of which 

 about seventy are represented. 



connective tissue, and pigment corpuscles, 

 continuation of the membrane of Bruch. 

 terminate abruptly at the margin of 

 the lens. The ciliary mmcle (13, 14, 

 and 15, fig. 567), takes origin at the 

 corneo-scleral junction. It is" a ring 

 of muscle, 3 mm. broad and 8 mm. 

 thick, made up of fibres running in 

 three directions. (a) Meridional 

 fibres near the sclerotic and passing 

 to the choroid; (b) radial fibres in- 

 serted into the choroid behind the 

 ciliary processes; and (c) circular 

 fibres (muscle of Miiller), more in- 

 ternal ; they constitute a sphincter. 



The Iris is a continuation of the 

 choroid inwards beyond the ciliary 

 processes. It is a fibro-muscular 

 membrane perforated by a central 

 aperture, the pupil. 



Posteriorly is a layer of pigment cells (uvea), which is a con- 

 tinuation forwards of the pigment layer of the retina. The structure 

 of the iris proper is made of connective tissue in front with corpuscles 

 which may or may not be pigmented, and behind of similar tissue 

 supporting blood - vessels. The pigment cells are usually well 

 developed here, as are also many nerve-fibres radiating towards 

 the pupil. Surrounding the pupil is a layer of circular unstriped 

 muscle, the sphincter pupillce. In some animals there are also 

 muscle-fibres which radiate from the sphincter in the substance 

 of the iris forming the dilator pupillce. The iris is covered 

 anteriorly by a layer of epithelium continued upon it from the 

 posterior surface of the cornea. 



The Lens is situated behind the iris, being enclosed in a distinct 

 capsule, the posterior layer of which is not so thick as the anterior. 

 It is supported in place by the suspensory ligament, fused to 

 the anterior surface of the capsule. The suspensory ligament is 

 derived from the hyaloid membrane, which encloses the vitreous 

 humour. 



The lens is made up of a series of concentric laminae (fig. 569), 

 which, when it has been hardened, can be peeled off like the coats of 

 an onion. The laminae consist of long ribbon-shaped fibres, which in 

 the course of development have originated from cells. 



The fibres near the margin have nuclei and are smooth, those 

 near the centre are without nuclei and have serrated edges. They 

 are hexagonal in transverse section. The fibres are united together 



