746 



THE EYE AND VISION. 



[CH. LV. 



The choroid coat ends in front in what are called the ciliary 

 processes (figs. 565, 566). These consist of from 70 to 80 meridion- 

 ally arranged radiating plaits, which consist of blood-vessels, fibrous 

 connective tissue, and pigment corpuscles. They are lined by a 

 continuation of the membrane of Bruch. The ciliary processes 



terminate abruptly at the margin 

 of the lens. The ciliary muscle 

 (13, 14 and 15, fig. 565), 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 ; (6) radial fibres 

 passing to be inserted into the 

 choroid behind the ciliary pro- 

 cesses ; and (c) circular fibres 

 (muscle of Mtiller), more inter- 

 nal ; they constitute a sphincter. 

 The Iris. The iris is a con- 

 tinuation of the choroid inwards 

 beyond the ciliary processes. It 



is a fibro-muscular membrane perforated by a central aperture, 

 the pupil. It is made up chiefly of blood-vessels and connective 

 tissue, ,with pigment-cells and unstriated muscle. 



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 enclosed in connective 

 tissue. 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 dilatator pupillce. The iris is covered anteriorly by a layer of 

 epithelium continued upon it from the posterior surf ace of the cornea. 

 The Lens. 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. 



Fig. 566. Ciliary processes, as seen from 

 behind, i, posterior surface of the iris, 

 with the sphincter muscle of the pupil ; 



2, anterior part of the choroid coat; 



3, one of the ciliary processes, of which 

 about seventy are represented. |. 



