CILIARY MUSCLE AND IRIS. 661 



albidvs), may be seen in its natural position outside the front of the 

 choroid coat. 



It consists of unstriated fibres, and forms a grayish layer, about y^th of 

 an inch wide, on the surface of the choroid coat (fig. 231, e) : there are 

 two sets of fibres in it, radiating and circular : 



The radiating or superficial, arise in front from the sclerotic coat (fig. 

 227, ), and the fibres of the posterior elastic layer of the cornea ; its 

 fibres are directed backwards, and are inserted into the choroid coat oppo- 

 site to, and rather behind the ciliary processes. The nerves to the iris 

 pierce it. 



The circular fibres (ciliary ligament, fig. 231, d) are internal to the 

 radiating, and form a ring about ? V n f ' nc ^ wide around the edge of the 

 iris : they are not attached to the sclerotic or the cornea. 



Use. The radiating fibres of the muscle draw forwards the fore part of 

 the choroid coat, and the ciliary processes, and thereby relax the suspen- 

 sory ligament of the lens. The circular fibres are said to compress the 

 outer part of the lens. 



The IRIS (fig. 231, c) is a vascular and muscular structure, about half 

 an inch in diameter, whose vessels are continuous with those of the cho- 

 roidal coat. Its position and connections may be observed in the different 

 dissections that have been prepared. 



Placed within the ring of the ciliary muscle, it is suspended in front of 

 the lens (fig. 227, /), and is pierced by an aperture for the transmission of 

 the light. It is circular in form, is variously colored in different persons, 

 and is immersed in the aqueous humor. By its circumference it is con- 

 nected with the choroid coat and the posterior elastic layer of the cor- 

 nea. The anterior surface is free in the aqueous humor, and is marked by 

 lines converging towards the pupil. The posterior surface is covered with 

 a thick layer of the pigment (fig. 227), to which the term uvea has been 

 applied. 



The aperture in it (fig. 231 ) is the pupil of the eye ; this is slightly in- 

 ternal to the centre, and is nearly circular in form, but its size is constantly 

 varying (from Jyth to ^ of an inch) by the contraction of the muscular 

 fibres, according to the degree of light acting on the optic nerve. 



Membrane of the pupil. In the foetus the aperture of the pupil is closed 

 by a vascular transparent membrane, which is attached to the edge of the 

 iris, and divides into two distinct chambers the space in which the iris is 

 suspended. The vessels in it are continuous behind with those of the iris 

 and the case of the lens. About the eighth month the vessels become im- 

 pervious, and at the time of birth only fragments of the structure remain. 



Structure (fig. 229). The stroma of the iris is composed of fibres of 

 areolar tissue, arranged circularly at the circumference, and as radiating 

 bundles passing like rays towards the pupil. In it are involuntary mus- 

 cular fibres, both circular and radiating, together with pigment cells. 

 Vessels and nerves ramify through the tissue. 



Muscular fibres. The sphincter of the pupil (fig. 229, d) is a narrow 

 band about ^th of an inch wide, which is close to the pupil, on the pos- 

 terior aspect of the iris. The dilator of the pupil (e) is said to begin at 

 the outer border of the iris, and to consist of bundles of fibres which join 

 one another, and end internally in the sphincter. 



Action. Enlargement of the pupil is effected by shortening of the radi- 

 ating fibres ; and diminution, by contraction of the circular ring. The 



