THE IRIS. 



719 



The pupil is nearly circular in form, and is placed a little to the inner 

 side of the centre of the iris. It varies in size according to the contraction 

 or relaxation of the muscular fibres, and this variation ranges from -g^th to 

 ^rd of an inch. The movements of the iris regulate the quantity of light 

 admitted to the eyeball, and are associated with convergence of the optic 

 axes, and with the focal adjustment of the eye. 



STRUCTURE. Fibrous and muscular tissues form the framework of the 

 iris, and pigment is scattered through the texture. In front and behind is 

 placed a distinct layer of pigment cells. It is still matter of discussion 

 whether or not in the adult a delicate epithelium is continued from the 

 margin of the cornea over the front of the iris : it is admitted to exist in 

 childhood. 



The fibrous stroma consists of fibres of connective tissue directed radiat- 

 ingly towards the pupil, and circularly at the circumference ; these, inter- 

 weaving with one another, form a net-like web which is less open towards 

 the surfaces. 



The muscular fibre is of the non-striated kind, and is disposed as a ring 

 (sphincter) around the pupil, and as rays (dilatator) from the centre to the 

 circumference. 



Fig. 465. 



Fig. 465. A SMALL PART OF THE 

 IRIS, SHOWING THE MUSCULAR 



STRUCTURE (from Kolliker). ass 



The specimen is from the albino- 

 rabbit, and has been treated with 

 acetic acid : a, the sphincter muscle 

 at the margin of the pupil ; 6, fas- 

 ciculi of the dilatator muscle ; c, 

 connective tissue with nuclear cells 

 rendered clear by the acid. 



The sphincter is the flat 

 narrow band on the posterior 

 surface of the iris, close to 

 the pupil, and w about ^th 

 of &n inch wide. At the 

 edge of the pupil the fibres 

 are close together, but at the 

 peripheral border they are 

 separated, and form less com- 

 plete rings. 



The dilatator, less apparent than the sphincter, begins at the ciliary or outer 

 margin of the iris, and its fibres, collected into bundles, are directed inwards 

 between the vessels and nerves, converging towards the pupil, and forming 

 a net-work by their intercommunications. At the pupil they blend with the 

 sphincter, some reaching near to its inner margin. 



Pigmentary elements. In the substance of the iris anteriorly and through- 

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

 in the choroid membrane. The pigment contained in them is yellow, or 

 of lighter or darker shades of brown, according to the colour of the eye. On 

 the fore part of the iris is a thin stratum of rather oval or rounded cells with 

 granular ramified offsets (an epithelial layer Kolliker). At the posterior 

 surface is a covering of dark pigment the uvea of authors ; this is con- 



