THE EYE 1515 



The circular fibres of the ciliary muscle are well developed in 

 cases of hypennetropia, but are deficient in cases of myopia. . 



3. Iris. — The iris forms the anterior part of the middle coat of the 

 eyeball. It is a coloured contractile curtain, which is suspended 

 in the aqueous humour between the cornea and the cr^'stalline 

 lens. It is perforated by an almost circular aperture, called the 

 pupil, which is situated slightly to the nasal or inner side of its 

 centre, and serves for the transmission of light. The margin which 

 surrounds the pupil is known as the pupillary margin. Its cir- 

 cumference is continuous with the ciliary body, and is cormected 

 with the posterior elastic lamina of the cornea by means of the 

 ligamentum pectinatum iridis. The circumference is known as 

 the ciliary margin. The surfaces of the iris are anterior and 

 posterior. The anterior surface is directed towards the cornea. 

 Its colour varies in different individuals, and it presents a striated 

 appearance, the striae converging towards the margin of the pupil, 

 and being produced by the underljdng vessels. The posterior 

 surface is directed towards the crystalline lens and ciliary processes. 

 It has a purple colour, and is covered by two layers of colunmar 

 epithelium, the cells of which contain dark pigment. These two 

 layers of pigmented cells constitute the pars iridica retinae {uvea), 

 which is continuous with the pars ciliaris retinae. The iris divides 

 the space between the cornea and the crystalline lens into two 

 compartments, the anterior chamber and posterior chamber, both 

 of which contain the aqueous humour. 



Structure. — The component parts of the iris are as follows : 

 (i) a layer of endothelium ; (2) a connective-tissue stroma, with 

 branched cells ; (3) muscular tissue ; and (4) pigment. 



The layer of endothelium covers the anterior surface of the iris, 

 and is continuous with the endothelium which lines the posterior 

 elastic lamina (membrane of Descemet) of the cornea. 



The connective-tissue stroma is composed of fibres which, for 

 the most part, pass in a radiating manner towards the pupillary 

 margin. Some, however, are disposed circularly at the ciliary 

 margin. They support the bloodvessels and ner\-es, and scattered 

 between their bundles there are branched cells. These cells con- 

 tain pigment in dark coloured eyes (black, grey, or brown), but in 

 blue eyes the\' are destitute of pigment. 



The muscular tissue is of the unstriped variety, and its fibres 

 are arranged in two sets, circular and radiating. The circular 

 fibres form a ring round the pupil, and are nearer the posterior 

 surface than the anterior. They are known as the sphincter 

 pupiilse. The radiating fibres converge from the ciliary margin of 

 the iris towards the pupillary margin, where they blend with the 

 circular fibres. The radiating fibres constitute the dilator pupiUse. 

 Some authorities regard the radiating fibres as being elastic, and 

 not muscular. 



The pigment of the iris is variously situated, according to the 

 colour of the eye. In the eyes of ailbinos there is no pigment. 



