IRIS STRUCTURE. 337 



dividing the front of the eye into two chambers, the ante- 

 rior and posterior. These chambers contain the aqueous 

 humor, in which the iris moves freely. The anterior 

 chamber extends from the cornea to the iris ; the posterior 

 from the iris to the lens. The iris is circular in its form 

 in the human eye, and oblong, either transversely or 

 vertically, in many of the inferior animals, li has in its 

 centre an opening called the pupil. The size of the pupil 

 varies according to the intensity of the light, the sen- 

 sibility of the retina, and the distance of the object. When 

 the light is strong, the object near, and the retina sensi- 

 ble, the pupil contracts, while in the opposite condition of 

 things it dilates. 



The external circumference of the iris is attached to the 

 ciliary ligament. Its internal circumference forms the 

 margin of the pupil. Its anterior surface is flat, and pre- 

 sents the variety of color from which it has been called iris 

 or rainbow. There is stated to be a resemblance between 

 the color of the iris and the hair. In most fish it is found 

 to present a metallic lustre. The posterior surface of the 

 iris is covered with a thick layer of dark pigment, and from 

 its resemblance to the ripe purple grape, is called uvea. 



Structure. The structure of the iris is regarded as essen- 

 tially muscular, and has been compared to the columnse 

 carneas and chordae tendineaa of the heart. 



On the anterior surface of the iris are seen projecting 

 lines and intervening depressions. Some of these lines sur- 

 round the pupil after the manner of a sphincter ; others 

 radiate from the pupil towards the circumference of the 

 iris, while others are described as interlacing and bifur- 

 cating, and ending in small projections. The radiated 

 fibres mingle with the circular somewhat after the manner 

 of the muscles of the mouth, with the orbicularis oris. 

 This arrangement of the muscular fibres satisfactorily ex- 

 plains the functions of the iris. When it is required that 

 the pupil shall contract, the sphincter or circular fibres 

 afford the means of doing it, and shutting out the light ; 

 when it is necessary to open the pupil, the dilatation is 

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