THE IRIS. 263 



other is left projecting at the edge, so as in this way 

 to be seen distinct from the other layers. But before 

 commencing the attempt at separation, the hexago- 

 nal pigment cells, which belong to the retina but 

 frequently adhere to the inner surface of the choroid, 

 must be entirely removed by gently brushing that 

 surface with a hair pencil. The separation and 

 brushing are performed under fluid (spirit), and will 

 be much facilitated by the use of a dissecting lens. 



Preparation 19. The musculature of the 

 iris. The circular and radiating plain muscular 

 fibres of the iris may be demonstrated in the albino 

 rabbit. The eye is cut in half and the anterior part 

 placed in spirit for a day or more. Then the lens is 

 removed, and the segment of the iris including its 

 whole width, from the pupillary aperture to the 

 ciliary processes of the choroid is cut out and 

 placed in dilute logwood. When sufficiently but 

 not too deeply stained, it is put into water for a 

 minute or two to remove the excess of staining fluid, 

 then passed through spirit and oil of cloves, and 

 finally mounted in dammar, with the posterior sur- 

 face uppermost. The thick ring of the sphincter is 

 easily seen in these preparations, and also the inter- 

 lacing bundles of plain muscular fibres of which tjrie 

 dilatator is composed ; they may be observed to bend 

 round near the pupil, and take the direction of and 

 blend with those of the sphincter. At the circum- 

 ference of the iris also, a similar bending round of 

 the radiating fibres is observed. 



Preparation 20. Human iris. Although, in 

 consequence of the presence of the uveal pigment, 

 more troublesome to prepare, it is nevertheless desir- 

 able to make a similar preparation of the human 

 iris, for the musculature is somewhat different, the 

 dilatator forming a uniform thin expansion, which 

 covers the posterior surface immediately under the 

 pigmented epithelium, and not distinct radially ar- 

 ranged interlacing bundles with intervening meshes 

 as in the rabbit. The specimen may be made from 



