THE EYE. 399 



of the iris, may be seen in sections and by careful dissection. 

 In the latter case, the inner wall of the " canal of Schlemm" 

 should be carefully removed, together with the iris and choroid, 

 and an attempt made to raise from it portions of the membrana 

 Demoursiana, which is not unfrequently successful. The uvea 

 offers no difficulty. The pigment-cells of the stroma, with 

 their processes, and the inner pigment are readily seen ; the 

 latter at the margin of folds and in carefully detached portions. 

 For the investigation of the ciliary muscle, a fresh eye is 

 requisite, as its elements very soon become unrecognisable. 

 The muscles of the iris should be studied in a blue eye, and 

 best in that of a child, after removal of the posterior pigment; 

 and also in the eye of a white Rabbit, in which the sphincter 

 pupillce may be readily seen without farther trouble on the 

 application of acetic acid. The same preparation should be 

 employed in order to examine the nerves of the iris, but a 

 perfectly fresh eye and a dilute solution of soda are indis- 

 pensable. The retina should be examined in the recent state, 

 on the surface, in vertical sections, and at the edges of folds, 

 moistened with aqueous humour, and without any covering 

 glass ; and also with the aid of slight compression and by the 

 teasing out of the tissue. Chromic acid preparations are very 

 important in the study of this structure. This reagent, it is 

 true, affects the "rods" to some extent, but preserves the 

 other parts so much the better, and without its aid Miiller and 

 I should never have arrived at the results above stated, 

 although Hannover, on account of its influence upon the 

 "rods," erroneously considered it an unfit agent to employ. 

 The most advantageous mode of applying it, is to treat a 

 fresh retina at once with chromic acid, and to trace all the 

 stages of its effect step by step. If the solution be much 

 diluted, the elements are very little changed, and in particular 

 may be easily isolated; and if more concentrated, sections 

 through the retina may be prepared, without which no complete 

 view of the structure of that tunic can be arrived at. I apply 

 it by extending a portion of the retina upon an object-bearer, 

 with a little chromic acid, in such a way that it should lie flat 

 and not float. Extremely fine slices may then be taken by a 

 sharp convex scalpel or razor from any sectional surface, by 

 pressing downwards, which, with a little pains, may be done 



