764 SPECIAL HISTOLOGY. 



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 inves- 

 tigation of the ciliary muscle, a fresh eye is requisite, as its elements 

 very soon become unrecognizable. 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 pupillse 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 indispensable. The retina should be 

 examined in the recent state, on the surface, in vertical sections, and at 

 the edges of folds, moistened with aqueous humor, 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 impor- 

 tant 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 Midler 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 con- 

 centrated, 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 easily enough. It is as well, 

 however, to guide the cutting scalpel by the handle of another held in 

 the other hand, until the edge of the former is brought immediately 

 over the border of the retina. When the nervous layers, which are 

 very well defined from each other, have been studied in sections of this 

 kind, which should be taken especially from the neighborhood of the 

 macula lutea, as well as from other situations in the transverse and 

 longitudinal directions, and which when useful necessarily exhibit only 

 a few layers of the elements, they may be carefully teased out or ren- 

 dered more transparent by soda, which last, however, is not generally 

 of much use, since it makes the elements pale. The hyaloid membrane 



