22 DR. KLEIN. 



be suitably employed) is rendered extremely difficult by the 

 fact that, with the exception of the cellular structures of the 

 proper tissue of the membrane, all its constituents which we 

 have enumerated possess nearly as much power of combina- 

 tion with the gold salt as do the nerves themselves. It is 

 thus only in some few cases that the finer nerves can be suc- 

 cessfully traced out. Among these cases are those in Avhich 

 the epithelium of both surfaces can be removed without 

 thereby seriously injuring the tissue of the membrane. In 

 the case of the cornea it is an easy matter to remove, over a 

 considerable area, the anterior epithelium with a thin layer 

 of the proper tissue, and even to prepare thin layers from the 

 residual tissue; but this is by no means the case with the 

 nictitating membrane. In it, indeed, although it is Avell 

 known not to exceed the cornea in thickness, we have first a 

 thin epithelium on either surface, and, besides this, the fun- 

 damental tissue possesses a peculiar structure, which is found 

 in the cornea, to a limited degree only, in its most posterior lay- 

 ers. This peculiar arrangement is as follows : — The stroma of 

 the nictitating membrane consists of connective- tissue bundles, 

 as a rule running parallel to each other, and arranged in three 

 layers, a middle and a superficial corresponding to each sur- 

 face. The bundles of the two last do not remain near the sur- 

 face during their whole course, but bend away from it, the 

 fibres of each bundle being compressed into a thin cord, and 

 running in a narrow cylindrical canal which pierces the 

 membrane in a vertical direction, so that they emerge at the 

 opposite surface once more, and there run parallel with it. 

 Whether these vertical bundles of connective tissue all extend 

 from the one surface to the other, or whether on their way 

 they are not in connection with the bundles of the middle 

 layer, I cannot distinctly say ; this much is certain, that 

 the nictitating membrane is pierced by regular and tolerably 

 compactly arranged canals, in which the bundles of the one 

 surface reach the other. 



From this it is plain that the attempt to split up the nicti- 

 tating membrane must be unsuccessful, and that the most we 

 can do is to detach small portions, which can be made 

 use of only for tracing the nerves in the epithelium. For 

 following up the nerves of the blood-vessels and glands, 

 and for determining the relations of the cellular structures, 

 the portions thus removed are of no importance worth men- 

 tioning, since no preparations are of any use for these pur- 

 poses except those in w^hich the tissue of the membrane is 

 present in toto Avithout the epithelium of the surfaces. Out 

 of a disproportionately large number of nictitating membranes 



