WITH THE MICROSCOPE. 193 



this was not the case. In such instances the real disposition of the 

 parts is only to be made out by a careful examination of the structure 

 under different circumstances and prepared in various ways. Thus 

 the idea that the tubes end by blind extremities may be shown to be 

 quite inconsistent with the appearances observed in some particular 

 mode of examining the texture. I am unable, however, to devote 

 much space to the consideration of this part of my subject, or I might 

 review the various methods in which a tissue is examined, and 

 show how by a consideration and comparison of the different facts 

 observed, one is enabled at length to embody the results arrived at 

 in several different enquiries, and form an idea of the real structure 

 of the part. 



On the Difficulty of Seeing Structures from their Transparency. 

 Another fallacy arises from the great transparency of certain structures. 

 Oftentimes a membrane may appear perfectly clear and transparent 

 when in reality it is covered with a delicate layer of epithelium, which 

 only becomes visible by being immersed in some special fluid or 

 treated with some particular chemical reagent. On the other hand, 

 there are instances in which an appearance resembling that produced 

 by the presence of a cellular investment is perceived where no cells 

 whatever exist. A peculiar corrugated state of uninjected capillaries, 

 and the nuclei in the walls of the capillary vessels themselves, some- 

 times give rise to these mistakes. Basement membrane, from its extreme 

 delicacy and transparency, is often only recognised by the folds into 

 which it is thrown, or by the debris and granular matter which is 

 accidentally adherent to it. Sometimes it becomes visible when 

 immersed in a slightly coloured solution, instead of in perfectly pure 

 water. Not only may blood and lymphatic vessels be completely 

 passed over from their transparency, but I could adduce instances 

 in which broad bands of connective tissue and bundles of nerve 

 fibres existed in a specimen in great numbers although they could 

 not be seen when the ordinary methods of demonstration were 

 employed. 



Fibres and Membranes Produced by the Action of Reagents arti- 

 fMally. On the other hand, by the action of reagents a fibrous 

 appearance is sometimes produced which, without care, may be mis- 

 taken for actual structure. 



The addition of acetic acid to many preparations frequently pro- 

 duces a swelling of the tissue, with the elevation of a clear mem- 

 brane-like structure, which might be termed basement membrane, but 

 which has really been formed in this manner. Thus the outer uncal- 

 cified portion of the cells of the enamel of a young tooth, may be 

 made to swell up into a transparent mass, which has been mistaken, 



o 



