MICROSCOPICAL TECHNIC 419 



objects are placed (depending on the variety) in 15 to 35 per cent 

 alcohol or in physiological salt solution which contains 2 cc. of 40 per 

 cent formaldehyd per liter, or into 0.1 per cent to 0.005 per cent 

 chromic acid, or 1 per cent osmic acid, dilute picric acid, 20 per cent 

 nitric acid, pure HC1, javelle water or many other solutions recom- 

 mended for particular purposes. Obviously, with the first mentioned 

 substances we may dissolve the connections by a differential shrinking 

 of the cell constituents, because as we shall see the identical substances 

 are employed in different concentrations for fixation and for harden- 

 ing. Substances such as 20 per cent nitric acid, pure HC1, etc., 

 obviously change the cement substances chemically. We understand 

 the action of digestive fluids (pepsin-HCl, pancreatin) to be similar, 

 yet they have not proven very satisfactory. After successful "mac- 

 eration" it is sometimes sufficient to shake violently the object which 

 has been treated to cause it to fall apart or it may be teased on the 

 slide with needles or a coarse brush. 



Fixing and Hardening. 



Whereas the methods previously described permit the recognition 

 of individual elements of a tissue, they do not permit a study of the 

 relations of the tissue elements, their connections and, in short, the 

 entire tissue structure. For this purpose a thin section of tissue 

 must be prepared and stained. Before doing this it is frequently 

 necessary to fix and harden the object to be studied. 



Fixation is undertaken for the purpose of making the partly fluid 

 and partly semifluid constituents firm, so that they stop changing, 

 neither swelling, shrinking, coagulating nor the like and so that their 

 appearance shall remain as nearly lifelike, or at least as fresh as 

 possible, in order that this condition shall be maintained through all 

 the later manipulations. By fixation, the relations between the dif- 

 ferent tissue elements are made permanent. The object is frequently 

 too soft to section, so that it must be subjected to a special proce- 

 dure, hardening. Viewed colloid-chemically, tissues may be consid- 

 ered to consist of (1) irreversible, slightly elastic gels, (2) reversible 

 elastic gels, (3) sols. All sorts of transition states exist. 



Accordingly, fixation renders each constituent completely insol- 

 uble, unshrinkable and incapable of swelling; the sols are changed 

 to gels, and no shrinking or swelling should occur during the fixation. 

 Finally, it must be possible to stain the objects well, and 'conse- 

 quently their chemical properties must not be too radically changed. 

 It is a problem almost impossible to solve. For the sake of compari- 

 son, every one knows how great and almost insuperable difficulties 



