CHAPTER XXIX. 363 



many globules contain a core of lipoids. surrounded by true fats. 

 In such a case examination in polarised light is helpful. Differentia- 

 tion by means of different solvents is also possible within limits. 

 Again, the size of the globules is, for obvious reasons, an important 

 factor in the methods which involve mordanting with bichromate. 

 But even if due consideration is given to these reservations, the 

 application of these staining reactions has yielded important 

 results and has profoundly modified our conceptions of the part 

 taken by the fatty substances as constituents of protoplasm. In 

 the first place, it is important to realise that no one single method is 

 a specific staining method, either for all fatty substances or for any 

 one group of them. Thus, Sudan III. or Scharlach R. do not stain 

 all fatty substances, as a glance at the table of the group reactions 

 shows. They stain intensely the true fats, and less intensely 

 cholesterinesters and cholesterin-fatty acid mixtures. But many 

 lipoids are not stained by these dyes, and an examination in polarised 

 light is necessary to detect their presence. Osmic acid alone has 

 the widest range as a reagent for fatty substances, and stains all the 

 different groups. With some substances, however, such as chole- 

 sterin or cerebrosides, it may give negative results if they are not 

 present in a state of colloidal solution in the cytoplasm. But since 

 osmic acid stains also substances which are not fatty in nature, 

 e.g., adrenalin in the cells of the adrenal medulla, it cannot be looked 

 upon as a specific stain. In the case mentioned, a differentiation 

 can still be effected by immersing the section in turpentine, which 

 dissolves the fatty substances, even after osmication. The method 

 giving the highest degree of specificity is the use of osmic acid after 

 bichromate. This will stain only the true fats or cholesterin-fatty 

 acid mixtures. Or, expressed in terms of everyday histological 

 technique, the presence of black cell globules in material fixed either in 

 bichromate-osmic mixtures, such as Altmann's, Champy's or 

 Flemming's fixatives, or first in bichromate fixatives, such as Midler's 

 or Zenker's fluids, and post-osmicated ( 691) indicates the presence 

 either of true fats or of cholesterin-fatty acid mixtures. These two 

 can then be differentiated by examination in polarised light. It is 

 perhaps equally important to be able to draw the opposite con- 

 clusion, when other facts have indicated the presence of fatty 

 substances. The absence of blackening of the globules under the 

 conditions just mentioned definitely excludes the presence of true 

 fats. Thus, true fats can be proved to be absent from the myelin 

 sheath of normal nerves. The significance of the staining of fatty 

 substances after mordanting with bichromate has already been 



