336 RETROGRESSIVE PROCESSES 



is particularly true of the kidney. Thus, the amount of fat 

 present in normal kidneys (dog) was found to vary between 

 18.5 per cent, and 29.12 per cent, of the dry weight, the 

 average being 21.8 per cent. ; whereas, after producing a typi- 

 cal " fatty degeneration " by means of phosphorus and other 

 poisons, the fat content was still found to be between 16.9 

 per cent, and 22.6 per cent. 1 In all instances the amount 

 of fat in kidneys showing typical fatty degeneration under the 

 microscope was found equal to or less than the normal amount 

 it was never increased. The same conditions were found to 

 obtain in human kidneys that showed fatty metamorphosis. 

 Microscopic examination of specimens stained with the specific 

 fat stains, 2 therefore, gives no indication of the amount of fat 

 contained in a degenerated kidney. A pathologic kidney con- 

 taining 16 per cent, of fat (18 per cent, is about the average 

 amount of fat in normal human kidneys) may show extreme 

 " fatty degeneration " under the microscope, whereas another 

 kidney may contain as much as 23 per cent, of fat, yet not 

 show any fat whatever by staining methods. 



The explanation of this remarkable discrepancy is as follows : 

 Every tissue and organ seems to contain a greater or less amount 

 of fat, varying from 5 per cent, to 20 per cent, of the total dry 

 weight of the organ in the case of most of the important tissues, 

 yet this fat is usually held in such a form that it cannot be stained 

 by any stains available for the purpose. Thus in the kidneys, 

 as before remarked, we may have as much as 23 per cent, of fat 

 present and yet be entirely unable to stain any of it. The 

 greater part of this fat seems to be essential to the cell, for it 

 cannot be removed by the most extreme starvation ; e. g., the 

 liver of the most emaciated dogs may contain 10 per cent, to 

 20 per cent, of fatty substances. Furthermore, the same resistance 



1 Concerning the normal intracellular fats see introductory chapter. 



2 Fat-staining involves several principles of interest in this connection. 

 Osmic acid (OsO 4 ), the longest used for this purpose, is reduced to OsO 2 by 

 oleic acid, imparting a black or dark-brown color to the fat ; but it does not 

 stain saturated fatty acids, such as palmitic or stearic acid. Thus, Christian 

 found in pneumonic exudates fat that stained by other methods but not by 

 osmic acid, apparently because it contained no oleic acid (Jour. Med. 

 Research, 1903 (10), 109). Sudan III and scarlet R (fat ponceau) are two 

 synthetic dyes which stain fat in a purely physical way, entering and remain- 

 ing in the fat-droplets because they are much more soluble in fat than they 

 are in water or alcohol. (Fully discussed by Michaelis (who introduced scar- 

 let R) in Virchow's Arch., 1901 (164), 263; and by Mann, "Physiological 

 Histology," p. 306.) These stains have the advantage of staining all sorts of 

 fats and not staining other substances that may reduce osmic acid. Fatty 

 acids and soaps may be stained with copper acetate, which forms a green cop- 

 per salt, and thus be distinguished from fats (Benda, Virchow's Arch., 1900 

 (161), 194. 



