556 METABOLISM. NUTRITION AND DIETETICS 



sebaceous glands, and of the mammary glands, may be produced 

 from protein by a transformation of the cell- substance. But abso- 

 lutely convincing proof is wanting. The old idea that the cells of 

 these glands underwent a physiological process of transformation 

 into fat analogous to the fatty degeneration of pathology, and then 

 broke down bodily into the secretion, has been long since disproved 

 for milk formation, and is probably erroneous also as regards the 

 secretion of sebum. The rule which experience has taught, that 

 a woman during lactation requires an excess of proteins in her food 

 corresponding not only to the proteins, but also to the fat given off 

 in the milk, suggests such an origin for the milk- fat, but does not 

 prove it. Other fat-containing secretions are the ear-wax formed 

 by glands in the wall of the external auditory meatus, and the 

 smegma formed by the glands of the prepuce, but nothing is known 

 of the sources from which the fatty substances are derived. 



The Intermediary Metabolism of Fat. The mechanism and the 

 stages of the transformation, including the migration, of fats is 

 not well understood indeed, not as well as that of the carbo- 

 hydrates. Many of the tissues contain intracellular, soluble, 

 fat-splitting ferments called lipases, especially the liver, the 

 active mammary gland, and the intestinal mucosa. We have 

 already seen that there is evidence that these lipases, like some 

 other enzymes, have a reversible action. They are either fat- 

 splitting or fat-forming ferments, according to the conditions 

 (Kastle and Loevenhart). It is stated that the perfectly aseptic 

 blood does not split ordinary neutral fats, although it contains a 

 ferment which splits up monobutyrin (glycerin butyrate) into 

 glycerin and butyric acid. 



| The question how the fat, after absorption from the intestine, 

 passes from the blood into the cells, and how it is enabled again to 

 pass out of the fat-cells when the needs of the tissues call for its 

 mobilization, cannot at present be definitely answered. It is 

 possible that just as fat is split in the lumen of the intestine before 

 being absorbed, and then rebuilt in the epithelium, so it is split in 

 the blood or in the lymph before being taken up by the fat-cells. 

 The lipase in these cells would then be capable of synthetizing the 

 glycerin and fatty acids to fat in their interior. When the fat is 

 about to pass out of the cells in response to the call, of whatever 

 nature it is, of the tissues for fat, it may again be split, resynthetized 

 in the blood, and again hydrolysed for entrance into the tissue 

 cells. Or it may be carried to the cells in the form of glycerin and 

 fatty acids, or soaps, in such small concentration as to be harmless, 

 and there built up again into the original fat, or transformed into 

 other fats characteristic of the particular tissues, including the fatty 

 acid components of the phosphatides, or utilized without synthesis 

 into fat. An alternative hypothesis avoids this series of decomposi- 



