550 METABOLISM, NUTRITION AND DIETETICS 



At this point in the discussion it is necessary to remark that a 

 distinction ought to be established between that store of surplus fat 

 laid down in the connective tissue which, in order to avoid com- 

 plicating the matter unduly, has hitherto been referred to as if it 

 constituted the whole of the body- fat, and the fat which is contained 

 in greater or less amount in all the tissue cells. The fat con- 

 tained in the tissue elements e.g., in the liver cells in the visible 

 form of droplets, and which can be easily extracted from them by 

 solvents such as chloroform, should also be distinguished from the 

 fat which is so intimately incorporated or combined with the cell 

 substance that it can only be extracted after this has been digested 

 by the aid of proteolytic ferments or acids. The latter fraction of 

 the body-fat is probably an integral and indispensable constituent 

 of the protoplasm. Now, it is in the great fat depots of the sub- 

 cutaneous tissue and the mesentery and omentum that variations 

 in the proportions of the various fatty acids corresponding to varia- 

 tions in the nature of the food-fat are most easily produced, or, at 

 least, most easily observed. These depots are laid down, not in 

 the interest of the fat cells themselves, but to serve the purpose of 

 a reserve of fat which may be drawn upon for the nutrition of the 

 body as a whole, just as the glycogen store of the liver forms a 

 general carbo-hydrate reserve. The free fat in the cells of the organs 

 is superficially analogous to the glycogen reserves of such tissues 

 as muscles and glands, and certain facts are known which might 

 be interpreted as indicating that this fraction of the body-fat, like 

 the fat of the connective tissue, is not a definite and specific mixture 

 of fats with an unvarying composition for each kind of animal, but 

 a mixture whose composition can be made to vary by altering the 

 nature of the fats in the food. On the other hand, the fat combined 

 in the tissues appears to preserve a certain specificity which is inde- 

 pendent of the fats supplied in the food. Thus, when dogs were 

 fed with rape oil, and had accumulated considerable quantities of 

 this fat of low melting-point in the subcutaneous and other fat 

 depots, 1 he fat combined in the organs remained in all respects the 

 same as normal dog's fat. This was also the case with animals 

 fed on fat of high melt ing-point, such as sheep's tallow (Abderhalden). 

 Although the liver appears to have a special relation to the metabo- 

 lism of fat, it is not known whether any particular organ is more 

 than the rest responsible for the manufacture of this specific mix- 

 ture of fats. It appears more probable that each cell has the power 

 of forming for itself the characteristic fats from the crude materials 

 represented by the food-fat directly absorbed from the tissue lymph, 

 or the fat of the depots after it has been mobilized and has found 

 its way again into the blood, or, finally, from other materials than 

 fats, such as dextrose or some of its decomposition products. 



Even in the case of the subcutaneous and similar collections of 



