476 THE METABOLIC PKOCESSES OF THE BODY. 



422. It is clear then that a construction of fat does occur in the body 

 somewhere. What limits can we place on the degree to which this construc- 

 tion is carried ? When the food contains sufficient actual fat to account for 

 the fat stored up in the body, does any construction of fat take place? In 

 the first place we find that when the food contains abnormal fats such as are 

 not present in the body, spermaceti for instance, or erucin (from rape-seed 

 oil), these fats are not to be found, or are found in very small quantity, in 

 the fat which is stored up in the body as a consequence of a large supply of 

 that food. In the second place we may call to mind the statement previously 

 made, that the composition of fat varies in different animals. The fat of a 

 man differs from the fat of a dog, even if both feed on exactly the same food, 

 fatty or otherwise. Were the fat which is taken as food stored up as adi- 

 pose tissue directly and without change, recourse being had to other sources 

 of food for the construction of fat only in cases where the fat in the food was 

 deficient, we should expect to find that the nature of the fat of the body 

 would vary greatly with the food. So far from this being the case, direct 

 experiments show that the fat of the dog is, as far as composition is con- 

 cerned, very largely independent of the food, that the normal constituents 

 of fat make their appearance very much as usual, and in very much their 

 appropriate proportion, though their proportion in the food may largely vary, 

 and though some of them may be wholly absent. Thus in one experiment 

 the fat of the body contained considerable quantities of stearin after a diet 

 free from stearin, and in another preserved the normal amount of olein after 

 a diet free from olein. 



Of course it is quite possible that in such cases as these, though the 

 stearin, or the olein, when absent from the food, was in some way or other 

 constructed anew, yet at the same time those constituents which were present 

 were simply stored up ; and the small quantity of erucin present in the fat 

 of the body after feeding on erucin must have been directly stored up. So 

 also, when an animal is rapidly fattened on a diet consisting of a small 

 quantity of proteid and a large quantity of fat, the amount of fat stored up 

 may be too great to have come from the proteids of the diet, in which case we 

 may infer that it was the actual fat of the food simply deposited in the fat-cells 

 of the body. But even in this case, as more distinctly in the others, it is also 

 open for us to suppose that all the fat taken as food was in some way or other 

 disposed of, and that all the new fat which made its appearance was constructed 

 anew. And the latter view is more perhaps in harmony with the histological 

 facts previously mentioned, as well as supported by other considerations. 



At the present, however, we may be content with the following conclu- 

 sions : 1. Fat is actually formed in the animal body, and the fat present at 

 any moment in the body is not exclusively, if at all, fat merely stored up 

 from the fat of the food. 2. The carbon elements of the newly-formed fat 

 may be supplied either from carbohydrate food, or from the carbon surplus of 

 proteid food, or from fats taken as food which are not the natural constitu- 

 ents of the body-fat. 3. The fat stored up appears as fat granules or drops 

 deposited in the cell- substance of certain cells, and the increase of the fat in 

 the cells is accompanied first by a growth, and subsequently by a consump- 

 tion of the cell substance ; but, as in the analogous case of glycogen, there 

 is no complete evidence to show whether the fat granules which appear are 

 simply deposited by the cell substance in a more or less mechanical manner, 

 without their forming an integral portion of that cell substance, the chief 

 stages of the manufacture of the fat having been gone through elsewhere, or 

 whether they arise from a breaking up, a functional metabolism of the cell 

 substance of the fat-cell itself; the latter view is on the whole, however, the 

 more probable. 



