454 ^ MANUAL OF PHYSIOLOGY 



a ready answer to these questions. The fat cells of adipose 

 tissue are apparently ordinary fixed connective-tissue cells 

 which have become filled with fat, the protoplasm being 

 reduced to a narrow ring, in which the nucleus is set like a 

 stone. It would, at first thought, seem natural to suppose 

 that the fat of the food is rapidly separated by these cells 

 from the blood, and slowly given up again as the needs of 

 the organism require, just as carbo-hydrate is stored in the 

 liver for gradual use. And it has been found that a lean 

 dog, fed with a diet containing much fat and little proteid, 

 puts on more fat, as estimated by direct analysis, or keeps 

 back more carbon, as estimated by measurements of the 

 respiratory interchange, than can be accounted for on the 

 supposition that even the whole of the carbon of the broken- 

 down proteid corresponding to the excreted nitrogen has 

 been laid up in the form of fat. Even with a diet of pure 

 fat and with such a diet digestion and absorption are 

 carried on under unfavourable conditions more carbon is 

 retained than can have come from the metabolism of the 

 proteids of the body, as measured by the nitrogen given off 

 in the urine and faeces : the fat passes rapidly from the 

 blood into the organs, and especially into the liver (Hofmann, 

 Pettenkofer and Voit). It is thus certain that some of the 

 absorbed fat may be stored up as fat in the body. 



This is borne out by the careful experiments of Munk and 

 Lebedeff, who find that when dogs are fed with excess of 

 foreign fat (linseed-oil, rape-oil, mutton-fat), a fat is laid down 

 which is quite different from dog's fat, and has the greatest 

 resemblance to the fat of the food. Thus, when rape-oil, 

 which contains a fatty acid, erucic acid, not found in animal 

 fat, was given, erucic acid could be detected in the fat laid 

 on. When the dogs were fed with mutton-fat, whose melt- 

 ing-point is much higher than that of dog's fat, the fat laid 

 on did not melt till it was heated to 40 C. or more. When 

 they were fed with linseed oil, the body-fat was found liquid 

 even at o C. We have already referred (p. 371) to the fact 

 that neutral fat can be built up in the wall of the intestine 

 from fatty acids given in the food. Munk has shown that 

 fat formed in this way can also be laid down as body-fat. 



