CH. XLL] METABOLISM OF FAT 619 



form of the usual glycerides. In many cases, the total fat present in 

 this concealed form (possibly in combination with protein) in the 

 cells of the heart, kidney, and liver may be greater than when with 

 disease it takes the form of droplets. 



With regard to the origin of glycerin, there is no doubt that the 

 cells are able to produce it, as was shown by Munk's experiments 

 on chyle, which are referred to on p. 548. There is no necessity to 

 suppose that it originates from protein, for if glycerin can be con- 

 verted into sugar, there is good ground for believing that the converse 

 also takes place. 



On katabolism, the fats yield the same ultimate products as the 

 carbohydrates, namely, carbonic acid and water. A great deal of 

 the oxygen we breathe in is used up in the burning of fats, and the 

 simultaneous liberation of heat and work. It is quite certain that 

 sugar is an important source of muscular energy, but the fats also 

 play the same role, and muscles which are perpetually at work, such 

 as the heart and the diaphragm, are particularly rich in fats. No 

 actual lessening of the fat has yet been demonstrated to occur in 

 excised muscles subjected to stimulation, but we have other and more 

 trustworthy evidence that it does take place. During muscular 

 work, the output of carbonic acid is increased, but the respiratory 

 quotient is almost unaltered ; if sugar alone was undergoing 

 combustion, this quotient would rise. Again, if the carbohydrate 

 stores of the body are depleted by inanition, or by giving phloridzin 

 to an animal, muscular work has but little influence on protein 

 katabolism, and therefore the necessary increased combustion must 

 fall on the fat. But how the long carbon chains of the fatty acids 

 are taken to pieces and burnt up is very largely a matter of guess- 

 work ; still the recent work of Leathes and Hartley, which is briefly 

 summarised on p. 541, has given us some insight into the way in 

 which the liver accomplishes this object. Much the same problem 

 is presented when we consider the fate of the proteins. We know 

 fairly accurately how the protein nitrogen is disposed of, but the 

 non-nitrogenous residue (which is chiefly lower fatty acid, and 

 which like a fat is used for combustion, and as a source of heat 

 and energy) is no doubt dealt with by the liver, as the fats them- 

 selves are. The existence of this non-nitrogenous and fat-like 

 component of protein should make physiologists hesitate before they 

 finally deny the possible conversion of the food-protein and tissue- 

 protein into fat. 



The study of acidosis or acidaemia referred to on p. 542 is 

 important, because it sheds light on the phenomena of fat katabolism ; 

 /3-hydroxybutyric acid and acids of similar molecular size are 

 probably normal intermediate products in the process; a healthy 

 man on a properly mixed diet is able to oxidise /3-hydroxybutyric 



