THE HISTORY OF FAT IN THE BODY 885 



fatty degeneration and was loaded with fat globules, contained 19 per 

 cent, of its dried weight of fat. A heart muscle taken from a normal 

 animal at the same time, presenting no visible fat globules, contained 

 17 per cent, of fat. 



COMPOSITION OF FAT 



The fats occur generally in the form of triglycerides of various 

 fatty acids. In adipose tissue the acids are chiefly stearic, palmitic, 

 and oleic, the consistency of the fat depending on the relative amount 

 present of triolein, with its low melting-point. In certain animals 

 the glycerides of more unsaturated fatty acids occur. Thus lard 

 contains about 10 per cent, of fats belonging to the linoleic series. 

 The fats of cows' milk, though consisting chiefly of the three above 

 mentioned, include also the esters of butyric and caproic acids in 

 fair amounts, and traces of the intermediate acids, caprylic, capric, 

 lauric, and myristic acids. 



The ' fat ' extracted from the tissues (e.g. heart muscle) includes a 

 considerable amount of ' phosphatides ' (lecithins, &c.). It also 

 contains a much larger proportion of unsaturated fatty acids, of the 

 linoleic and even lower series, so that its ' iodine value ' is generally 

 found relatively high (120 as compared with 40 to 60 in adipose 

 tissue). 



FUNCTIONS OF FAT 



First and foremost must be mentioned the significance of fat as a 

 reserve food store. The power of the organism to store up reserve 

 carbohydrate is strictly limited. The liver of man can probably not 

 accommodate more than 150 grm. of glycogen, and assuming that the 

 muscles of the body may contain an equal amount, 300 grm. represents 

 the extreme limit of storage of carbohydrates in the body. On the 

 other hand, in most animals there is practically no limit to the amount 

 of fat which can be laid down, and over-feeding, whether with carbo- 

 hydrates or fats, leads to the deposition of fat. This fat does not 

 enter into the normal metabolism of the body, but is available for 

 use whenever the needs of the body are increased above its income. 



As to the part taken by fat, especially the hidden fat of the working 

 cells, in the chemical processes which determine the life of the cell, 

 our knowledge is still very scanty. Fats enter into the constitution 

 of the complex bodies lecithin and myelin, which form important 

 constituents of the limiting membrane of every living cell. As con- 

 stituents of the membrane itself, fatty substances therefore have a 

 protective action, and also regulate the passage of substances into 

 the cell across the membranes. 



The presence of lecithin as an integral constituent of all protoplasm, 

 and of the first products of disintegration of protoplasm, suggests 



