24 HUMAN PHYSIOLOGY 



olein o. According to the proportion of stearin and palma- 

 tin on the one hand and olein on the other, the natural fats 

 are solid at ordinary temperature, as tallow and butter, or 

 liquid, in which condition they are called oils. 



In small quantities there are present in animal fats the glycerides 

 of butyric acid, C 4 H 8 O 2 , caproic acid, C 6 H 12 O 2 , caprylic acid, 

 C 8 H 16 O 2 , capric acid, C 10 H 20 O a , and myristic acid, C M H 28 O a . 



Fats are insoluble in water and cold alcohol, but readily 

 soluble in hot alcohol and in ether. Stearin and palmitin 

 solidify in needle-shaped crystals. On heating, especially 

 with the anhydrid of phosphoric acid, the fats, in contra- 

 distinction from free fatty acids, yield the offensive-smelling 

 acrolein, a decomposition product of glycerin. The fats are 

 stained black by osmic acid. By boiling with alkalies, 

 especially in alcoholic solutions, also by the action of many 

 ferments (steapsin of the pancreatic juice) they are split up, 

 under the assumption of water into glycerin and free fatty 

 acids. The fatty acids unite with the alkali present, forming 

 salts of fatty acids, the soaps (sodium soap or hard soap, 

 potassium soap or soft soap). 



If the fats contain free fatty acid (rancid fats), they can, 

 on melting, form an emulsion with water and a little soda; 

 in this process of emulsion the fats are finely divided, forming 

 a milky fluid. As emulsification is dependent upon the 

 presence of soap, formed by the union of fatty acid and 

 alkali, a purely neutral fat cannot be emulsified. The 

 emulsification of fat is of importance in the absorption of fat 

 in the food. 



Fats are found in all parts of the body, generally stored 

 up in cells. The percentage of fat in the tissues varies very 

 much, as it is dependent upon the state of nutrition. In 

 lean meat there is but little above \% fat, while the quantity 

 of fat in a fattened animal may be above 30$. The tissues 

 containing the most fat are the subcutaneous tissue, the 

 mesentery, the bone marrow (adipose tissue), which may con- 

 tain about Sofo fat. 



