1232 



LACTIC ACIDS. 



the free fatty or oleic acid to form a salt, leaving the glycerin 

 in solution. These salts are called soaps, being soluble in water 

 if the metal is an alkali, insoluble if it is calcium, lead, or other 

 similar metal. The reaction which takes place during the above 

 saponification is as follows. 



Tri-stearin. Potassium stearate. Glycerin. 



C.H a (C 17 H . CO . 0), + 3 KHO = 3(C 17 H M . COOK) + C 3 H 5 (OH) 3 . 



A similar decomposition into glycerin and free fatty acid can 

 be effected by pancreatic juice (see p. 1193), the acid uniting 

 with the alkali of the juice or of the bile to form a soap. This 

 decomposition is however quantitatively inconsiderable but 

 qualitatively of great importance for the absorption of fats, 

 owing to the extraordinarily great emulsifying power of a 

 mixture of bile, free fatty acids and soluble soaps. The same 

 decomposition takes place when fats, more especially butter, 

 turn rancid. 



III. ACIDS OF THE GLYCOLIC AND OXALIC SERIES. 



GLYCOLIC ACID SERIES. 

 Lactic (a-hydroxy-propionic) acid. C 3 H 6 O 3 . 



This, after carbonic acid, is to the physiologist the most 

 important acid of the series. 



If lactic acid is regarded as derived from propionic acid, 

 CH 8 . CH 2 . COOH, it may be noticed at once that two iso- 

 meric lactic acids must be capable of being formed from it. 

 These acids will have the following formulae respectively; 

 CH 3 .CH(OH). COOH and CH 2 (OH) . CH 2 . COOH. Of 

 these the first is known as ethylidene-lactic acid, the second 

 as hydracrylic acid. 



In addition to the above a third acid, isomeric with ethyl- 

 idene-lactic acid, is known, namely sarcolactic or paralactic 

 acid. Of these three acids only two occur in the body, hydra- 

 crylic being absent. A fourth acid, to which the name of 

 ethylene -lactic acid has been given, has also been described 

 as isomeric with hydracrylic acid. It is however probable that 

 this acid is really acetyl-lactic acid, hydracrylic acid being the 

 true ethylene-lactic acid. (See below.) 



The several forms of lactic acid are all sirupy colourless 

 fluids, soluble in all proportions in water and in alcohol, and to 

 a slight extent in ether. They possess an intensely sour taste, 

 and a strong acid reaction. When heated in solution they may 

 partially distil over in the escaping vapour but are usually 

 decomposed during the process. They form salts with metals, 

 of which those with the alkalis are very soluble and crystallize 

 with difficulty. The calcium and zinc salts are of the greatest 



