CHEMICAL BASIS OF THE ANIMAL BODY. 125 



GLYCOLIC ACID SERIES. 

 Lactic (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 3 . CH 2 . COOH, it may be noticed at once that two isomeric 

 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 ethy- 

 lidene-lactic acid, the second as hydracrylic acid. 



In addition to the above a third acid, isomeric with ethylidene 

 lactic acid is known, namely sarcolactic or paralactic acid. Of these 

 three acids only two occur in the body, hydracrylic 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 syrupy 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 crystallise with difficulty. The calcium and zinc salts are 

 of the greatest importance, as will be seen later on, inasmuch as by 

 their varying solubilities they afford a means of separating the several 

 acids each from the other. 



1 This acid is frequently classed in the preceding group of acids as the first of 

 the glycolic series. 



