CHAP. IX.] THE CONTRACTILE TISSUES. 361 



v/' ; ' 



The isomeric Lactic Acids. 



At least three acids are known to chemists which have the 

 composition expressed by the formula C 3 H 6 O 3 . These acids all agree 

 in being syrupy, colourless, liquids of strongly acid reaction, soluble 

 in water, alcohol and ether, and yielding when heated first lactic 

 anhydride (C 6 H 10 5 ) and afterwards lactide (C 8 H 4 2 ). Though pos- 

 sessed of many common characters, a careful examination of their 

 behaviour to polarized light, of the crystalline form and the amount 

 of water of crystallization of their salts, and of their products of 

 decomposition, has clearly established the existence of three perfectly 

 distinct lactic acids. 



These three acids are (1) sarcolactic or paralactic acid, the chief 

 acid of dead muscle : (2) ordinary lactic acid : (3) ethylene-lactic acid. 

 The two first of these acids appear to possess the same chemical 

 constitution, and they may be spoken of as ethylidene lactic acids ; 

 still they exhibit certain well-marked differences, the first being, 

 for instance, dextrogyrous whilst the second is optically inactive, and 

 the salts of the two acids differing in the amount of their water of 

 crystallization, and in their solubility. 



a. Sarcolactic Acid. (Optically active ethylidene lactic acid.) 



1. Liebig's extract of meat is dissolved in four parts of 



of^actf^acids Warm water and 8 P arts of 90 P er cent< alcoho1 are theu 

 from muscle. added to the liquid which is continually stirred. The 

 mixture is allowed to stand until the insoluble matter 

 has subsided and a clear supernatant liquid is obtained, and the latter is 

 then separated by decantation. In order to separate any lactic acid from 

 the insoluble residue, the latter is mixed with twice its weight of warm 

 water, and then precipitated with four or five times its volume of 

 alcohol. The alcoholic fluids obtained by these two operations are then 

 evaporated on the water-bath to the consistence of a thin syrup, and the 

 ' latter is again precipitated by the addition of three or four times its volume of 

 alcohol ; the insoluble matter may be kept for the preparation of creatine, 

 hypoxanthine, &c. The alcoholic solution is now evaporated to dryness, 

 the residue is mixed with water, some dilute sulphuric acid added, and 

 then shaken up several times with ether. On evaporating this liquid, a 

 residue is obtained which consists of a mixture of sarcolactic and ethylene- 

 lactic acids. 



2. Instead of employing the above method, the syrupy liquid from 

 which creatine has crystallized, in Liebig's method of preparing creatine, is 

 acidulated with sulphuric acid and then shaken with ether, and the ether 

 evaporated. 



Separation Having obtained a mixture of the two acids, their 



of sarcolactic separation is effected by converting them into zinc salts, and 



from ethylene- the separation of the two salts is based upon their different 



solubility in alcohol. With this object, the mixture of 



