CHEMICAL BASIS OF Till: ANIMAL P,<>I>Y. 125 



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

 will have the following formula; respectively: CH 8 .CH(OH). 

 COOH an.l riL (nil,. ril a . COOH, Of these the first is known 



ethylidene-lactic acid, the second as hydraerylic acid. 



In adilitiiiu to the above a third acid, isoineric with ethylidene- 

 lactic acid is known, namely sarcolactic or para lactic 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 

 ha- been given, has also been described as isomeric with hydra- 

 i iv lie acid. It is however probable that this acid is really acetyl- 

 lactic acid, hydracrylic acid being the true ethylene-lactic acid, 

 below.) 



The several forms of lactic acid are all syrupy colourless fluids, 

 soluble in all proportions in water and in alcuhol, and to a slight 

 extent iii ether. They possess an intensely sour taste, and a 

 stmng 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 

 at lord a means of separating the several acids each from the other. 



1. Ethylidene-lactic acid. CH, . CH(OH) . COOH. 



This is the ordinary form of the acid, obtained characteristically 

 as the chief product of the lactic fermentation of sugars (see p. 105). 



From this source it may be readily prepared by adding a little old 

 chei-M.' and sour milk to a solution of cane sugar t<> \\hirli some car- 

 lionatcof /inc is added. The whole is kept warmed to 40 or 45 for 

 ten day> or a fortnight, hcing vigorously stirred at frequent intrnals. 

 Tin- lactic acil i- fixed as a lactate by the /inc salt as fast as it is 

 formed. thi> removal of free acid heing essential to the progress of the 

 fermentation which does not take place in an acid solution. The 

 crusts of /inc-lactate formed during the above process are purified by 

 -tallisini:, the acid is then liberated from the compound 1>\ the 

 action ,,)" >iilphurettcd hydrogen, and extracted hy shaking up with 

 ether, in which it is soluble. By a similar process lactic acid may In- 

 read ily o I it aiued from lactose. 



Lactic acid occurs in the contents of the stomach and intestine, 

 more particularly during " ( li ( 't rich in carbohydrates, and maybe 

 readily funned by the digestion of gastric mucous membrane with 

 solutions of dextrose or saccharose. 1 According to Heint/ 2 it is 

 found also in muscles, and according to Gscheidleu 3 in the 

 ganglionic cells of the grey substance of the brain. 



1 Maly, Ann. d. Chrm. n. Phnrm. H.I. ci.xxm. (1S74), S. i'27. 



2 Ann. <1. </,. . /'>,<,,-,. IM . i vi! f|v7l i. S. 314. 

 Pfliiger'a Archiv, Bd. vin. (1873-74), S. 171. 



