68 DTTEODUCTION. 



time supposed, in fresh milk, but only after it has become 

 sour. Its composition (C 6 H 5 O 5 + HO) assimilates it to the 

 sugars, and indicates how it may be formed theoretically from 

 them by transposition of their atoms ; milk sugar having for 

 its composition C 12 H 12 O 12 , which is also the formula for an- 

 hydrous glucose. 



It is a constant constituent of the gastric juice, and is 

 indispensable to the digestive properties of this secretion. 



Lactic acid has been demonstrated by Liebig in the 

 juice of muscular tissue. 1 



Sources and Function. This principle may be formed, in 

 minute quantity, in the intestines, from the saccharine and 

 amylaceous articles of food ; but it is in greatest part pro- 

 duced in the economy as an element of secretion. It is 

 thought that a great portion of the sugar which passes in the 

 blood from the liver to the lungs is converted into lactic acid. 

 If this be the case, it unites with bases and is almost imme- 

 diately decomposed and lost. Lactates in the blood are very 

 readily converted into carbonates, as has been shown by the 

 experiments of Lehmann, 2 who took into the stomach half an 

 ounce of dry lactate of soda, and in thirteen minutes his 

 urine had an alkaline reaction from the presence of carbon- 

 ates. Alkalinity of the urine from this cause is often pro- 

 duced by the ingestion of combinations of the vegetable acids 

 in fruits, etc. 



The most marked function of lactic acid is in the gastric 

 juice, and will be considered under the head of digestion. 



Pneumic Acid and Pneumate of Soda. Pneumic acid 

 was discovered and extracted from the tissue of the lungs by 

 Yerdeil in 185 1. 3 Its ultimate composition is not given. 

 According to this author, it exists in the lungs of the mam- 



1 LEHMANN, Physiological Chemistry, Philadelphia, 1855, voL i., p. 00. 



3 Ibid, p. 97. 



8 KOBIN and VERDEIL, op. ctt., tome ii., p. 466. 



