988 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



which the biliary salts return to the blood from the liver, they are burned in 

 the body, sometimes so completely that none appear in the urine. They have 

 the power of dissolving haemoglobin from the blood-corpuscles, and in con- 

 sequence the urine may be highly colored, perhaps from bilirubin. 1 



Pettenkofer, experimenting once on the conversion of sugar into fat, warmed together 

 cane-sugar, bile, and concentrated sulphuric acid. He obtained no fat, but a strong violet 

 coloration. This is " Pettenkofer' s test" for biliary acids (cholic acid, fellic acid, etc.). 

 This coloration is likewise given by proteid, oleic acid, and other bodies. The test of Neu- 

 komm, however, is said to be absolutely characteristic. Here a drop of a substance con- 

 taining biliary acids is placed on a small white porcelain cover, with a drop of dilute 

 cane-sugar solution, and one of dilute sulphuric acid ; the mixture is then very carefully 

 evaporated over a flame and leaves a brilliant violet stain. 



OXY- FATTY ACIDS, LACTIC-ACID GROUP. 



These are diatomic monobasic acids of the glycols. A glycol is a diatomic 

 alcohol. The oxy- fatty acids have the general formula C n H 2n O 3 , and include : 



Carbonic acid, CH 2 O 3 . Oxy-butyric acid, C 4 H 8 O 3 . 



Glycollic acid, C 2 H 4 O 3 . Oxy-valerianic acid, C 5 H 10 O 3 . 



Lactic acid, C 3 H 6 O 3 . etc. 



Carbonic Acid, or Oxy-fonnic Acid, HO.CO.OH. This is, in reality, 

 a dibasic acid on account of the symmetric structure of the two OH radicals. 

 It has already been considered (see p. 1003). 



Lactic Acids, or Oxy-propionic Acids. Of these there are two isomeres, 

 which vary in the position of their OH group, the a- and ft- lactic acids. 

 Physiology is concerned only with the first. 



a-Lactic Acid, or Ethidene Lactic Acid, CH 3 .CHOH.COOH. This 

 is called fermentation lactic acid, being a product of the fermentation of carbo- 

 hydrates (see p. 982) : 



C 6 H 12 6 = 2C 3 H 6 3 . 



On lactic fermentation of milk-sugar depends the souring of milk. This fer- 

 mentation does not take place in the presence of sufficiently acid gastric juice, 

 but it is very active in the more nearly neutral (or alkaline) intestine. After 

 a meal which includes carbohydrates the intestinal contents may remain quite 

 distinctly acid down to the ileo-caecal valve, due to acetic and lactic acid pro- 

 duction, to such an extent even that proteid putrefaction is inhibited, as indicated 

 by the total absence of leucin and tyrosin. 2 It has been noticed that the fecal 

 excrements after a carbohydrate diet react acid, after proteid diet alkaline. 

 The acid reaction is due chiefly if not wholly to acetic acid, since lactic acid, 

 being the stronger acid, is first neutralized by the intestinal alkali. Lactic 

 acid, when absorbed, is completely burned in the body. Lactic-acid fermenta- 

 tion between the teeth dissolves the enamel, and gives bacteria access to the 

 interior. The fermentation lactic acid is inactive to polarized light, and, since 



1 Hoppe-Seyler : Physiologische Chemie, 1877, p. 476. 



2 Macfadyen, Nencki, und Sieber : Archiv fur exper. Pathologic und Pharmakologie, 1891, 

 Bd. 28, p. 347 



