60 IMMEDIATE PRINCIPLES OF THE TISSUES. 



lysis. Some f them, however, are previously converted into lactic, 

 uric, hippuric, or pneumic acid. 



Since the tissues are formed mainly from the immediate princi- 

 ples of the third class, and those of the second class result from the 

 waste of the tissues, it follows that the last-mentioned principles 

 represent the amount of chemical elements of the third class which 

 have ceased to be a part of the living organism. Hence, though 

 very important to the physiologist, they need not occupy much 

 space in a work on histology. The most important alone will, 

 therefore, be particularly mentioned, and these as briefly as possible. 



FIRST DIVISION. 



Acid or Saline Immediate Principles of Organic Origin. 



For the twenty-three compounds in this division, the reader is 

 referred to the table, page 40. They are found in a notable quan- 

 tity only in the excrementitious fluids, or in the urine alone, or in 

 morbid products, except the inosate of potassa and the lactic and 

 pneumic acids. Only these two acids, together with the uric and 

 hippuric, and the oxalate of lime, will be here described. 1 



1. Lactic Acid. (C 6 H 5 5 .HO.) 



In its most concentrated state, lactic acid is a colorless, inodorous, 

 thick, syrupy fluid, not solidifiable by the most intense cold. It 

 exists in sour milk, resulting from the fermentation of its sugar. 

 In the human body it is always found in the urine when the oxalate 

 of lime is, and while one is living on a strictly animal, diet; as it is 

 in all circumstances in the urine of carnivorous animals. It is also 

 abundant in the "muscular juice;" so much so, indeed, as to be 

 more than sufficient, Liebig asserts, to saturate the alkali of all the 

 alkaline fluids in the body. Lehmann has also found that lactic 

 acid and the lactates exist in the human gastric juice and in the 

 small intestines. 



Origin. The lactic acid in the stomach and small intestines pro- 

 ceeds partly from the gastric fluid, and partly from the starch and 

 sugar in the food, by fermentation. It enters the urine, of course, 

 from the blood ; and the latter from the alimentary canal on the one 

 hand, and from the muscular juice on the other. 



1 The last four salts of soda in this class (see the table), are peculiar to the bile. 



