96 CHEMICAL COMPOSITION OF THE BODY. 



LACTIC ACID, C 3 H 6 O 3 , is a compound of considerable chemical and 

 physiological interest, for it is related not only to the saccharine, but also 

 to the oleaginous and albuminous groups. It may be obtained from the 

 fermentation of milk sugar, in the form of a colorless syrupy fluid, of pure 

 acid taste, but free from smell, which dissolves in all proportions in alcohol, 

 ether, and water. It also crystallizes in the form of brushes of very fine 

 needles. It is not volatile, and can therefore displace some of the stronger 

 mineral acids, as the hydrochloric, at a high temperature; when still more 

 strongly heated, however, it yields lactide, carbonic acid, and carbonic oxide 

 gases. It is connected with the albuminous compounds, of which it may 

 be regarded as a product of the regressive metamorphosis, through alanin, 

 which is isomeric with sarcosiu, a derivative of kreatin. To the oleagi- 

 nous compounds it is related by the similarity of its composition to propi- 

 ouic acid, which is a product of the oxidation of oleic acid, and it is thus 

 associated with formic and butyric acids. Proceeding from one or other of 

 these sources, it is found widely distributed through the tissues of the body, 

 and is of almost constant occurrence in the various parenchymatous juices, 

 especially in that of muscle, the proportion present being considerably in- 

 creased after exercise. It appears to be rapidly decomposed in the blood 

 (carbonic acid being left in combination with the bases), since a few minutes 



after the introduction of considerable quantities 

 FIG. ss. of the alkaline lactates into the stomach the urine 



is found to be alkaline from the presence of their 

 carbonates. 



SUCCINIC ACID, C 4 H 6 O 4 . A very stable or- 

 ganic acid, crystallizing in rhombic prisms and 

 rhombohedric plates ; it is tasteless, and dissolves 

 easily in water. It occurs in the parenchymatous 

 juices of the spleen, thymus, and thyroid glands, 

 of the ox, in the urine of the herbivora, in the 

 contents of cysts containing echinococci, in man, 

 in the fluid of hydrocele, and in considerable 

 Succinic acid. quantity in the blood of the rabbit, goat, ox, and 



horse. It appears to belong to a series of acids 



obtained by the action of powerful oxidizing agents on fats, and is one of 

 the terminal products of their disintegration. It can be obtained from the 

 oxidation of butyric acid, malic acid, and asparagin, to the use of which 

 last substance its occurrence in the urine is probably usually due. 



OXALIC ACID, C 2 O 2 (OH.,), is frequently found in urine after the use of 

 certain kinds of vegetable food. In combination with lime, it constitutes a 

 formidable variety of calculus. It is a constantly-occurring product of the 

 oxidation of organic bodies, and stands in close relation with uric acid. 



56. OLEAGINOUS COMPOUNDS. The animal fats are glycerides, /. e., com- 

 pound ethers of the trivalent alcohol glycerin, and the fatty acids. Those 

 usually present are tripalmitiu, tristearin, and triolein, which are repre- 

 sented by the formula : 



Tristearin (Tristearic Acid Glyceride), ( C &0 8 ) I O 3 



^:t rl f, J 



Tripalmitin (Tripalmitinic Acid Glyceride), ( C A'^ 1 O., 



35 } 



Trioleiu (Trioleiuic Acid Glyceride), ( C iA> O s) \ Q 3 



^3^5 ) 



These are associated in human fat in the proportion of about three-fourths of 

 stearin and palmitin and one-fourth of oleiu. The substance formerly called 



