14 THE HUMAN BODY 



of carbonic acid, having the formula CO< MR 2 ; the relationship of urea to 



2 OH 



carbonic acid is clear when the formula for the latter is written thus: CO < ~. 



OH. 



Fully 30 grams of urea are eliminated daily from the body of an adult man. 



Creatinine (C^NaO) is an interesting member of the group of extractives 

 because the amount of it that is eliminated from the body daily is very con- 

 stant, regardless of changes in amount of food or exercise taken, and seems 

 to depend closely upon the amount of muscle tissue present in the Body; 

 persons of great muscular development have a larger daily creatinine output 

 than those of smaller build. 



Creatine (C-iHgNsC^) is closely related chemically to creatinine, but appears 

 to play a very different part in the Body. Creatinine is undoubtedly a waste 

 product of protein decomposition, being merely an incidental product of the 

 vital processes which go on within the organism. Creatine, on the other hand, 

 seems to be an essential constituent of living protoplasm, although just what 

 purpose it serves is not clear. About one per cent of the solid substance of 

 muscle is creatine. 



The purin bodies, of which uric acid (CsH^N^s) is the most familiar 

 example, are derived chiefly, if not wholly, from the decomposition of nucleo- 

 proteins and are therefore interesting as being the end products of the vital 

 activities of the cell nuclei. 



Pigments. The most important of these that occur in the 

 Body are: 



Hemochromogen, an iron containing pigment which in combination with 

 the histon globin forms hemoglobin, the red coloring matter of the blood. 

 When hemochromogen is in the presence of oxygen it combines with it to 

 form hematin. 



Bilirubin and biliverdin are the bile pigments and give to bile its color. 

 Bilirubin is yellow and biliverdin is green. The former usually predominates 

 in the bile of man and the carnivora, making such bile yellow; the latter is 

 the dominant color in the bile of herbivorous animals, which is green. They 

 are closely related chemically and are derived from the decomposition of 

 hemoglobin. 



Urobilin is formed in the intestine as the result of the putrefaction there 

 of the bile pigments. It is absorbed thence into the blood and excreted by 

 the kidneys, and imparts to the urine its characteristic yellow color. 



Enzyms are a group of substances which seem to be allied in 

 chemical composition to the true proteins, but it is so difficult to 

 be sure of the purity of any specimen that their composition is still 

 in doubt. The enzyms have the power, even when present in very 

 small quantity, of bringing about extensive changes in other sub- 



