12 THE HUMAN BODY. 



proteids, the nearest chemical allies of which they seem 

 to be, they are only known in or derived from living 

 beings. Gelatin, obtained from bones and ligaments by 

 boiling, is a typical albuminoid; as is chondrin, which is 

 obtained similarly from gristle. Mucin, which gives their 

 glairy tenacious character to the secretions of the mouth 

 and nose, is another albuminoid. 



Crystalline Nitrogenous Substances. These are a 

 heterogeneous group, the great majority of them being 

 materials which have done their work in the Body and are 

 about to be got rid of. Nitrogen enters the Body in foods 

 for the most part in the chemically complex form of some 

 proteid. In the vital processes these proteids are broken 

 down into simpler substances, their carbon being partly 

 combined with oxygen and passed out through the lungs 

 as carbon dioxide; their hydrogen is similarly in large part 

 combined with oxygen and passed out as water; while their 

 nitrogen, with some carbon and hydrogen and oxygen, is 

 usually passed out in the form of a crystalline compound, 

 containing what chemists call an "ammonium residue.'' 



CO) 

 Of these the most important is urea (Carbamide H 2 > X 2 ), 



H-) 



which is eliminated through the kidneys. Uric acid is an- 

 other nitrogenous waste product, and many others, such as 

 fareatin and Tcreatinin, seem to be intermediate stages be- 

 'tween the proteids which enter the body and the urea and 

 uric acid which leave it. 



In the bile or gall, two crystallizable nitrogen-contain- 

 ing bodies, glycocliolic and tauroclwlic acids, are found com- 

 bined with soda. 



Nitrogenous Coloring Matters. These form an arti- 

 ficial group whose constitution and origin are ill known. 

 Among the most important are the following: 



HcBmatin, derived from the red corpuscles of the blood 

 in which a residue of it is combined with a proteid residue 

 to form haemoglobin. 



Bilirubin and biliverdin, which exist in the bile ; the 

 former predominating in the bile of man and of carnivo- 



