8 THE HUMAN BODY 



formation of the Human Body. These are carbon, hydrogen, 

 nitrogen, oxygen, sulphur, phosphorus, chlorin, fluorin, iodin, 

 silicon, sodium, potassium, lithium, calcium, magnesium, iron, and 

 manganese. Copper and lead have sometimes been found in small 

 quantities, but are probably accidental and occasional. 



Uncombined Elements. Only a very small number of the 

 above elements exist in the Body uncombined. Oxygen is found 

 in small quantity dissolved in the blood ; but even there most of it 

 is in a state of loose chemical combination. It is also found in the 

 cavities of the lungs and alimentary canal, being derived from the 

 inspired air or swallowed with food and saliva; but while con- 

 tained in these spaces it can hardly be said to form a part of the 

 Body. Nitrogen also exists uncombined in the lungs and alimen- 

 tary canal, and in small quantity in solution in the blood. Free 

 hydrogen has also been found in the alimentary canal, being there 

 evolved by the fermentation of certain foods. 



Chemical Compounds. The number of these which may be 

 obtained from the Body is very great; but with regard to very 

 many of them we do not know that the form in which we extract 

 them is really that in which the elements they contain were united 

 while in the living Body; since the methods of chemical analysis 

 are such as always break down the more complex forms of living 

 matter and leave us only its debris for examination. We know in 

 fact, tolerably accurately, what compounds enter the Body as 

 food and what finally leave it as waste; but the intermediate con- 

 ditions of the elements contained in these compounds during their 

 sojourn inside the Body we know very little about; more especially 

 their state of combination during that part of their stay when they 

 do not exist dissolved in the bodily liquids, but form part of a solid 

 living tissue. 



For present purposes the chemical compounds existing in or de- 

 rived from the Body may be classified as organic and inorganic, 

 and the former be subdivided into those which contain nitrogen 

 and those which do not. 



Nitrogenous Organic Compounds. These fall into several main 

 groups : proteins * subdivided into simple proteins, conjugated pro- 



* The classification of proteins here given is that recommended by the joint 

 committee on protein nomenclature of the American Physiological Society 

 and the American Society of Biological Chemists, 1907. 



