8 THE HUMAN BODY. 



of Physiological Chemistry. For the present we may confine 

 ourselves to the more important substances derived from or 

 known to exist in the Body, leaving questions concerning the 

 chemical changes taking place within it for consideration 

 along with those functions which are performed in connection 

 with them. 



Elements Composing the Body. Of the elements known 

 to chemists only sixteen have been found to take part in the 

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

 nitrogen, oxygen, sulphur, phosphorus, chlorine, fluorine, 

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

 and manganese. Copper and lead have sometimes been found 

 in small quantities, but are probably accidental and occa- 

 sional. 



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 contained in these spaces it can hardly 

 be said to form a part of the Body. Nitrogen also exists un- 

 combined in the lungs and alimentary 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 de- 

 bris 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 conditions of the ele- 

 ments 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 



