GENERAL STRUCTURE AND COMPOSITION 9 



material, and these are inseparably connected with its functions, 

 the great majority of which are in the long run dependent upon 

 chemical changes. From this point of view, then, the chemical 

 study of the Body presents physiological problems, and might be 

 called Chemical Physiology. At present it is customary to include 

 under the term Biological Chemistry the study of the chemical 

 structure of living matter and of the chemical changes occurring 

 in it. At this point we may confine ourselves to the more im- 

 portant 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 seventeen have been found to take part in the 

 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- 



