236 ANIMAL BIOLOGY 



This gas is a common product of living bodies, since it 

 results from the action of oxygen on the carbon contained 

 in living tissue. Carbon occurs in all tissues and in all 

 foods. 



HYDROGEN 



This element naturally occurs in all organisms since it 

 is one of the constituents of water, but it is found also 

 in other combinations in all living substance. It is a 

 very light, transparent, odorless gas that enters very 

 freely into composition with oxygen, chlorine, carbon and 

 a number of other elements. 



NITROGEN 



This element is a rather inert gas, transparent and 

 odorless, as we might infer -from its constituting about 

 four-fifths of the atmosphere. 



Other elements contained in living bodies are sodium, 

 potassium, calcium, sulphur, phosphorus, chlorine, iron, 

 iodine, and in some cases silicon, manganese and copper. 

 Many of these elements occur in the form of salts which, 

 while not commonly classed as foods, are nevertheless 

 necessary to maintain the life of the body. The elements 

 of the living body are for the most part combined to form 

 substances of a good deal of complexity. Most of the 

 compounds formed are not found elsewhere in nature, 

 and they are consequently known as organic compounds. 

 It was formerly held that organic compounds could be 

 formed only through the agency of life, but chemists 

 have succeeded in making a good many of them artificially 

 in the laboratory. The very complex and unstable com- 

 pounds more immediately associated with the phenomena 

 of life it is still impossible to fabricate. The body of the 

 simplest organism is a chemical laboratory in which 



