428 PHYSIOLOGICAL CHEMISTRY. 



and pancreatic juice, etc., together with bones, teeth, hair, and many 

 others. 



Most of these substances (some excretions, such as milk and others, 

 excepted) suffer a constant oxidation in the system, and are finally 

 eliminated as waste products ; in regard to the intermediate com- 

 pounds formed in the tissues we know little, but it is highly probable 

 that the reduction of the complicated food material to the simple 

 forms of the waste products is very gradual. There are three chan- 

 nels through which the waste products are given off; they are the 

 lungs, the skin, and the kidneys. By the lungs are eliminated 

 chiefly carbon dioxide and some water, by the kidneys urine (which 

 is a weak aqueous solution of urea, uric acid, urates, phosphates, 

 chlorides, and sulphates of calcium, magnesium, sodium, potassium, 

 etc.), and by the skin are constantly eliminated carbon dioxide and 

 water, and during the process of sweating also more or less of the 

 constituents of urine. 



Chemical changes after death. After the death of either a 

 plant or an animal, a chemical decomposition commences which finally 

 results in the formation of those inorganic compounds from which 

 the plant originally derived its food, viz., carbon dioxide, water, 

 ammonia, sulphates, phosphates, etc. This decomposition of a dead 

 body is generally a simultaneous fermentation or putrefaction, aided 

 by decay or slow combustion. 



There are numerous intermediate products formed, which differ 

 according to the nature of the decomposing substance, or according 

 to the conditions (degree of temperature, amount of moisture and air 

 present, etc.) under which the decomposition takes place. 



During the decomposition of dead vegetable matter (especially of 

 moist wood) the intermediate products are frequently called humus, 

 which substance (or better, mixture of substances) forms the chief 

 part of the organic matter in the soil. 



During the decomposition of dead animals, the sulphur is first 

 eliminated as hydrogen sulphide, and a number of other intermediate 

 products have been shown to be formed ; among them certain organic 



QUESTIONS. 511. What is the difference between vegetable and animal life 

 from a chemieaLpoint of view ? 512. Mention the chief substances serving as 

 plant food. 513. Explain the formation of organic substances in the plant. 



514. What elements enter into the animal system as necessary constituents ? 



515. The members of which three groups of organic substances are chiefly used 

 as food by animals ? 516. Give a full explanation of respiration. 517. Explain 



