CHEMICAL CHANGES IN PLANTS AND ANIMALS. 353 



whilst the undissolved and undigested portions of the food are 

 further propelled until they leave the organism asfeces. 



The chyle, after having passed through the walls of the in- 

 testines, is absorbed and carried in the blood to the lungs, where 

 it is acted upon by the inhaled oxygen, absorbing a certain 

 quantity of this element, whilst about an equal volume of carbon 

 dioxide is given off. The blood, thus regenerated by the ab- 

 sorption of chyle and oxygen, returns to the system as arterial 

 (oxidized) blood, which in its course through the body comes in 

 contact with all the various parts of the system, nourishing and 

 regenerating them by leaving behind albuminous (and other) 

 matter, whilst, at the same time, it oxidizes another quantity of 

 matter which had been previously left behind, had assumed 

 cellular structure, and had been rendered useless during the 

 performance of its functions. 



Waste products of animal life. The changes which the food 

 suffers after having been absorbed by the animal system are ex- 

 tremely complicated, and far from being thoroughly understood. 

 Numerous products and organs are formed and nourished from 

 and by the blood; among them muscular-, nerve-, and brain- 

 substance, excretions and secretions, such as milk, saliva, bile, 

 gastric and pancreatic juice, etc., also bones, teeth, hair, and 

 many others. 



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

 others, exceptcd) suffer a constant oxidation in the system, and 

 are finally eliminated as waste products. 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 elimi- 

 nated 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, magne- 

 sium, 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 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 



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