30 INTRODUCTION. 



go different chemical alterations in its interior, and are finally discharged 

 from it under other forms. If these changes be prevented from taking 

 place, life is immediately extinguished. Thus animals constantly absorb, 

 on the one hand, water, oxygen, salts, albumen, oil, sugar, etc., and give 

 up, on the other hand, to the surrounding media, carbonic acid, water, 

 creatine, the urates, urea, and the like ; while between these two extreme 

 points, of absorption and exhalation, there take place a multitude of 

 different transformations which are essential to the continuance of life. 



Some of these chemical actions are the same with those which are 

 seen outside the body ; but most of them are peculiar, and do not take 

 place anywhere else. This, again, is not because there is anything excep- 

 tional in their nature, but because the conditions necessary for their 

 accomplishment exist in the body, and do not exist elsewhere. All 

 chemical phenomena are liable to be modified by surrounding conditions. 

 Many reactions which will take place at a high temperature will not 

 take place at a low temperature, and vice versa. Some will take place 

 in the light, but not in the dark ; others will take place in the dark, but 

 not in the light. Because a chemical reaction, therefore, takes place 

 under one set of conditions, we cannot be at all sure that it will take 

 place under others which are different. 



The chemical conditions of the living body are exceedingly compli- 

 cated. In the animal solids and fluids, there are many substances 

 mingled together in varying quantities, which modify or interfere with 

 each other's reactions. New substances are constantly entering by 

 absorption, and old ones leaving by exhalation; while the circulating 

 fluids are incessantly passing from one part of the body to another, 

 and coming in contact with different organs of different texture and 

 composition. All these conditions are peculiar, and so modif}^ the 

 chemical actions taking place in the body that they are often unlike 

 those met with elsewhere. 



If starch and iodine be mingled together in a watery solution, they 

 unite with each other, and strike a deep blue color; but if they be 

 mingled in the blood, no such reaction takes place, because it is pre- 

 vented by the presence of certain organic substances which interfere 

 with it. 



If dead animal matter be exposed to warmth, air, and moisture, it 

 putrefies; but if introduced into the living stomach, this process is pre- 

 vented, because the fluids of the stomach cause the animal substance to 

 undergo a peculiar transformation (digestion), after which the blood- 

 vessels immediately remove it by absorption. There are also certain 

 substances which make their appearance in the living body of animals 

 or vegetables, and which are not found elsewhere ; such as fibrine, albu- 

 men, caseine, the biliary salts, hemoglobine, chlorophyll, morphine, etc. 

 These substances cannot be manufactured artificially, simply because 

 we are unable to imitate the necessary conditions. They require for 

 their production the presence of a living organism. 



The chemical phenomena of the living body are, therefore, not different 



