RESPIRATION. 335 



The blood, with its complex constitution, becomes in this way 

 the principal medium for all the phenomena of nutrition. It is 

 known to be collecting, in its course, for its own recoiistitution, cer^ 

 tain materials elaborated by the digestive passages and then deposit 

 ing assimilable principles in the various tissues. The blood repre- 

 sents, therefore, a reparatory fluid whose continual renewal and 

 destruction, intrusted to digestion and respiration, constitute the twc 

 inseparable conditions for existence of the higher animals. 



When air is fed to the wood in the firebox of a boiler a process 

 known as burning takes place. It is a real chemical process: the 

 oxygen unites with the carbon and hydrogen of the wood, so that 

 both the wood and oxygen disappear as such. The carbon and a por- 

 tion of the oxygen unite to form carbonic-acid gas. The hydrogen 

 and the remainder of the oxygen by their union form water. The 

 two substances thus formed pass off in the smoke, leaving behind as 

 the debris, or ashes, the mineral part of the wood. By this burning 

 also termed oxidation, heat and a flame are produced. 



Within the body there occurs an analogous process, also termed 

 oxidation, whereby the oxygen inhaled into the body slowly burns the 

 protoplasm of cells in a manner similar to the burning of the wood in 

 the boiler. This process within the body is performed so slowly that 

 there is no appearance of a flame, but there is yielded the same 

 amount of heat as would be produced were the same materials burned 

 within a furnace or stove. Some of this heat is utilized to give warmth 

 to the body, while the remainder of it is converted into power and 

 energy, so that the body may do work, either of motion, thought, or 

 manufacturing the various products of the body. Oxidation is the 

 essential process of life ; when it ceases, life ends. It occurs in every 

 cell of the economy. Its degrees of oxidation in the living cells can be 

 heightened or lowered according to the needs of the body. The end- 

 products of body-oxidation are also carbonic-acid gas, water, and ashes, 

 or urea as occurred in furnace oxidation. 



From studies in general physiology it is known that the peculiar 

 form of energy which is called life exists only in association with 

 living cells or living organisms. It is liberated during a catab- 

 olism, or destructive metabolism of living cell-protoplasm, and this 

 metabolism is possible only in the presence of oxygen. During these 

 catabolic metabolisms the living protoplasm of the cell, the deeply 

 complex protoplastic molecule, is split up into two, perhaps more, 

 simpler molecules; these last, which probably represent proteids, 

 may again separate into still simpler ones. Each change from a 



