GENEEAL CONSIDEKATIONS. 109 



chiefly in the change of venous into arterial blood ; but experiments have 

 demonstrated that the tissues themselves, detached from the body and placed 

 in an atmosphere of oxygen, will absorb this gas and exhale carbon diox- 

 ide. Under these conditions they certainly respire ; and it is evident, there- 

 fore, that in this process, the intervention of the blood is not an absolute 

 necessity. 



The tide of air in the lungs does not strictly constitute respiration. 

 These organs merely serve to facilitate the introduction of oxygen into the 

 blood and the exhalation of carbon dioxide. If the system be drained of 

 blood or if the blood be rendered incapable of interchanging its gases with 

 the air, respiration ceases, and all the phenomena of asphyxia are presented, 

 although air be introduced into the lungs with perfect regularity. It must 

 be remembered that the essential processes of respiration take place in all 

 the tissues and organs of the system and not in the lungs. Respiration is a 

 process similar to what are known as the processes of nutrition ; and although 

 it is much more active and uniform than are the ordinary nutritive changes, 

 it is inseparably connected with and strictly a part of the general process. 

 As in the nutrition of the tissues the nitrogenized constituents of the blood, 

 united with inorganic substances, are transformed into the tissue itself, 

 finally changed into excrementitious products, such as the urinary mat- 

 ters, and discharged from the body, so the oxygen of the blood is appro- 

 priated, and carbon dioxide, which is an excrementitious substance, is pro- 

 duced, whenever tissues are worn out and regenerated. There is a necessary 

 and inseparable connection between all these processes ; and they must be 

 considered, not as distinct in themselves, but as different parts of the general 

 function of nutrition. 



As physiologists are unable to follow out all the intermediate changes 

 which take place between the appropriation of nutritive materials from the 

 blood and the production of effete, or excrementitious substances, it is impos- 

 sible to say precisely how oxygen is used by the tissues and how carbon dioxide 

 is produced. It is known only that more or less oxygen is necessary to the 

 nutrition of all tissues, in all animals, high or low in the scale, and that the 

 tissues produce a certain quantity of carbon dioxide. The fact that oxygen 

 is consumed with much greater rapidity than any other nutritive substance 

 and that the production of carbon dioxide is correspondingly active, as com- 

 pared with other effete products, points to a connection between the absorp- 

 tion of the one and the production of the other. 



The essential conditions for respiration in animals which have a circulat- 

 ing nutritive fluid are air and blood separated by a membrane which will 

 allow the passage of gases. The effete products of respiration contained in 

 the blood, the most important of which is carbon dioxide, pass out and vitiate 

 the air. The air is deprived of a certain portion of its oxygen, which passes 

 into the blood, to be conveyed to the tissues. Thus the air must be changed 

 to supply fresh oxygen and get rid of the carbon dioxide. The rapidity of 

 this change is in proportion to the nutritive activity of the animal and the 

 rapidity of the circulation of the blood. 

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