SECTION YI. 



RESPIRATION. 



The study of the metabolism of muscle has taught that a 

 process of oxidation is constantly going on in the living 

 tissues for which oxygen is constantly required, and by 

 which carbon dioxide is constantly being produced. 



In the lowliest animals a direct exchange of gases takes 

 place between the cells and the surrounding medium. 



In the higher and more complex animals a special 

 mechanism has been evolved for carrying the oxygen from 

 outside to the tissues, and of transporting the carbon dioxide 

 from the tissues to the exterior. 



This is the Respiratory Mechanism. 



In mammals it consists of arrangements by which — 



1. Air is brought into relationship with the blood. 



2. The exchange of gases between air and blood takes 

 place. 



3. The blood carries the oxygen to the tissues, and the 

 carbon dioxide from the tissues. 



4. The oxygen is passed from the blood to the tissues, 

 and the carbon dioxide from the tissues to the blood. 



5. The oxygen brings about combustion in the tissues. 



The first two constitute the process of External Respira- 

 tion, the third and fourth that of Intermediate Respiration, 

 and the last that of Internal Respiration. This last has been 

 already dealt with in the study of muscle (p. 255) and of meta- 

 bolism, and it dominates the other parts of the process. 



A. EXTERNAL RESPIRATION. 



I. STRUCTURE OF THE RESPIRATORY 

 MECHANISM. 



In aquatic animals the mechanism by which this process 

 is carried on is a gill or gills. Each consists of a process 

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