CHAPTER XVII 

 TISSUES OF RESPIRATION 



The respiratory tissues form those organs by means of which an ani- 

 mal acquires its principal supply of oxygen, a gas that is absolutely 

 needful in liberal and constant supply for the support of life. Unlike 

 some food materials, oxygen cannot be stored for any length of time in 

 the body, and therefore the organs of respiration are in constant use, 

 even during sleep. In some forms sufficient air can be stored in these 

 organs to last for a short time while the animal suspends breathing 

 temporarily. Also, in other forms, an extremely slow form of respira- 

 tion takes place during a state called hibernation. 



The oxygen is always derived from the free supply of this gas in the 

 atmosphere. When water is breathed, the oxygen is also obtained from 

 the air that is dissolved in the water to which it has access, and not from 

 the oxygen that constitutes a part of the water chemically. The respira- 

 tory tissues also serve as a medium through which carbon dioxide, a 

 gas resulting from the use of the oxygen by the cells, is passed out of the 

 body. The exchange of these two gases constitutes respiration. 



The specific cell of respiration is a thin cell. Besides being thin in 

 body, a feature of its cytoplasm is its clearness and non-staining property, 

 especially of that part of it which is most directly used to transmit the 

 gases. This probably comes from the absence of all materials or 

 structures that might impede the passage of the oxygen and carbon 

 dioxide. It is always placed between the air or water that supplies the 

 oxygen and the tissue of the body that receives it. This receiving tissue 

 is usually the blood. 



The exchange of gases is probably not due to any specific physiological 

 action of these cells, but rather to physical and chemical laws acting 

 almost unrestrainedly through the body of the cell whose function seems 

 to be one of self-elimination in the processes that are going on. This 

 is not because protoplasm has not the power of handling gases phy- 

 siologically and of operating with them against the activities of the 

 ordinary physical and chemical laws (read next chapter, XVIII), 

 but rather for the apparent reason that, since these necessary processes 

 will go on by themselves, it would be a loss of energy to the organism to 

 do it physiologically. 



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