CHAPTER XL 

 CIRCULATION 



Vocabulary 



Transportation, carrying from place to place. 

 Plasma, liquid portion of blood tissue. 

 Auricles, upper, receiving chambers of the heart. 

 Ventricles, lower, sending chambers of the heart. 



The function of any circulatory system is transportation; the 

 blood is the carrier, the blood vessels are the roads, and the heart 

 is the motive power. Digested food is carried from the digestive 

 organs to the tissues, oxygen from the lungs to the tissues, waste 

 matters from the tissues to the lungs, skin, and kidneys, and in- 

 ternal secretions from their glands to places where they are used. 



Development of Circulation. A circulatory system is not found 

 in very simple animals like protozoa, sponges, and hydra, because 

 they have so few cells that each can obtain its own food and oxy- 

 gen and throw off its waste, without the need of a set of organs for 

 carrying them. We do not find a transportation system within 

 our own home, nor even in a small village, for each individual 

 does his own carrying. In larger cities street railways are neces- 

 sary, while to care for a whole state, numerous railroads and canals 

 are required. 



It is the same in animal structure. The simple forms have no 

 circulatory transportation; in higher types there are simple cir- 

 culatory organs (earthworm) . In still more complicated organisms, 

 a heart and blood vessels are required (crayfish), while in the ver- 

 tebrates, especially birds and mammals with their very highly 

 specialized organs, there is needed a very complete and complex 

 transportation system, in order that each cell may be supplied. 



Now we may carry our comparison between cell functions and 

 life on Crusoe's island a step further and find another result of 



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