CHAPTER XII 



CIRCULATION AND RESPIRATION IN ANIMALS 



I finally saw that the blood, forced by the action of the left 

 ventricle into the arteries, was distributed to the body at large, 

 and its several parts, in the same manner as it is sent through 

 the lungs, impelled by the right ventricle into the pulmonary 

 artery, and that it then passed through the veins and along 

 the vena cava, and so round to the left ventricle . . . which 

 motion we may be allowed to call circular. Harvey, 1628. 



THE crucial points of contact between the higher animal 

 and its environment, in so far as the intake of matter and 

 energy is concerned, are the membranes which line the 

 digestive tract and a large diverticulum from it, the lungs. 

 Through the former must pass all the materials which are to 

 be assembled as integral parts of the organism and the fuel 

 which is to supply the energy for the vital processes, while 

 through the latter must pass the oxygen which is to release 

 this energy. Only when these membranes have been passed 

 are the materials really within the body and at its disposal 

 for distribution by the CIRCULATORY SYSTEM to the individual 

 cells of the various organs which are to use them. In addition 

 to carrying the fuel and the oxygen, the circulatory system 

 must remove the waste products of metabolism from the cells 

 and deliver them to the proper excretory organs, such as the 

 lungs or kidneys, to be passed to the outside world. The cir- 

 culatory system is therefore the essential connecting link be- 

 tween the points of intake, utilization, and outgo of materials 

 a distributing system which in cooperation with the nerv- 

 ous system unifies the organs into an organism. 



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