306 



THE MAINTENANCE OF THE INDIVIDUAL 



and that the amount and rate of flow of the blood from the heart makes 

 it necessary to assume that most of it must return to the heart. This 

 latter fact was shown by assuming that the ventricle held only two 

 ounces ; then, if the pulse beats 72 times per minute, in an hour it 

 would force 72 X 60 X 2, or 8640 ounces, or 540 pounds, into the aorta, 

 which is considerably more than the weight of man. The return of 

 the blood to the heart is accomplished by veins, thus completing the 

 circuit. This summarizes briefly the gist of Harvey's contributions 

 on circulation. Small wonder that after so many misleading beliefs 

 this master should be acclaimed for his careful thinking and his 

 accurate observations upon the action of the heart. His study in- 

 volved examinations of about forty species of animals, and ulti- 

 mately led to the fundamental concept of the circulation of blood. 



The Heart 



The vertebrate heart is really a pumping station which in its 

 simplest form, as found in the fishes, consists of a receiving auricle 

 and a pumping ventricle. Back flow is prevented by a series of valves 

 placed at strategic points. Ascending the vertebrate scale and leav- 

 ing behind water-inhabiting forms, we find the circulatory system 



ampl-^ibian Tept.il<2. 



hlrd. and, 

 raccynmod 



Evolution of four-chambered heart. Contrast situation in fish and amphibia 



with reptiles, birds, and mammals. 



becoming more complicated and the heart evolving from a two- 

 chambered form, typical of fish, to a four-chambered type found in 

 birds and mammals. Intermediate stages in this progression appear 

 in the amphibia and reptiles. 



The heart of man is a cone-shaped, muscular organ about the size 

 of the fist. It is surrounded by a loose membranous bag called the 



