. Biology; the story of living things. 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
. Biology; the story of living things. 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 ;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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may no
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