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THE POPULAR SCIENCE MONTHLY 



forcing pure blood to the body ; and a pulmonary, forcing impure 

 blood 'to the lungs. The pure and impure blood arc never mingled, 

 and all the blood has to pass through the lungs and be oxygenated 

 every time it makes the complete circuit. This perfect circulation, 

 with aerial respiration, produces more rapid chemical changes in 

 the blood and tissues, and consequently the higher temperature of 

 the " warm-blooded " animals. In the embryonic ctages of the heart, 

 the septum dividing the auricles is slowly formed, and an aper- 

 ture exists for a time, called the forcmicn ovale. Cases rarely occur 

 of human subjects in which the opening persists. Such persons are 

 physiologically reduced to the condition of a reptile. It is stated that 

 human infants have lived several days with a circulation as mixed as 

 that of a frog. 



To economize space and muscular effort, these two hearts are 

 formed of the same circular muscles, and are inclosed by a lubricating 

 membrane called the pericardium. In the dugong, however, the two 

 ventricles are quite separate, showing a structural distinction corre- 

 sponding with the functional difference. 



On account of the structural union, the 

 two hearts contract and dilate in unison, joro- 

 ducing the " beating " of the heart. The 

 cause of the first sound in the heart-beat is 

 uncertain, but it occurs at the time of the 

 ventricle contraction. The second sound is 

 produced when the ventricles dilr.te, by the 

 flapping back of the semilunar valves, those 

 placed at the origin of the arteries to pre- 

 vent the regurgitation of the blood. 



Each half of the heart of birds or mam- 

 mals is, like the entire heart of the fishes, a 

 double force-pump, with perfection of valves 

 and tubes and surpassing efficiency. The 

 power is enormous. It has been estimated 

 that, while an engine can lift its own weight 

 three thousand feet in an hour, and an active climber can ascend four 

 thousand feet, the human heart performs hourly a labor equal to lifting 

 its own weight twenty thousand feet. Its daily work is also estimated 

 at seventy-five thousand kilogramme-metres. We can otherwise gain 

 an idea of the power of Nature's enginery, by observing what the heart 

 actually performs. The quantity of blood in the human body is at least 

 six quarts. In its course it has to traverse many feet of tubing and 

 two sets of capillaries, and, notwithstanding the friction and loss of 

 power, all the blood completes the circulation in about thirty-two 

 heart-beats. We should further observe that the heart never rests, 

 but is ceaseless from birth to death. Its cessation is death. 



The necessity of uninterrupted action of the heart requires that it 



F:g. 0. Dorp AT. View op the 

 Heart of the Du(;ong KjJali- 

 core), ITS Cavities being laid 



OPEN, showing the SEPARATION 



OF THE Ventricles. R t\ right 

 ventricle ; L v, lelt ventricle. 



