CIRCULATION. 71 



The capacities of the different cavities of the heart are said to 

 vary slightly, as well as the thickness of the walls; the right auricle 

 and ventricle are said to be most capacious, while the left auricle 

 and ventricle have the advantage in thickness of their walls ; the 

 left ventricle having its parietes nearly three times as thick as the 

 right, since it has to exercise more force in sending the blood to the 

 distant parts of the system. 



Each cavity will hold about two fluidounces, but it is probable 

 that the ventricles do not entirely empty themselves at each stroke ; 

 they will therefore discharge about 1^ ounces at each pulsation. 

 Reckoning 75 pulsations in a minute, there will pass through the 

 heart in this time, 112 oz., or 7 lbs. of blood. The whole quan- 

 tity of blood in the human body is equal to about ^th of its weight, 

 or 28 lbs. in a person weighing 140 lbs. This quantity would there- 

 fore pass through the heart once in four minutes, and would circu- 

 late about fifteen or twenty times in an hour. 



Recent experiments would seem to show that this is below the esti- 

 mate, since substances introduced into the venous circulation have 

 been detected in the remotest parts of the arterial system in less 

 than 30 seconds. 



T\\e force with which the heart propels its blood has been variously 

 stated. According to Hales, the usual force of a man's heart would 

 sustain a column of blood 7J feet high, the weight of which would 

 be about 4 lbs. 6 oz. According to Poisseuille, who caused the blood 

 to act upon a column of mercury, the force of the heart is equal to 

 4 lbs. 3 oz. The true estimate of this force is found by multiplying 

 the pressure of blood in the aorta into the surface of a plane passing 

 through the base and apex of the left ventricle ; according to which 

 it is found to be about 1 3 lbs. 



The number of contractions of the heart in a minute, is about 70 

 or 75. The frequency of its action gradually diminishes from the 

 commencement to the end of life. Just after birth, it ranges from 

 140 to 130; in old age, 65 to 50. Age, sex, muscular exertion, 

 emotions, and temperament, exert a controlling influence over the 

 heart's action. In persons of sanguine temperament, the heart beats 

 more frequently than in those of the phlegmatic, and in the female 

 sex more frequently than in the male. Its action is also increased 

 after a meal, and by rising from a recumbent, to a sitting or stand- 

 ing posture. The time of day also affects it ; the pulse is more fre- 

 quent in the morning, and becomes gradually slower as the day 

 advances. 



In inflammations and fevers, the pulse is much more frequent than 

 during health. When the vital powers decline, it becomes frequent 

 and feeble. In nervous affections, with more oppression than ex- 

 haustion of the forces, the pulse is often remarkably slow. 



Movement of the blood in the arteries and capillaries, — By the 



