46 CIRCULATION OF THE BLOOD. 



its character. The abrupt descent of the lever showed that the relaxation was almost 

 instantaneous. 



4. The impulse of the heart, as marked by the third lever, was shown to be absolute- 

 ly synchronous with the ventricular systole. 



Condensing the general results obtained by Marey, which are of course subject to a 

 certain amount of variation, we have, dividing the action of the heart into ten equal 

 parts, three distinct periods, which occur in the following order : 



Auricular Systole. This occupies two-tenths of the heart's action. It is feeble as 

 compared with the ventricular systole, and relaxation immediately follows the contraction. 



Ventricular Systole. This occupies four-tenths of the heart's action. The contrac- 

 tion is powerful and the relaxation, sudden. It is absolutely synchronous with the im- 

 pulse of the heart. 



Auricular Diastole. This occupies four-tenths of the heart's action. 



Force of the Heart. There are few points in physiology concerning which opinions 

 have been more widely divergent than the question of the force employed by the heart at 

 each contraction. Borelli, who was the first to give a definite estimate of this force, put 

 it at 180,000 pounds, while the calculations of Keill give only 5 ounces. These estimates, 

 however, were made on purely theoretical grounds. Borelli estimated the force em- 

 ployed by the deltoid in sustaining a given weight held at arm's length, and formed his 

 estimate of the power of the heart by comparing the weight of the organ with that of 

 the deltoid. Keill made his estimate from a calculation of the rapidity of the current 

 of blood in the arteries. Hales was the first to investigate the question experimentally, 

 by the application of the cardiometer. He showed that the pressure of blood in the 

 aorta could be measured by the height to which the fluid would rise in a tube connected 

 with that vessel, and estimated the force of the left ventricle by multiplying the press- 

 ure in the aorta by the area of the internal surface of the ventricle. The cardiometer 

 has undergone various improvements and modifications, but the above is the principle 

 which is so extensively made use of at the present day in estimating the pressure of the 

 blood in different parts of the circulatory system. First we have the improvement of 

 Poiseuille, who substituted a U-tube partly filled with mercury for the long straight tube 

 of Hales; and then, the various forms of cardiometers constructed by Magendie, Ber- 

 nard, Marey, and others, which will be more fully discussed in connection with the arte- 

 rial circulation. These instruments have beeji made use of by Marey, with very good 

 results, in investigating the relative force exerted by the different divisions of the heart. 



Hales estimated, from experiments upon living animals, the height to which the blood 

 would rise in a tube connected with the aorta of the human subject, at 7 feet 6 inches, 

 and gives the area of the left ventricle as 15 square inches. From this he calculates the 

 force of the left ventricle as equal to 51'5 pounds. Although this estimate is only an ap- 

 proximation, it seems to be based on more reasonable data than any other. 



The apparatus of Marey for registering the contractions of the different cavities of 

 the heart enabled him to ascertain the comparative force of the two ventricles and the 

 right auricle ; the situation of the left auricle precluding the possibility of introducing a 

 sound into its cavity. By first subjecting the bags to known degrees of pressure, the de- 

 gree of elevation of a lever may be graduated so as to represent the degrees of the car- 

 diometer. In analyzing traces made by the left ventricle, the right ventricle, and right 

 auricle, in the horse, Marey found that, as a general rule, tjie comparative force of the 

 right and left ventricles is as one to three. The force of the right auricle is comparatively 

 insignificant, being in one case, as compared with the right ventricle, only as one to ten. 



Action of the Valves. "We have already indicated the course of the blood through 

 the cavities of the heart, and it has been apparent that the necessities of the circulation 

 demand some arrangement by which the current shall always be in one direction. The 



