198 CIRCULATION. 



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 ex- 

 perimentally, by the application of the cardiometer. He 

 showed that the pressure of blood in the aorta could be meas- 

 ured 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 pressure in the aorta by the 

 area of the internal surface of the ventricle. The cardiometer 

 has undergone various improvements and modifications, but 

 this 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 Magen- 

 die, Bernard, Marey, and others, which will be more fully 

 discussed in connection with the arterial circulation. These 

 instruments have been made use of by Marey, with very 

 good results, in investigating the relative force exerted by 

 the different divisions of the heart. 



Hales estimates, 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 estimates the force of the left ventricle at 51*5 pounds. 1 



Though this estimate is only an approximation, it seems 

 based on more reasonable data than any other. 



The apparatus of Marey for registering the contrac- 

 tions of the different cavities of the heart enabled him to as- 

 certain, also, the comparative force of the two ventricles and 

 the right auricle ; the situation of the left auricle as yet pre- 

 cluding the possibility of introducing a sound into its cavity. 



1 STEPHEN HALES, B.D., F.R.S., &c., Statical Essays : Containing HcemastaticJcs, 

 &?., London, 1733. Vol. II., p. 40. 



