42 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



A little bag, stretched over two metallic buttons separated by a central rod, 

 is then secured in the cavity thus formed and is connected by an elastic tube 

 with the registering apparatus. All the tubes are provided with stop-cocks, 

 so that each initial bag may be made to communicate with its lever at will. 

 AVhen the operation is completed and the sound is firmly secured in place by 

 a ligature around the vein, the animal experiences no inconvenience, is able 

 to walk about, eat etc., and there is every evidence that the circulation is 

 not interfered with. The cylinder which carries the paper destined to receive 

 the traces is arranged to move by clock-work at a given rate. The paper 

 may also be ruled in lines, the distances between which represent certain frac- 

 tions of a second. Fig. 21 represents the apparatus reduced to one-sixth of 

 its actual size. Two of the levers are connected with the double sound for 

 the right auricle and ventricle, and one is connected with the bag destined to 

 receive the impulse of the heart. In an experiment upon a horse, the move- 

 ments of the three levers produced traces upon the paper which were inter- 

 preted as follows : 



The auricular systole, marked by the first lever, immediately preceded the 

 ventricular systole and occupied about two-tenths of a second. The eleva- 

 tion of the lever indicated that it was much more feeble than the ventricu- 

 lar systole, and sudden in its character ; the contraction, when it had arrived 

 at the maximum, being immediately followed by relaxation. 



The ventricular systole, marked by the second lever, immediately followed 

 the auricular systole and occupied about four-tenths of a second. The almost 

 vertical direction of the trace and the degree of elevation showed that it was 

 sudden and powerful in its character. The abrupt descent of the lever 

 showed that the relaxation was almost instantaneous. 



The impulse of the heart, marked by the third lever, was shown to be ab- 

 solutely synchronous with the ventricular systole. 



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

 subject to some variation, and dividing the action of the heart into ten equal 

 parts, three distinct periods are observed, 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 contraction is powerful and the relaxation is sudden. It is absolutely 

 synchronous with the impulse of the heart. 



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



Force of the Heart. Hales (1733) was the first to investigate experiment- 

 ally the question of the force exerted by the heart, 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 pressure in the aorta by the area of the internal surface of the ventricle. 

 The cardiometer has since undergone various improvements and modifica- 



