302 



OF THE CIRCULATION OF THE BLOOD. 



of which rests on the membrane, whilst its head is surmounted by an ivory 

 knob. The tympanum is provided with three adjusting screws, by which 

 when in use it rests on the wall of the chest with its face parallel to the sur- 

 face, and can be approximated or withdrawn at will. 

 FIG. 123. The cavity of the disk communicates by a vulcanized 



india-rubber tube with a second tympanum represented 

 by b in Fig. 123, in such a way that the two tympana 

 and the tube inclose an air-tight cavity. The movement 

 of the second tympanum is magnified and inscribed on a 

 registering cylinder by a lever. The causes of the im- 

 pulse according to Sibson are, first, the rigidity of the 

 muscular walls during contraction; secondly, a lever 

 movement forward, due to the thickening of the muscle, 

 which occurs during contraction, being resisted by the 

 fibrous bed of the pericardium on which the heart rests ; 

 and lastly, to the outward pressure of the blood in the 

 ventricles on the walls by which it is expelled, and through 

 those walls on the ribs and intercostal spaces. The me- 

 chanical force put forth by the heart is wonderfully great. 

 Houghtou 1 estimates the total daily work of both Ven- 

 tricles at 124.208 foot-tons, or admitting the weight of 

 the heart to be nine ounces, it is capable of raising 20.576 

 pounds one foot in height per minute for every ounce of 

 its weight, which considerably exceds the work done by 

 the muscles in a boat race. Dr. Andrew Buchanan,- pro- 

 ceeding upon somewhat different data, estimates the force 

 exerted by the heart per diem to be 42.3 foot-tons in the 



FIG. 124. 



Tracing obtained by the Cardiograph of MM. Chauveau and Marey. 

 In this tracing o represents the short and abrupt contraction of the Auri- 

 cles which immediately precedes the more energetic and protracted con- 

 traction of the Ventricles, represented by the line v. The line c shows the 

 impulse of the Heart to be coincident with the Ventricular contraction. 



twenty-four hours. Helmholtz" calculates that the heart 

 can raise its own weight, through 20,250 feet in one hour, 

 and Ranke' estimates that the force developed by the Ven- 

 tricle alone amounts to 64,800 kilogrammes per diem, 

 or that it would be sufficient to raise 142,560 Ibs. avoir. 

 through the space of 39.37 inches; the force of the Right 

 Ventricle he estimates at one-third of the above. Whichever estimate is 

 taken the power exerted is enormous, and greatly in excess of any effort of 



1 Principles of Animal Mechanics, 1873, p. 145. 

 1 Lancet, 1870, vol. ii, p. (iiiii. a 



4 GrundzOge der Physiologic, 1868, p. 332. 



Times and Guz., 1864, p. 629. 



