350 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



to be half that of the left, he would estimate the power of the latter 

 at one-hundredth part of the weight of the body. This would give 

 a force of upwards of three pounds for the left ventricle for a man 

 weighing eleven stone, and half of that for the right. 



Now Hales had long ago (1769) shown that under the pressure 

 of a column of water nine feet and a half in height, fluid might be 

 made to pass from the carotid artery to the jugular vein through 

 the capillary system. And it is well known to anatomists that 

 when the vessels are free from coagulated blood or other mechanical 

 obstruction, thin fluids may be transmitted by a very slight force 

 from the arteries to the veins. 



Dr. Sharpey's experiments* indicate the exact amount of force 

 necessary for this purpose. A syringe with a hsemadynamometer, 

 to show the amount of pressure used, was adapted to the thoracic 

 aorta of a dog just killed, the abdominal aorta having been pre- 

 viously tied immediately above the renal arteries, and the inferior 

 vena cava opened just as it passes through the diaphragm. Fresh 

 defibrinated bullock's blood was injected with a pressure of three 

 and a half inches of mercury, and passed through the double capil- 

 lary system of the intestines and the liver out of the veins with a 

 full stream. When the pressure was increased to five inches, the 

 blood spirted from the vein in a full jet. When the aorta was not 

 tied above the renal arteries, the same pressure sufficed to drive the 

 blood through the vessels of the lower extremities, and it was made 

 to traverse the capillary system of the lungs by a pressure of from 

 one and a half to two inches of mercury, so as to flow freely 

 through the pulmonary veins. Allowing one pound for every two 

 inches of mercury, it would thus appear that a pressure of two 

 pounds was sufficient to complete the circulation through the two 

 abdominal capillary systems and of one pound for the pulmonary 

 circulation. 



Unless, then, we assume that there are obstacles to the flow of 

 blood through the vascular system, which during life are much 

 greater than those after death, it must be granted that the heart's 

 force, which in man does not probably exceed three pounds, is 

 sufficient to drive the blood throughout the three systems of blood- 

 vessels, and to maintain the current of the circulation ; and that 

 this force alone is capable of producing all the grand phenomena of 

 the circulation. 



It remains, then, to inquire whether the vis a tergo of the heart 

 is the sole force by which the circulation is maintained, or whether 

 * See Williams's "Elements of Medicine/ 'p. 185. 



