CIRCUMSTANCES AFFECTING RATE OF PULSE. 325 



each ventricle of the heart in that time ; consequently nearly three 

 minutes would be required for the passage of the entire mass of the 

 blood through the whole circle of its movement, if its rate be entirely 

 determined by the impulses it receives from this central organ. But it 

 appears, from various experiments, that the rate of circulation is much 

 more rapid than this. For if a solution of any salt, easily detectible in 

 the blood be injected into one of the large veins near the heart, it may 

 be traced in the arterial circulation in from 15 to 20 seconds after- 

 wards ; during which interval it must have traversed the whole pul- 

 monary system of vessels, and passed through both sides of the heart. 

 And if the salt be one, which acts powerfully on the heart itself, as is 

 the case with Nitrate of Baryta or Nitrate of Potass, this action is 

 manifested almost at the same moment with the appearance of the salt 

 in the arteries of other parts ; thus showing that it has been conveyed 

 by the coronary arteries into the capillaries of the heart itself. The 

 period required for the transmission of a saline substance from the 

 veins of the upper part of the body to those of the lower, which can 

 scarcely be accomplished through any more direct channel than the 

 current that returns to the heart, then passes through the lungs back 

 to the heart again, and then flows through the systemic arteries and 

 capillaries to the veins, is accomplished in little more than 20 seconds, 

 even in an animal so large as a Horse. It appears, then, that even 

 the vigorous and constant action of the Heart is not alone sufficient 

 to maintain the circulation at its ordinary rate ; and we are not 

 justified, therefore, in excluding those sources of movement in the 

 higher animals, which evidently exert so important an influence in the 

 lower. 



578. The force with which the heart propels the blood is such, that 

 if a vertical pipe be inserted into the Carotid artery of a horse, the 

 blood will sometimes rise in it to a height of 10 feet. From com- 

 parative experiments upon other animals, it has been estimated that 

 the vigorous action of the heart in Man would sustain a column of 

 blood in his aorta about 7J feet high ; or, in other words, that the 

 force with which the heart ordinarily propels the blood through the 

 aorta, is equal to that which would be generated by the weight of a 

 column of blood of the same size, and 7J feet high; which weight 

 would be about 4^- Ibs. But the force which must be exerted by the 

 heart to sustain such a column, may be shown, upon physical principles, 

 to be as much greater than this, as the area of a plane passing 

 through the base and apex of the left ventricle is greater than that 

 of the transverse section of the aorta ; and as the proportion of these 

 arese is about 3 : 1, the real force of the heart may be stated at about 

 13 Ibs. 



579. The number of contractions of the heart, in a given time, is 

 liable to great variations within the limits of health, from several 

 causes: the chief of which are diversities of Age and Sex, amount 

 of Muscular exertion, the condition of the Mind, the state of the 

 Digestive system, and the period of the Day. The following are the 

 points of greatest importance, in regard to the action of these several 

 influences. 



