250 PHYSIOLOGY AT THE FARM. 



time than usual ; hence the left cavities of the heart contract 

 more frequently, and send the blood all over the body at a far 

 more rapid rate than in ordinary circumstances. To this view 

 other effects might be added ; for example, that when the 

 blood is made to move more rapidly in the veins, the resistance to 

 the passage of the blood from the aortic system into the system 

 of the vence cavce is very much diminished, so that, under this 

 violent exercise, the moving power, or the heart's contraction, 

 acts more efficiently at the same time that the resistance to 

 be overcome undergoes a great diminution. 



The circulation of the blood, even when the body is at rest, 

 takes place in a wonderfully short time. Many experiments 

 concur in proof of this fact. An easy mode of conceiving this 

 effect is the following : In the human body the left ventricle 

 is assumed to throw two ounces of blood into the aorta at each 

 stroke of the heart ; the average number of strokes in the 

 minute may be taken at 70, whence the ventricle throws into 

 the aorta 140 ounces of blood per minute, or nearly 9 imperial 

 pounds in that space of time ; but the calculated amount of 

 blood in the whole body does not exceed from J 8 to 27 pounds, 

 so that the left ventricle pours out a quantity of blood equal to 

 the whole blood in the body in from two minutes to two 

 minutes and a half. Now this period is plainly the average 

 time which the particle of blood which performs the circulation 

 a number of times takes to complete one circuit. 



Plethora. When an animal is in the highest degree robust 

 and vigorous, there is a certain harmonic balance between the 

 proportion of blood contained in the arteries and that contained 

 in the veins. Naturally the venous system is much more capa- 

 cious than the arterial system. A common estimate is, that the 

 capacity of the system of the vence cavce is about three times 

 greater than that of the aortic system ; in correspondence with 

 which proportions it is believed that the average velocity of 



