CIRCULATION OF THE BLOOD. 107 



between their great trunk and the heart; and there is 

 nothing but this .to prevent the backward flow of the blood. 

 When the left ventricle beats, and forceps its blood into the 

 arteries, they expand suddenly to admit the increased quan- 

 tity. All the arteries in the body expand and beat at very 

 nearly the same moment that the heart beats. We can feel 

 this beating of the arteries at the wrist, in the temples, the 

 sides of the neck, and wherever else they come near the. 

 surface. Some can perceive it and count their pulsations in 

 the brain. 



231. While the-heart is pressing the blood into the artery, 

 the valve is opened, and the yessel expands. But as soon as 

 this pressure ceases, the valve closes, and the artery begins 

 to contract and force the contents out and onward through 

 its minute extremities. The blood is then thrown into the 

 arteries by a muscular power of contraction, and it is moved 

 through these vessels merely by the elastic power of their 

 coats. 



232. The left ventricle of the heart will, in a man of av- 

 erage size, contain about two ounces, or one eighth of a 

 pint. Every time the heart beats, this cavity is filled and 

 emptied ; therefore, two ounces of blood are forced into the 

 arteries at every pulsation. In ordinary health, the heart of 

 a man beats about seventy-five times in a minute, or a little 

 more than once in a second. The quantity of blood in a 

 man of average size is estimated to be about twenty-eight 

 pounds, or four hundred and forty-eight ounces ; and if two 

 ounces pass through the heart at every beat, and one hundred 

 and fifty ounces every minute, then the whole blood of the 

 human body must pass through that organ, and through the 

 whole system, once in three minutes. Seventy gallons of 

 blood flow through a man's heart in the course of an hour, 

 and sixteen hundred and eighty-eight gallons in the course 

 of a day. 



233. This is the usual rate of the circulation ; but it varies 

 with many circumstances. It is more rapid in most diseases 



