CHAPTER XIII 

 THE CIRCULATION 



WE must postpone still further our account of the ab- 

 sorption of the products of digestion until we shall have 

 made clear the general course followed by the circulating 

 blood. We have quoted the estimate that the body con- 

 tains 8 or 10 pounds of blood. At a given moment about 

 one-fourth of this may be assumed to be in the thorax 

 (the heart, the lungs, and the great blood-vessels), a 

 fourth in the skeletal muscles, a fourth in the liver, and the 

 remaining fourth elsewhere. The ceaseless movement of 

 this large volume of liquid is maintained by the beating of 

 the heart. 



This organ consists of two halves, right and left, com- 

 pletely separated, so far as their cavities are concerned, by 

 a middle partition. Regarded as a mass of muscle, the 

 heart is single; considered as a pump, it is double. Each 

 half is, in a literal sense, a force-pump. Each side shows 

 us two communicating chambers, an auricle above and a 

 ventricle below. The auricles receive the blood, which the 

 corresponding ventricles will shortly discharge. The 

 vessels leading to the auricles are called veins; those which 

 convey the blood from the ventricles are called arteries. 

 The auricles have thin walls, while the ventricles are 

 fitted for their task by heavy muscular development. 

 The left ventricle has much more power than its fellow, 

 and the necessity for this will soon be evident. 



A single great artery, the aorta, springs from the left 

 ventricle. Its branches reach all parts of the body. Sub- 

 dividing repeatedly, they introduce the blood at last into 

 the capillaries, the innumerable vessels of the smallest 

 order through whose exquisitely thin walls take place the 



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