88 THE CIRCULATION. 



CHAPTER VI. 



CIRCULATION OF THE BLOOD. 



THE body is divided into two chief cavities the chest or 

 thorax and abdomen, by a curved muscular partition, called 

 the diaphragm (Fig. 31). The chest is almost entirely filled 

 by the lungs and heart ; the latter being fitted in, so to speak, 

 between the two lungs, nearer the front than the back of the 

 chest, and partly overlapped by them (Fig. 31). Each of 

 these organs is contained in a distinct bag, called respectively 

 the right and left pleura and the pericardium, the latter being 

 fibrous in the main, but lined on the inner aspect by a smooth 

 shining epithelial covering, on which can glide, with but little 

 friction, the equally smooth surface of the heart enveloped by 

 it. In Fig. 31 the containing bags of pleura and pericardium 

 are supposed to have been removed. Entering the chest from 

 above is a large and long air-tube, called the trachea, which 

 divides into two branches, one for each lung, and through 

 which air passes and repasses in respiration. Springing from 

 the upper part or base of the heart may be seen the large ves- 

 sels, arteries, and veins, which convey blood either to or from 

 this organ. 



In the living body the heart and lungs are in constant 

 rhythmic movement, the result of which is an unceasing stream 

 of air through the trachea alternately into and out of the 

 lungs, and an unceasing stream of blood into and out of the 

 heart. 



It is with this last event that we are concerned especially in 

 this chapter, with the means, that is to say, by which the 

 blood which at one moment is forced out of the heart, is in a 

 few moments more returned to it, again to depart, and again 

 pass through the body in course of what is technically called 

 the circulation. The purposes for which this unceasing cur- 

 rent is maintained, are indicated in the uses of the blood enu- 

 merated in the preceding chapter. 



The blood is conveyed away from the heart by the arteries, 

 and returned to it by the veins; the arteries and veins being 

 continuous with each* other, at one end by means of the heart, 

 and at the other by a fine network of vessels called the capil- 

 laries. The blood, therefore, in its passage from the heart 



