THE CIRCULATION. 99 



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 diapliraym (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. 3 1 

 the containing bags of pleura and pericardium are sup- 

 posed to have been removed. Entering the chest from 

 above is a krge 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 vessels, 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 



