CHAPTER IV. 



THE CIRCULATION OF THE BLOOD. 



THE Heart is a hollow muscular organ consisting of four chambers, 

 two auricles and two ventricles, arranged in pairs. On the right and 

 left sides of the heart is an auricle joined to and communicating with a 

 ventricle, but the chambers on the right side do not directly communi- 

 cate with those on the left side. The circulation of the blood is chiefly 



FIG. 87. Diagram of the circulation. The unshaded part of the figure to the right indicates 

 the district of the circulation of arterial blood; the dark part to the left the district of venous blood. 



carried on by the contraction or systole of the muscular walls of the 

 chambers of the heart : the auricles contracting simultaneously, and 

 their contraction being followed by the simultaneous contraction of the 

 ventricles. The blood is conveyed away from the left side of the heart 

 (as in the diagram, Fig. 87) by the arteries, and returned to the right 



