vi FORM AND FUNCTION 69 



but there arc openings from the upper into the lower 

 which may be closed by valves. The two lower 

 chambers are called ventricles and the two upper ones 

 auricles. Before explaining the working of the 

 valves, I shall trace the circulation of the blood. The 

 left ventricle, which is the strongest and most muscular 

 part of the heart, opens into the aorta, the largest of 

 all the arteries. Thence it is distributed into branch 

 arteries and from these into smaller branches : these, 

 in turn, lead into smaller channels called capillaries, 

 varying in diameter, in man, from ^Vo to T gVo °f an 

 inch. It is when it reaches these extremely minute 

 vessels that the blood does its work of nourishing all 

 the tissues of the body. The capillaries unite to form 

 larger vessels called veins, and these finally form two 

 great trunk veins which carry the blood into the right 

 auricle. From the right auricle it passes to the right 

 ventricle. Thence it is driven into the lungs, from the 

 lungs it passes into the left auricle, and thence into 

 the left ventricle where the same process begins 

 again. Thus the blood in , the right chambers of 

 the heart can reach the left only through the 

 lungs : that in the left can find its way to the 

 right only through the arteries and veins of the 

 body. The pure arterial blood is all on the left side, 

 the impure venous blood on the right. The former 

 may be known by its bright red colour, the latter is 

 blue-black. The following diagram will make clear 

 the course of circulation. 



When the blood has passed through the arteries 

 into the capillaries and from them into the veins, it 

 finds a new contrivance to assist in driving it on. In 



