VI FORM AND FUNCTION 69 
but there are 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 ga/55 to ze55 Of an 
inch. Itis 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 
