The smallest arteries pass into the capillaries, which have very thin walls, and form very close networks in nearly all parts of the body; their immense number compensating for their smaller size. The average diameter of a capillary vessel is so small that only two or three blood-corpuscles can pass through it abreast, and in many parts the capillaries lie so close together that a pin's point could not be inserted between two of them, as, for instance, in the deep layers of the skin which can hardly be pricked anywhere with a needle without drawing blood. It is while flowing in these delicate tubes that the blood does its nutritive work, the arteries being merely supply-tubes for the capillaries, through whose delicate walls liquid containing nourishment exudes from the blood to bathe the various tissues. Imagine a piece of the finest lace, with all its threads consisting of hollow tubes, and diminished twenty times in size, and you will have some idea of the capillaries.

Into what vessels does the abdominal aorta ultimately divide? What is the femoral artery derived from? To what point does it run? What is the popliteal artery? Into what branches does the popliteal artery divide? What parts do they supply with blood?

What main facts are to be borne in mind in connection with the arteries? How is the quantity of blood which an artery will let pass regulated?

What is found when the arteries are followed to the ends of their smallest branches? Describe the structure, arrangement and size of the capillaries.