The left side of the heart pumps this blood out into the great main delivery-pipe for pure blood, known as the _aorta_, and this begins to give off branches to the different parts of the body, within a few inches of where it leaves the heart.
[Illustration: SURFACE VEINS AND DEEP-LYING ARTERIES OF INNER SIDE OF RIGHT ARM AND HAND The deep-lying veins that run parallel to the arteries have been omitted; so have the veins of three of the fingers.] One of the first of these branches to be given off by the aorta is a large blood pipe, or artery, to supply the shoulder and arm; this artery runs across the chest, thence across the armpit, and down the arm to the elbow.

Here it divides into two branches, one to supply the right, and the other the left, side of the forearm and hand.

These branches have by this time got down to about the size of a wheat straw; the one supplying the right side is the artery which we feel throbbing in the wrist, and which we use in counting the pulse.

From it run off smaller branches to supply the thumb and fingers.

These branches break up again into still smaller branches, and they into a multitude of tiny capillaries, which run in every direction among all the muscle cells, delivering the food and oxygen at their very doors, as it were.