As the heart has to pump all the blood in the body twice,--once around and through the lungs, and once around and through the whole of the body,--it has become divided into two halves, a right half, which pumps the blood through the lungs and is slightly the smaller and the thinner walled of the two; and a left half, which pumps the purified blood, after it has come back from the lungs, all over the rest of the body. [Illustration: THE EXTERIOR OF THE HEART Showing the strands of muscle that compose it, the arteries and veins that feed and drain the muscle coat, and fat protecting these.] Each half, or side, of the heart has again divided itself into a receiving cavity, or pouch, known as the _auricle_; and a pumping or delivering pouch, known as the _ventricle_. And another set of valves has grown up between the auricle and the ventricle on each side of the heart. These valves have become very strong and tough, and are tied back in a curious and ingenious manner by tough little guy ropes of tendon, or fibrous tissues, such as you can see quite plainly in the heart of an ox. It is important for you to remember this much about them, because, as we shall see in the next chapter, these valves are one of the parts of the heart most likely to wear out, or become diseased. [Illustration: DIAGRAM OF VALVES IN THE VEINS AND HEART In _A_ the blood flows forward naturally. In _B_ and _C_ is shown what would happen were the blood to reverse its course, as it does when it meets an obstruction: the pockets would fill until they met and closed the passageway.] Heart Beat and Pulse. The heart fills and empties itself about eighty times a minute, varying from one hundred and twenty times for a baby, and ninety for a child of seven, to eighty for a woman, and seventy-two for a full-grown man. When the walls of the ventricles squeeze down to drive out their blood into the lungs and around the body, like all other muscles they harden as they contract and thump the pointed lower end, or _apex_, of the heart against the wall of the chest, thus making what is known as the _beat_ of the heart, which you can readily feel by laying your hand upon the left side of your chest, especially after you have been running or going quickly upstairs. As each time the heart beats, it throws out half a teacupful of blood into the aorta, this jet sends a wave of swelling down the arteries all over the body, which can be felt clearly as far away as the small arteries of the wrist and the ankle. This wave of swelling, which, of course, occurs as often as the heart beats, is called the _pulse_; and we "take" it, or count and feel its force and fullness, to estimate how fast the heart is beating and how well it is doing its work. We generally use an artery in the wrist (_radial_) for this purpose because it is one of the largest arteries in the body which run close to the surface and can be easily reached.