towards the hand. But the veins are under pressure still; notice what is happening. Blood is entering through the arteries; it cannot escape through the veins because of the pressure of the bandage. As a result the hand grows red and swollen from its unusual supply. Release the bandage entirely, and in almost no time those veins have relieved themselves. Blood is once more streaming upwards.

Such experiments as these ... and others led Harvey to his second announcement. He declared to his astonished friends that:

The heart receives its entire blood supply from the veins.

To complete this account, turn to the heart again and remember the following facts about it:

B

D

THE RIGHT AURICLE AND
VENTRICLE

E

vein that brings blood to the auricle;

B, auricle; C, valves that are forced

open by the blood as it passes into

the ventricle; D, ventricle; E, tube through which blood goes to the lungs to be purified

1. The heart is a powerful muscle. It does its work by contracting and relaxing.

2. The heart is made up of two halves; and the wall of muscle between these separate halves is so firmly closed that after birth, and after the heart is in good working order, not a drop of

blood ever passes through it from one side to the other.

3. Each half of the heart has two divisions, the smaller called the auricle, the larger called the ventricle.

Two VIEWS OF THE SAME VENTRICLE TO SHOW THE VALVES

On the left blood enters; on the right the ventricle contracts
and forces the blood onward

4. Each auricle and each ventricle has its own opening, its own tube for blood, and its own valves to prevent the blood from running the wrong way.

5. The auricle in each half of the heart always receives the blood and sends it into its own ventricle. 6. Each ventricle receives blood from its own auricle and sends it off to its own district of the body.

At this point we reach a most interesting fact about this process of circulation; yet it may be given in a few

easy words. One side of the heart receives blood from the body and sends it to the lungs; the other side of the heart receives blood from

the lungs and sends it to the body. We see, then, that one side always deals with pure blood alone, for all that comes to it is fresh from the lungs and is sent onward in the same condition; while the other side deals with impure blood alone,

for all that comes to it is from the body after it has been used, and it goes onward to the lungs in that condition to be purified.

A, auricle; B, ventricle; C, outline of the heart; D, D, blood vessels

Thus the entire blood supply of the body, on each journey round, passes through both sides of the heart and through the lungs before it goes back to nourish the body. blood from the lungs and sends it This was Harvey's great dis

The dark side receives impure blood from the body and sends it to the lungs; the light side receives pure

to the body

covery about the circulation of the blood. Even for him, however, there was a mystery which the microscope alone could solve. The next chapter will speak about it,

CHAPTER XI

TO THE CAPILLARIES AND BACK

To complete the proof about blood which makes its regular journey from the heart round the body and back again, scientists have the testimony of the blood itself. They have taken a syringe as slender as a needle, and by its use have pricked some harmless chemical into a vein on one side of the body of a horse or of a man. They have then examined blood drop by drop from the corresponding vein on the opposite side of the body until the same chemical has appeared there.

By comparing the time when the substance was put in, with the time when they find it again, they know how long it takes for blood to make the entire circuit of the body. The following table gives results:

For a horse, twenty-five seconds.

For a full-grown man, twenty-three seconds.

For a child of fourteen, eighteen seconds.
For a child of three, fifteen seconds.

Evidently each set of tubes and each heart does its work more or less rapidly, according to the distance which the blood has to travel. But for each one of us the road which the blood takes is ever the same. The

Black tubes represent veins through which impure blood goes to the right side of the heart from all parts of the body; light-colored tubes represent arteries through which pure blood from the left side of the heart goes to all parts of the body. Notice that the large tubes of each kind lie near one another