CIRCULATION OF THE BLOOD.

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many years he durst not let the public know his ideas and opinions on this subject; and when he did, he was much opposed, very few believing his new principles.

19. Nearly every discovery that has been made in physiology or philosophy has met with very bitter opposition when first introduced. Some will not believe any new doctrine, even when there are light and evidence enough to prove it to be correct, but adhere tenaciously to the old system of things. But Harvey lived long enough to see his principles admitted by the scientific; and though he was much persecuted for many years, yet he had the pleasure to know that he was correct in his belief. Among the many proofs which led him to make this discovery were the following.

20. First; If the chest of a cold-blooded animal, a toad or a frog, be opened, the heart can be seen to contract and dilate. Then it remains an instant at rest, and again dilates and contracts, and raises itself a little, the same as in our own bodies, and causes beating. There would be no cause for this expansion and contraction, if the blood did not flow in and pass out of the heart.

21. Secondly; Another reason is, that in all the arteries there are valves, which permit the blood to pass into them, but prevent its passage backward toward the heart; also, in all the veins, the valves allow the blood to go toward the heart, but not in the opposite direction;

18. Did he make his discovery known? 19. How has every discovery in science been received by the public? To what do some always adhere? Was the opposition and prejudice finally removed? 20. What was the first proof of the circulation of the blood? 21. What was the second proof of the circulation of the blood?

which facts alone would clearly prove to every candid mind, that they would not have been constructed in this manner, and so well adapted for the circulation of the blood, unless it were for that specific purpose, and for that alone.

22. Thirdly; It has been said that the circulation of the blood through the veins and arteries may be seen by a microscope-an instrument by which the smallest object may be seen in some of the delicate parts of different animals, as in the web of a frog's foot.

23. Fourthly; The way in which bleeding from a vein is performed, is another proof of the circulation of the blood. A tight bandage is placed around the arm above the place where the vein is to be opened. The blood in returning through it toward the heart, is interrupted in its passage, and as the artery underneath is not compressed, the vein is filled with blood, and is swelled. If the vein be then opened below the bandage, the blood will flow freely; but if the bandage be tight enough to compress the artery, the blood cannot pass through it from the heart, so there will soon be none in the vein; or, if the opening be made above the bandage no blood is obtained.

24. Fifthly; In the amputation, or cutting off, of a limb, the surgeon ties only the ends of the arteries together. As these carry the blood from the heart to all parts of the body, the patient would soon bleed so as to cause

21. What should the present construction of the veins and arteries prove clearly? 22. What is the third proof of the circulation of the blood? What is a microscope? 23. What is the fourth proof of the circulation of the blood? How is bleeding from a vein performed? What would be the result if the bandage be too tightly bound? 24. What is the fifth proof of the circulation of the blood? Why do not the veins require to be tied ?

death, unless some means were taken to prevent it. The veins which carry the blood back to the heart do not bleed, and therefore need not be tied.

25. From these five reasons or proefs of Dr. Harvey we cannot but believe that the blood passes from the heart, through the arteries, into the veins, and is returned by them into the heart; or, in other words, that the blood continually circulates, or is distributed through our bodies

25. To what conclusion should these five reasons of Dr. Harvey lead us?

CHAPTER V.

SECRETION.

1. THE word secretion means a separation of fluids When it is used in physiology, it has reference to a certain process by which various substances are separated from the blood, without being changed in their separation. We call this act of separation secretion; and we also call the substance that is separated secretion; as we say that by secretion the gastric juice is formed in the stomach, and we say also that the gastric juice is a secretion of the stomach. Both are correct.

2. Without secretion there would be no possible way for the different parts of the body to be nourished, and many injurious substances would not be discharged from the blood; therefore it is very important for our health that these secretions should not be interrupted. These substances, though derived and separated from the blood, are very different in appearance and composition from the blood, as we shall see.

3. There are three kinds of organs for the purpose of secretion, which I wish you to remember, when I have explained them to you. First-THE EXHALANT VESsels.

What is the subject of chapter fifth ? 1. What does secretion mean? How is the term used in physiology? What two things does this word comprehend? What is an example? 2. What are the advantages of secretion? How important is secretion to health? Are the secreted substances similar to the blood? 3. How many kinds of secretory organs are there? What is the first kind?

FAT SYNOVIA-MARROW.

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Second-THe follicles.

Third-THE GLANDS.

4. You may recollect that I have informed you that there are a great many little vessels through which the chyle passes over the intestines. These are called capillaries, and it is thought by many that the exhalant vessels, which are very small, are connected with them. These exhalations are internal, when they take place in the body. The head, chest, and stomach are all lined with a thin covering, which throws out a little fluid, sufficient to keep them moist, and enable them to move easily.

5. The fat, of which I have heretofore spoken, is caused by exhalation, or is one form of secretion. It is first an oily, greasy fluid, then becomes hard, and serves as a protection to the skin. Then the ligaments around the joints are lined with the same kind of membrane as the stomach; this secretes the synovia, which enables the joints to move easily.

6. The marrow that is in the long bones is another secreted exhalation. The use of the marrow is not known. Many supposed it were to make the bones less liable to be broken; but there is more of it in the bones of aged than in those of younger persons, and their bones are much more easily broken.

7. The external exhalations are those which take place out of the body. These are the sweat or perspi

3. What is the second kind? What is the third kind? 4. With what are the exhalant vessels connected? What is meant by internal exhalations? Where are some of the internal exhalations found? 5. By what is the fat caused? How is it formed? What is the synovia, and where is it secreted? 6. What secretion is contained in the long bones? What is the ase of the marrow? How do we know that it does not strengthen the bones? 7. What is meant by external exhalations?

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