The quantities I have mentioned are the average amounts excreted by the kidneys of an adult in twenty-four hours.

One more thing we have to consider, and that is how this contrivance acts. The blood is brought by the kidney artery into the ball of capillaries, and the surplus water of the blood, with certain substances in solution, exudes from these capillaries and passes through the fine membrane (just as it would through a piece of filtering paper) into the tubule. Other matters are excreted through the capillaries which surround the tubule, and are then washed away by the watery excretion coming from the capsule.

A few words about the general changes that go on in the blood. The blood, you see, is being continually renewed from the nutritious parts of the food, partly by absorption from the walls of the small intestines directly into the capillary blood-vessels of the villi, and partly indirectly by absorption from the walls of the small intestines into the lacteal vessels, which, by means of the thoracic duct, convey their fluid into the blood.

It is renewed with nutritious substances from the food; the white corpuscles, from which we know the red ones are ultimately formed, are continually being formed in the lymphatic glands, and in the spleen, and a few other so-called ductless glands, and perhaps in the liver; these white corpuscles are being continually made, and being added to the venous blood.

To this blood is also added the blood that Has been purified in the way I have described in the kidneys, and all this mixture goes to the right side of the heart, is then pumped through the lungs, undergoes the alteration I have described there, especially loses carbonic acid and gains oxygen, then goes to the left side of the heart, and is pumped all over the body into the capillary vessels in the tissues; certain portions of the blood exude from the capillary vessels into the tissues; each tissue takes out what it requires for itself, and leaves the remainder, and, besides that, adds to the remainder the decayed portions of itself.

Whatever blood is not taken up by each tissue goes on into the veins, and to that is also added a certain quantity of the waste substance that comes through the capillary walls from the tissues into the blood; the remainder, viz., the fluid that was not required by the tissues, and some of the waste parts of the tissues, are taken up by the lymphatic vessels, and are conveyed by them into the thoracic duct, and so into the blood again.

While all this is going on, the oxygen gas that has been absorbed into the blood through the lungs is combining, especially in the capillary vessels in the different tissues, with the substances in the blood which are capable of combining with it; and it is combining notably with certain waste substances that the tissues have added to the blood; and I have told you that this combination is attended with the production of heat (all chemical combinations are attended with the production of heat), and this is the way in which the warmth of our bodies is kept up. It is by the conversion of this animal heat, as we call it, into various forces, that we are able to move about, to think, and to do all the various acts that we perform. This combination of the oxygen of the air with substances in the blood produces the waste substances that we get rid of by the organs I have been speaking about, - by means, then, of the combination of oxygen with certain substances in the blood, heat is produced, and the waste parts of the tissues of the body are converted into substances which can be separated from the blood and from the body by means of the organs just described.

Now, a word on the importance of the regular and proper action of these excretory organs, and of the intestinal canal. The former separate substances from the blood that are hurtful if they are kept in the blood. The waste substances that are got rid of by the intestinal canal include the parts of the food that are not digested, and certain secretions from the intestinal canal, especially from the large part of the intestine. These sub-stances are injurious if left in the body, as certain por-tions of them are reabsorbed into the blood, especially the foul organic matter in them, so that if these various excretory organs do not perform their functions in a proper manner, waste substances are either not separated from the blood or are reabsorbed into it, and poison it, and as the blood is distributed to the various tissues of the body they are not properly nourished, and they become degenerated, weak, and incapable of performing their proper functions; so that the regular action of these excretory organs of the body is of the greatest importance with regard to health, for not a single tissue of the body can be kept in a proper condition if the waste substances are not got rid of in the manner they should be.