From the surface of the skin, just as from the lungs, besides the substances that are got rid of from the blood, heat is lost. I told you that a considerable quantity of heat is lost from the lungs, so, too, a considerable quantity is lost from the surface of the skin. Now, this action I have just described of the perspiration glands in the skin is an action of extreme importance, and so it is of the greatest importance that we should keep the surface of our skin continually freed from the secretion of these glands which is being continually poured out. The action of these glands is continually going on although we do not notice it Water is continually evaporating from the surface of the skin.

The continual action of these glands goes by the name of insensible perspiration. When we visibly perspire, it is because, from some reason or other, these glands act more than usual, and secrete so much fluid on the surface of the skin that it cannot be got rid of by evaporation as quickly as it is formed. It is of extreme importance, however, that it should not accumulate, and so choke up the pores of the skin, as is commonly and quite rightly said. If this is allowed, then much of the action of the skin in separating waste substances from the blood is thrown upon the other excretory organs - viz., the lungs and the kidneys - and these organs get too much to do, and so become diseased. This is one cause why towards the latter end of life those organs frequently become diseased. You can actually as certainly kill an animal by varnishing his skin over as by cutting his throat.

The Kidneys are two organs situated in the abdomen, one on each side of the vertebral column; their shape is well known, and needs no description. These organs are true glands; each has a duct which indirectly communicates with the external air.

If we cut a kidney across, we find that the duct (which goes by the name of the ureter) is funnel-shaped where it leaves the kidney, and pointing into that funnel-shaped head of the duct, there are some conical masses of kidney substance; they go by the name of the pyramids of the kidney. When these are examined very carefully, it is found that there are an immense number of fine tubes running through them, called the tubules of the kidney, and they open out at the end of these pyramids, and run straight through the interior substance of the kidney, and then into the outer part of the kidney, where they twist about a great deal These tubules, when examined under a microscope, are found to end in little bags or capsules. The interior of these bags or capsules is in indirect communication with the external air, because from these little capsules the tubules of the kidneys start, and ultimately open, on the surface of the pyramids, into the funnel-shaped beginning of the ureter - the tube which leaves the kidneys and which is indirectly connected with the external air. Now, an artery comes to each kidney - we will call it the kidney artery; - it subdivides into small branches; these small branches go through the substance of the kidney until they come to the outer part of it, and then they end in very small branches, which go straight into the capsules just mentioned; each small branch of the artery goes into one of the capsules, and immediately breaks up into a bunch of capillaries, forming a little ball in the interior of that capsule, and that little ball, as it were, pushes the lining membrane of the capsule before it, so that between that little ball of capillaries and the interior of the tubule connected with the capsule there is nothing but that very fine lining membrane. Then the capillaries in that little ball run together, forming a small vein that goes out from the capsule. These small veins do not join together to form the vein that leaves the kidney; they do like the portal vein - they break up into capillary vessels surrounding the tubule; these capillary vessels then join together and form a little vein, and then numbers of these little veins join together and form the kidney vein which leaves the kidney.

Now, I want you to see that the kidney is built upon the same theory as the other two excretory organs - the skin and the lungs.

In the kidney, blood is brought by the kidney artery into the little capillary vessels inside the capsules, and so the blood in these capillary vessels is separated from the interior of tubes connected indirectly with the external air merely by a fine, moist membrane. That is the plan of construction in all these excretory organs; they are all contrivances by means of which blood in the capillary vessels is brought into the nearest possible connection with the external air, brought into connection with tubes connected with the external air, only being separated from the interior of these tubes by a fine moist membrane; and so there are points in all these excretory organs which are similar - the waste substances go through the walls of capillary vessels, and through fine membranes ultimately into the external air. By means of the kidneys, the following things are got rid of from the blood: in the first place, water, to the extent of about fifty ounces, or two and a half imperial pints, in the twenty-four hours; that water contains certain things in solution: it contains a substance which goes by the name of urea, a waste organic substance, of which I shall have more to say when speaking about foods; the kidneys get rid of this substance to the extent of 500 grains in the twenty-four hours, and of another organic body called uric acid, in much smaller quantity: besides this, there are mineral salts, such as common salt, and carbonate and phosphate of lime, and a good many other salts of, perhaps, less importance; so that you see we get rid of the same substances from the kidneys as from the lungs and skin - water, carbonic acid in the form of carbonate, and organic matter in very large quantities, and also large quantities of mineral salts.