Page:Popular Science Monthly Volume 40.djvu/839

Rh walls of these minute vessels, bathing and feeding the whole surrounding tissue. Thus, as somebody has said, the whole of the new and living body is in solution in this wonderful food-stream of the blood, which, by a very subtle mechanism of nerves, distributes its good gifts in proportion to the needs of each separate part. But the blood is not simply a food-stream, it is also a sewage-stream, and it is as such that we are specially interested in it. Where no growth or storing of flesh material of any kind is taking place in the system, it is evident that that part of the daily food which is turned into tissue measures not only the daily construction that is taking place within us, but also the daily destruction or waste. In fact we—if we may so speak of the particles of which we are composed—are forever living and dying within ourselves—making a new self, and getting rid of an old self; and just as the new living body is in solution in the blood, so also is the old dead body, that has done its work and has to be got rid of. Now, of this dead body a large part has to escape through our lungs and through our skin.

About this process of waste very little is known. We know, while certain temporary forms of waste are found in muscle, such as kreatin (Gr. Kreas, flesh), which, whether again made use of or not (M. Foster, page 154), is supposed to be eventually changed in some complex manner into urea in the liver (M. Foster, page 755), and an acid called sarcolactic (Gr. sarx, flesh; gala, milk), which is also supposed to be decomposed in the liver into carbonic acid and water (M. Foster, page 836), that all our dead tissue is, with a certain slight but most important exception, got rid of safely at last, as urea, carbonic acid, and water. These are the final forms which the waste that passes from the tissue into the blood takes—the urea being separated from the blood and got rid of by the kidneys, the carbonic acid both by the skin and the lungs, and the water by all three channels of separation.

But we said that urea, carbonic acid, and water did not account for quite all the waste tissue; and among the part not so accounted for are the very hurtful poisons which escape from lungs and skin. What are these poisons? Have they a connection with or a resemblance to the poisons which, as we know, exist at all times within the system on a large scale. Dead or waste tissue probably passes through many forms before it reaches the safe final forms of carbonic acid and water, and we must conclude