pin in anywhere for any distance without striking against one or more of them. The capillaries are spread so thickly that there are not many places in the body where living cells are more than a very small fraction of an inch from one of them. The cells do not, however, lie right against the capillaries, but are separated from them by tiny spaces filled with tissue fluid. In order for material to get from the blood to any living cell, then, it must pass through the wall of the capillary into the fluid which fills the tissue space and from that in turn to the cell itself. The wall of the capillaries is so delicate that if the blood flowing through any capillary contains more of any substance than is present in the tissue fluid surrounding that capillary, some of it will pass out through the wall and into the tissue fluid, just about as freely as though there were no wall there at all. The arrangement can be illustrated by a familiar example; if a drop of ink is allowed to fall into a glass of water, it will color only a small part of the water at first, but quickly spreads out until each part of the water is as deeply colored as any other part. If the glass of water were to be divided in half by a very delicate membrane, and the ink dropped in on one side, it would spread out in the same way, passing through the membrane in so doing, until again all the water in the glass was equally colored. Of course, the quickness with which the ink could pass through the membrane would depend on how delicate the membrane was. We could imagine membranes which would not let any ink at all through, and every degree from that up to membranes so delicate as to offer no obstruction at all to the passage of the ink. The walls of the capillaries rank as membranes of such delicacy as apparently to offer almost no obstruction to the passage of materials through them. They hold back the red corpuscles and the platelets fairly well, so that they do not pass out of the blood and into the tissue spaces, unless the capillaries are actually injured. The colorless corpuscles are able to make their way through the capillary walls and so also do nearly all the substances that are dissolved in the blood. It is an interesting fact that the blood proteins do not pass freely through the capillary walls, although the digestion products of food proteins, do. It will be remembered that in the chapter on Body Fluids the importance of the sticky quality of the blood proteins was spoken of. It is now believed that it is because of this gelatinous nature that the blood proteins are not able to pass out through the capillary walls, and this is supposed to be important in the proper working of the circulation. In fact there is a condition of greatly lowered vitality to which the name “shock” is applied, in which the blood proteins escape through the capillary walls to so great an extent as to interfere with the proper working of the body. It has been found possible to prevent this in a