Still another interesting thing about this whole matter is that every kind of infecting organism has its own kind of poison, which differs from that of the other kinds, and so the chemical effect of the poison upon the cells is not the same for one infection as for another. The functional metabolism of the cells in turn is adjusted to the kind of poison, so that the material they pour out is suitable to neutralize the particular poison which aroused them to activity in the first place, and in most cases no other. If one gets well from any infection, there is a surplus of the neutralizing material left in his body fluids, and, as long as it remains, he is secure from another infection of the same kind. This condition is defined as immunity. Since the neutralizing materials are different for different infections, immunity against one is in most cases of no avail against another. One may be immune against scarlet fever, but be just as likely to catch pneumonia as a person who has never suffered from any infection at all. It follows that an individual who has had and recovered from a great many infections has a correspondingly large assortment of neutralizing materials in his body fluids. Some of these appear to persist throughout life, others disappear fairly soon. The next group of permanent constituents to be described consists of some materials which seem to have nothing at all to do as long as everything is going well: the body fluids bathing the cells, or, in the case of the blood, circulating about from part to part through the blood vessels. These come into play only when, as the result of injury, the fluids begin to escape; namely, in the case of bleeding, or, as it is technically called, hemorrhage. It is clear that if any injury is started sufficient to allow the fluid to escape, there would be no more reason why it should stop running out, unless prevented, than there is why water should stop running out of an open faucet of itself. Since, as we know, we do not bleed to death every time we get a slight cut, but after a long or shorter time the bleeding stops of itself, there must be some automatic arrangement by which the opening is plugged. All of us are familiar with the way in which this is done. We know that in the course of a few minutes after the bleeding begins, the blood tends to set into a firm jelly, which is called the _clot_. This clotting is the result of a chemical transformation which goes on in the blood as the result of its escape from the blood vessels and its exposure to the outside. The details of the chemical processes are too complicated to be described here; all we need to remember is that the blood within the body contains certain special materials which are soluble and therefore float in solution along with the other soluble materials. When any blood vessel is injured, so that the blood begins to escape, a series of chemical changes is started automatically by which this soluble material