interesting fact that both the number of colorless corpuscles in the body and the vigor with which each colorless corpuscle attacks foreign organisms vary from time to time, so that we are much more secure against infection at some times than at others. In general we may say that when the body is in a condition of vigorous health the colorless corpuscles will be efficient. When we are run down, on the other hand, these cells share with all other body cells this state of low vitality. This explains why people who allow themselves to become run down are more likely to fall victims to infection than those who are in vigorous health, and emphasizes, of course, the importance of a habit of life that shall keep the body vigorously healthy at all times. It should be understood that the colorless corpuscles do not show the same effectiveness against all kinds of foreign organisms; some kinds of disease germs are able to bring about infection in the body quite regardless of the activity of the colorless corpuscles. From other kinds, on the other hand, they give the body very complete protection. The most familiar example of the action of the colorless corpuscles is in the formation of what we all know as pus. There are a few kinds of organisms that, instead of getting into the body and becoming scattered through its fluids, establish themselves at certain points and by growth and multiplication accumulate at those places in large numbers. Examples are pimples and boils. In these cases the pus-forming organisms have located just under the skin and are multiplying there at a great rate. They produce poison which is absorbed from the place where they are and distributed through the body. This poison appears to have some sort of chemical attraction for the colorless corpuscles; at any rate the corpuscles gather from all around to the place where these organisms are located and engulf as many of them as they are able, but in so doing they themselves are destroyed, and pus, as we know it, is simply a mass made up of the dead bodies of the colorless corpuscles along with the organisms which they have destroyed and which in turn have destroyed them. In the ordinary pimple or boil the colorless corpuscles ultimately win the combat and the infection is completely overcome. It happens occasionally, however, that a pus-forming infection becomes established in some place where it manages to maintain itself in spite of the attacks of the colorless corpuscles. This happens very frequently in the tonsils, so that persons who have infected tonsils may have pus formation going on month in and month out. This may also take place at the roots of the teeth. In fact it is now commonly believed that whenever the nerve to the tooth dies, infection is certain sooner or later to become established at the tip of the root. These places where pus formation is going on continually are known as “pus pockets.” For a long time very little attention was paid to them.