to enter the deeper tissues, and cause dangerous abscesses and appendicitis. The carriers of disease germs thus become a very important subject of study. There are carriers which make no selection, but are, so to speak, "casual" in their proceedings, and there are others which have the most special and elaborate relations to some one kind of disease-causing microbe for which alone they are responsible, and to the life of which they are necessary. Let us look first at the more casual group. Man himself is a great carrier and distributor of his own diseases. Unless and until he has learned to be careful and guard against thoughtless proceedings, he is always spreading the microbes of his diseases and passing them on to his fellow men. He pollutes the waters, rivers, lakes, and pools from which others drink. He manures his crops, and then eats some of them uncooked. His hands are polluted by disease-causing microbes, and he handles (to an alarming and unnecessary extent) the food, such as bread and fruit, which is swallowed by his fellows, without cleansing it by heat. It has lately been shown that apparently healthy men and women often harbour within them the microbes of typhoid fever or of cholera (and probably other diseases), without themselves suffering in health, and that unsuspected they thus become distributing centres of these diseases. The names "typhoid carrier" and "cholera carrier" have actually been introduced to describe the condition of such persons. Then, again, by his breath, and by coughing and spitting, a man acts as a carrier to others of disease-microbes already lodged in him, as well as by actual contact in the case of those infections which are called "contagious." The numerous animals which surround and are associated with man act very largely as casual carriers and distributors of disease microbes. Thus dogs and even the cleanly cat are frequently carriers of disease. But more especially those creatures which visit man's food stores and food ready for consumption (such as bread, fruits, cold meat, etc.) are active carriers. Rats and mice run over such stores and pollute them. But the most widely active in this way is the common house-fly.