The bacillus of lockjaw is found in great abundance around stables, and this explains the fact that hostlers, drivers, cavalrymen, all of whom had to do with horses, are especially liable to attacks of lockjaw. Moreover, certain bacteria thrive best when exposed to the open air. Other bacteria, and among them the bacilli of lockjaw, thrive best when the air is excluded, and this explains the danger of treading on a rusty nail, which is popularly and rightly known as peculiarly liable to produce lockjaw. The reason is not because it is a nail, nor because it is old, nor because it is rusty, but because from the earth in which it lies it is most apt to be the means of introducing into a punctured wound the bacilli of lockjaw. Such a wound bleeds but very little, the blood soon crusts and excludes the air, and if any of the bacilli of lockjaw have been carried into the body, they find in such a closed wound, from which the air is excluded, the most favorable conditions for growth and infection of the whole body. Knowing these facts from experiment, the treatment is clear. Lay open such a wound and disinfect it. These two forms, the “cocci,” or berry-like bacteria, and the “bacilli,” or rod-like bacteria, comprise the great majority of dangerous bacteria. It must be remembered that there is an enormous number of bacteria which are not dangerous; some of them are entirely harmless even if introduced into the human body. Others are the bacteria of decomposition, or putrefaction, which are known as “saprophytic” bacteria. All of the harmless ones are known as “non-pathogenic,” that is, non-producers of disease. Those which produce disease are known as “pathogenic,” and those which produce suppuration as “pyogenic” or pus-producing bacteria. All of these bacteria are plants, and not, as is very frequently supposed, animals of a low form. The danger from their introduction into the body can be best appreciated, perhaps, by the statement of Belfield, who estimated that a single bacterium which weighs, approximately, only the 1-40,000,000 part of a grain, if given plenty of food and plenty of “elbow room,” would so rapidly develop that in three days it would form a mass weighing 800 tons! It is the old story of the blacksmith who was to get a penny for the first nail, two for the second, four for the third, and so on till a set of shoes would cost more than Crœsus could pay for.