29).  This bacillus is commonly inoculated into the flesh of the victim by a wound made with some object which has been lying upon the earth where the bacillus lives.  The bacillus grows readily after being inoculated, but it is localized at the point of the wound, without invading the tissue to any extent.  It produces, however, during its growth several poisons which have been separated and studied.  Among them are some of the most violent poisons of which we have any knowledge.  While the bacillus grows in the tissues around the wound it secretes these poisons, which are then absorbed by the body generally.  Their poisoning effects produce the violent symptoms of the disease.  Of much the same nature is Asiatic cholera.  This is caused by a bacillus which is able to grow rapidly in the intestines, feeding perhaps in part on the food in the intestines and perhaps in part upon the body secretions.  To a slight extent also it appears to be able to invade the tissues of the body, for the bacilli are found in the walls of the intestines.  But it is not a proper parasite, and the fatal disease it produces is the result of the absorption of the poisons secreted in the intestines.

It is but a step from this to the true parasites.  Typhoid fever, for example, is a disease produced by bacteria which grow in the intestines, but which also invade the tissues more extensively than the cholera germs (Fig. 30).  They do not invade the body generally, however, but become somewhat localized in special glands like the liver, the spleen, etc.  Even here they do not appear to find a very favourable condition, for they do not grow extensively in these places.  They are likely to be found in the spleen in small groups or centres, but not generally distributed through it.  Wherever they grow they produce poison, which has been called typhotoxine, and it is this poison chiefly which gives rise to the fever.

Quite a considerable number of the pathogenic germs are, like the typhoid bacillus, more or less confined to special places.  Instead of distributing themselves through the body after they find entrance, they are restricted to special organs.  The most common example of a parasite of this sort is the tuberculosis bacillus, the cause of consumption, scrofula, white swelling, lupus, etc.  (Fig. 31).  Although this bacillus is very common and is able to attack almost any organ in the body, it is usually very restricted in growth.  It may become localized in a small gland, a single joint, a small spot in the lungs, or in the glands of the mesentery, the other parts of the body remaining free from infection.  Not infrequently the whole trouble is thus confined to such a small locality that nothing serious results.  But in other instances the bacilli may after a time slowly or rapidly distribute themselves from these centres, attacking more and more of the body until perhaps fatal results follow in the end.  This disease is therefore commonly of very slow progress.