This class of pathogenic bacteria includes those which actually invade the body and feed upon its tissues instead of living simply upon swallowed food. It is difficult, however, to draw any sharp line separating the two classes. The bacteria which cause diphtheria (Fig. 28), for instance, do not really invade the body. They grow in the throat, attached to its walls, and are confined to this external location or to the superficial tissues. This bacillus is, in short, only found in the mouth and throat, and is practically confined to the so-called false membranes. It never enters any of the tissues of the body, although attached to the mucous membrane. It grows vigorously in this membrane, and there secretes or in some way produces extremely violent poisons. These poisons are then absorbed by the body and give rise to the general symptoms of the disease. Much the same is true of the bacillus which causes tetanus or lockjaw (Fig. 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.