distribution of the organisms, or there may be infection of the blood and fluids of the body.  The injuries which are produced depend upon the nature of the infecting organisms.  The most common lesion consists in the death of the tissue about the infecting organisms.  In most cases the sum of the changes are so characteristic that from them the nature of the infection is easily determined, and these changes often give names to the disease; thus tuberculosis is a disease characterized by the formation of tubercles or little nodules in the body.  The situation of the foci of disease is determined by many conditions, the most important being the varying resistance of the different organs of the body to the growth of bacteria.  Certain organs, such as the central nervous system, the muscles, the testicles and the ovaries, have a high resistance to the growth of bacteria.  The disease may be localized in certain organs because only in these do the bacteria find favorable conditions for growth.  In spite of a high general resistance to infection the lesions in chronic glanders are most marked in the muscles, those of poliomyelitis in the spinal cord.  There are few bacterial diseases which are localized in the blood, but many of the diseases caused by protozoa have this localization.  In every infection some organisms enter the blood, which acts as a carrier and deposits them in the organs.

Bacteria cause disease by producing substances called toxines which are poisonous to the cells, and of which two sorts are distinguished.  One form of toxines is produced by the bacteria as a sort of secretion, and is formed both in the body and when the bacteria are growing in cultures.  Substances of this character, many of them highly poisonous, are produced both by animals and plants.  They may serve the purpose both of offence and defence, as in the case of the snake venom, and in other cases they seem to benefit their producers in no way whatever, and may even be injurious to them.  After the different cereals have been grown for succeeding years in the same place, growth finally diminishes not from the exhaustion of the soil, but from the accumulation in it of substances produced by the plants.  Beneath certain trees, as the Norway maple, grass will not grow, and it has been shown that the tree produces substances which inhibit the growth of grass.  When bacteria are grown in a culture flask, growth ceases long before the nutritive material has been consumed, from the accumulation of waste products in the fluid.  The other class of toxic substances, called endotoxines, are not secretion products, but are contained in the bacterial substance and become active by the destruction and disintegration of the bacteria.  They can be artificially produced by grinding up masses of bacteria, and in the body the destruction and solution of bacteria which is constantly taking place sets them free.  The toxines and the endotoxines are of an albuminous nature, and act only when they come in contact with the living cells within the body.  When taken into the alimentary canal they are either not absorbed or so changed by the digestive fluids as to be innocuous.  Many of the ordinary food substances, even a material apparently so simple as the white of an egg, are highly injurious if they reach the tissues in an unchanged form.