and apparently so mysteriously is expressed biologically in various ways besides in that of greater infective power: virulent bacteria may prove incapable of being charged with opsonin so that they can not be ingested by phagocytes; they may show unusual power to resist plasma or serum destruction; they may drive away or repel or act negatively in respect to chemical attraction on the phagocytes; and being thus unopposable they tend to multiply quickly and with little restraint and thus still further to break down and render ineffective the normal defensive mechanisms, and ultimately to damage seriously the sensitive cells of the organs. This constitutes disease.
Another power resides in the body that should be regarded, namely, the power to neutralize or destroy poisons as distinct from parasites; for the body is exposed to the deleterious action of poisons generated by living parasites that do not themselves penetrate within the body. Some of these poisons are generated away from the body, as is the case with certain food poisons; some by bacteria in the intestinal canal that do not seek to invade the blood; some by bacteria, like the diphtheria bacillus, that first kill tissue, usually of the mucous membranes, and then develop in the dead tissue and send the poison into the body. And besides this every bacterial disease resolves itself ultimately into a process of poisoning—of intoxication. In typhoid fever, in pneumonia, in meningitis and in the multitude of other bacterial invasive diseases of man and the lower animals, the severe symptoms are caused by the poisons liberated through disintegration of the invading bacteria which, however, continue by multiplication to recruit their numbers.
The condition of susceptibility to poisons varies with different races and species, very much as bacterial susceptibility does. The cold-blooded animals are indifferent to poisons that are very injurious to warm-blooded animals, but not all cold-blooded animals behave alike. Tetanus toxin is alike innocuous for the frog and the alligator; but by raising the temperature artificially the frog develops tetanus, but the alligator does not. Sometimes the effects depend merely upon the mode of entrance of the poison into the body. Tetanus toxin, diphtheria toxin and snake venom have no effect on mammals when swallowed unless the intestinal epithelium has been injured. These poisons can not pass through the epithelium to reach the blood, where alone they can exert their action. The toxin of the dysentery bacillus passes readily in the rabbit from the blood into the intestine, which it injures, but can not pass from the intestine into the blood. Tetanus toxin can be injected into the circulation of the hen but does no harm. Injected into the brain it produces tetanus. Introduced into the blood it remains there for many weeks, hence the failure to act can not be due to destruction, but probably is due to inability to pass through the blood vessels in order to reach the cells of the central nervous system in a sufficient state of concentration. The physiological state of the animal also exerts an influence: certain hibernating species are sus-
