i2 THE POPULAR SCIENCE MONTHLY 



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 pneu- 

 monia, 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 tc- 

 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 th< 

 animal also exerts an influence: certain hibernating species are sus- 



