812 THE POPULAR SCIENCE MONTHLY. 



its peculiar site for ingress, its locality for operation, and its pe- 

 culiar way of accomplishing the destructive work. Some produce 

 harmful if not fatal changes in the blood by appropriating some 

 of its vital qualities, leaving the system robbed and impoverished. 

 Others seem to obstruct the minute vessels by their immense 

 numbers, and thus do harm in a mechanical way. Some attack 

 the blood-cells, penetrate their walls, and absorb their contents. 

 Another and most important action of microbes is the production 

 of poisons of deadly intensity, tending not only to the destruction 

 of the infected person, but of themselves as well. 



These products of the pathogenic germs, called ptomaines, 

 seem to be the means of the suicidal limitation of germ-life in 

 certain instances — where, having gained access to the healthy 

 tissues, they nourish for a time, destroying as they go ; but pres- 

 ently they lose their vitality, poisoned by their own venom, 

 which may be sufficient in quantity and intensity to destroy the 

 individual infected. This fortunate tendency to self-destruction 

 of microbic life seems to belong to the infectious diseases. 



A remarkable and important fact here is the exemption ac- 

 quired by the individual once having a disease from all future 

 attacks. The explanation is as yet difficult. By some patholo- 

 gists it is supposed that the change in the system is due to the 

 permanent retention of a sufficient amount of the ptomaines gen- 

 erated by the first microbic invasion to prevent a reintroduction 

 of the same species. In that case the ptomaine would prove no 

 hindrance to the successful attack of other species. Some sup- 

 pose that an essential nutritive principle in the system becomes 

 completely consumed by the first attack, and may never be repro- 

 duced to support a second one. But recent, observations on the 

 behavior of certain cells furnish a means, at once the most plaus- 

 ible and remarkable, for explaining the acquired disease-immu- 

 nity, as well as a variable degree of original protection. These 

 cells, called leucocytes and phagocytes, seem possessed of an in- 

 stinctive, independent existence and behavior, suggestive of in- 

 tellection. They are capable of locomotion, and a change of size 

 and form — being constructed of elastic cell- walls of most filmy 

 attenuation. 



Their purpose, in part at least, seems to be to protect the sys- 

 tem from harm within the blood, organs, and tissues. They are 

 found where they may render the most ready and efficient serv- 

 ice, particularly in the blood and in the air-cells and bronchioles 

 of the lungs. Like a light guard in peaceful times, they are not 

 conspicuously numerous ; but in time of an attack they present 

 themselves in great numbers and efficiency, and their energy 

 in defense seems increased by any opposition not quite over- 

 whelming. 



