its peculiar site for ingress, its locality for operation, and its peculiar 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 presently 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 acquired by the individual once having a disease from all future attacks. The explanation is as yet difficult. By some pathologists it is supposed that the change in the system is due to the permanent retention of a sufficient amount of the ptomaines generated 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 suppose that an essential nutritive principle in the system becomes completely consumed by the first attack, and may never be reproduced to support a second one. But recent, observations on the behavior of certain cells furnish a means, at once the most plausible and remarkable, for explaining the acquired disease-immunity, as well as a variable degree of original protection. These cells, called leucocytes and phagocytes, seem possessed of an instinctive, independent existence and behavior, suggestive of intellection. 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 system from harm within the blood, organs, and tissues. They are found where they may render the most ready and efficient service, 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 overwhelming.