Page:A Text-book of Animal Physiology.djvu/49

Rh Bacteria grow and reproduce in Pasteur's solution, rendering it opaque, as well as in almost all fluids that abound in proteid matter. That such fluids readily putrefy is owing to the presence of bacteria, the vital action of which suffices to break asunder complex chemical compounds and produce new ones. Some of the bacteria require oxygen, as Bacillus anthracis, while others do not, as the organism of putrefaction. Bacterium termo.

Bacteria are not so sensitive to slight variations in temperature as most other organisms. They can, many of them, withstand freezing and high temperatures. All bacteria and all germs of bacteria are killed by boiling water, though the spores are much more resistant than the mature organisms themselves. Some spores can resist a dry heat of 140° C.

The spores, like Torula and Protococcus, bear drying, without loss of vitality, for considerable periods.

That different groups of bacteria have a somewhat different life-history is evident from the fact that the presence of one checks the other in the same fluid, and that successive swarms of different kinds may flourish where others have ceased to live.

That these organisms are enemies of the constituent cells of the tissues of the highest mammals has now been abundantly demonstrated. That they interfere with the normal working of the organism in a great variety of ways is also clear; and certain it is that the harm they do leads to aberration in cell-life, however that may be manifested. They rob the tissues of their nutriment and oxygen, and poison them by the products of the decompositions they produce. But apart from this, their very presence as foreign agents must hamper and derange the delicate mechanism of cell-life.

These organisms seem to people the air, land, and waters with invisible hosts far more numerous than the forms of life we behold. Fortunately, they are not all dangerous to the higher forms of mammalian life; but that a large proportion of the diseases which afflict both man and the domestic animals are directly caused, in the sense of being invariably associated with, the presence of such forms of life, is now beyond doubt.

The facts stated above explain why that should be so; why certain maladies should be infectious; how the germs of disease may be transported to a friend wrapped up in the folds of a letter.

Disease thus caused, it must not be forgotten, is an