90 MANUAL OF PHYSIOLOGY. 



plasm cannot be detected in the minute glistening mass which 

 makes up their body. 



They are so certain to appear in a couple of days in organic 

 infusions, or in any fluid prone to putrefaction, and they multiply 

 with such astounding rapidity, that they have been supposed by 

 some to develop spontaneously. But this is now known not to be 

 a fact. Bacteria do not appear without progenitors, any more 

 than any other form of living thing. They float lifeless and dry 

 in multitudes through our atmosphere, and adhere to all sub- 

 stances to which the air has free access. However, the moment 

 they light upon a suitable soil, they burst into prodigious activity, 

 at first forming masses or colonies, which may be seen as a jelly- 

 like scum on the fluid. Such a soil is supplied by an organic 

 substance capable of spontaneous decomposition, for which pro- 

 cess, as is well known, the great requirements for life, moisture 

 and warmth, to a certain degree are necessary. Vast varieties 

 of these organisms are now known. They differ slightly in shape, 

 in their habitat, and in their properties. Some are obviously 

 composed of two distinct layers, some are provided with a fine 

 hair-like process, by the lash-like motions of which they move 

 rapidly in a definite direction. 



They are known to be inseparable from putrefactive changes 

 in organic materials, in fact, without them no putrefaction can go 

 on, since this process is but the product of their living activity. 

 Intense heat kills them, too great cold or dryness checks their 

 activity and stops putrefaction. When an organic substance is 

 absolutely protected from their presence by exclusion of the air, 

 etc., no putrefaction occurs, even though it be prone to spontane- 

 ous decomposition, and be placed under favorable circumstances 

 as to warmth and moisture. 



Bacteria would not deserve so much notice here were it not for 

 the remarkable influence they have on the higher forms of life. 

 We do not know that they are necessary for any of the more 

 important processes that normally go on in the human body, 

 though they are constantly present in the intestinal tract, and are 

 inseparable from at least one change taking place there that may 

 be regarded as physiological. It is their relation to the diseased 



