PRODUCTS OF VITAL ACTIVITY. 139 



organisms present. One or the other of the bacteria concerned will 

 take the precedence when circumstances favor its growth. Thus the 

 aerobic bacteria cannot grow unless the putrefying material is freely 

 exposed to atmospheric oxygen ; the anaerobic species require its 

 exclusion. Some saprophytic bacteria grow at a comparatively low 

 temperature, others take the precedence when the temperature is 

 high ; some, no doubt, thrive only in presence of products evolved 

 by other species, and are consequently associated with and depend- 

 ent upon these species ; some are restrained in their growth sooner 

 than others by the products evolved as a result of their own vital 

 activity or that of associated organisms ; some grow in the presence 

 of acids and give rise to an acid fermentation which wholly prevents 

 the development of other species. 



At the outset putrefaction is often attended with the presence 

 of several species of micrococci and certain large bacilli, which are 

 displaced later by short motile bacteria belonging to a group which 

 includes several bacilli formerly described under the common name 

 of Bacterium termo. 



The malodorous volatile products of putrefaction are to a consid- 

 erable extent produced by anaerobic species. For this reason these 

 odors are more pronounced when masses of albuminous material 

 undergo putrefaction in situations where the oxygen of the air has 

 not free access or where it is displaced by carbon dioxide. The 

 body of a dead animal, although freely exposed to the air, furnishes 

 in its interior a suitable nidus for these anaerobic gas-forming spe- 

 cies, and they may give rise to products of one kind, while aerobic 

 species upon the surface of the mass induce different forms of putre- 

 factive fermentation. In the bodies of living animals these anaero- 

 bic microorganisms are constantly present in the intestine, and after 

 death they quickly invade the body and multiply at its expense 

 under favorable conditions as to temperature. The surface decom- 

 position due to aerobic bacteria occurs later and is not attended 

 with the same putrefactive odors, the products evolved being of a 

 simpler chemical composition CO 2 , HN 3 . No doubt these aerobic 

 bacteria, by consuming the oxygen and forming an atmosphere of 

 carbon dioxide, help to make the conditions favorable for the con- 

 tinued development of the aiiaerobics in the interior of the organic 

 mass ; at the same time they find a suitable pabulum in some of the 

 more complex products of decomposition occurring in the absence 

 of oxygen. The gases produced in the interior of a putrefying mass 

 are mainly CH 4 , H 2 S, and H. 



Many of the bacteria of putrefaction are facultative aiiaerobics 

 that is to say, they are able to multiply either in the presence of oxy- 

 gen or in its absence. The products evolved by these differ, no 



