142 : PRODUCTS OF VITAL ACTIVITY. 
tion of a great variety of products, some of which are volatile and 
are characterized by their offensive odors. According to Fligge, the 
first change which occurs consists in the transformation of the albu- 
mins into peptone, and this may be effected by a large number of 
different bacteria. Among the products of putrefactive fermenta- 
tion known to chemists are the following substances : Carbon diox- 
ide, hydrogen, nitrogen, hydrosulphuric acid (H,S), phosphoretted 
hydrogen (PH,), methane, formic acid, acetic acid, butyric acid, 
yalerianic acid, palmitic acid, crotonic acid, glycolic acid, oxalic 
acid, succinic acid, propionic acid, lactic acid, amidostearic acid, 
leucin, ammonia, ammonium carbonate, ammonium sulphide, tri- 
methylamine, propylamine, indol, skatol, tyrosin,neuridin, cadaverin, 
putrescin, cholin, neurin, peptotoxin, and various other volatile 
acids, ptomaines, etc. 
' The special products of putrefaction vary according to the nature 
of the material, the conditions in which it is placed, and the micro- 
érganisms present. One or the other of the bacteria concerned will 
take the precedence when circumstances favor its growth. Thus the 
aérobic bacteria cannot grow unless the putrefying material is freely 
exposed to atmospheric oxygen; the anaérobic 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 anaérobic 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 anaérobic gas-forming spe- 
cies, and they may give rise to products of one kind, while aérobic 
species upon the surface of the mass induce different forms of putre- 
factive fermentation. In the bodies of living animals these anaéro- 
