I need scarcely say that putrefaction is not a specific fermentation like alcoholic or acetic fermentation, but that it is extremely complex. In any putrefying matter, such as gelatin or albumin, a large number of different species of bacteria may be observed as well as monads and infusoria, and in some cases moulds, all of which take part in the process. The first stage is a process of oxidation in the presence of air, in which ærobic bacteria use up the oxygen present and only simple inorganic compounds are formed, carbon dioxide, nitrates and sulphates; this part of the process is generally without odour. The second stage, or true putrefaction, takes place in the absence of oxygen by anærobic bacteria, and is a process of reduction. It has been shown that there are no bacteria in healthy tissues, and if a muscle or any organ is taken from an animal under antiseptic conditions it may be preserved indefinitely in a sterile vessel to which filtered air has free access. Solid matter is usually liquefied by organisms like B. liquefaciens magnus, which are invariably present in the air, and which prepare the way for more specifically putrefactive bacteria, such as Proteus vulgaris and B. putrificus, but if one observes a number of putrefactions of the same kind of matter under natural conditions, scarcely any two follow the same course. The modern study of putrefaction dates from Hauser (58), who, in 1885, isolated from putrefying flesh the three organisms--Proteus vulgaris, P. mirabilis, and P. zenkeri. He studied the action of these in pure cultures, and came to the following conclusions:-- That Bacterium termo (Ehr.) is not a single definite species; various forms and stages of other organisms have been described under this name. The various species of Proteus go through a wide range of forms during their development in which cocci, short and long rods, thread forms, vibrios, spirilli, and spirochætæ occur. Under special nutritive conditions Proteus goes through a swarm stage, in which condition it is capable of moving over the surface and in the solid gelatin. The Proteus bacteria are facultatively anærobic, they all cause putrefaction; P. vulgaris and P. mirabilis are the commonest and most active of all putrefactive bacteria. They do not secrete an unorganised ferment, but decompose albuminous bodies by direct action. They also produce a powerful poison, of which small quantities injected into animals produce septicæmia.