With the air and saliva which are swallowed mixed with the food, large numbers of the lower organisms existing in them are introduced into the alimentary canal.

The effect of these organisms is to produce certain fermentative changes quite distinct from the action of the special ferments of the digestive fluids.

This is proved by the composition of the gases found in the intestine. Atmospheric air only is introduced from without, and this is not found in any part of the alimentary tract, the oxygen soon being absorbed and the nitrogen left, while a quantity of carbonic anhydride and hydrogen from the fermentation of the sugar are set free, lactic and butyric acids being produced at the same time.

Indol and skatol are also formed by putrefactive fermentation of the leucin and tyrosin.

It is in the large intestine that putrefactive fermentations have greatest effect, the acid reaction being caused by the various acids thus produced.

With regard to the interesting question, why the digestive juices do not dissolve the tissues of the organs in which they are contained, we cannot speak positively. We can no longer say that the "vital principle" has a protective influence, for we know that the fact of a tissue being alive is not sufficient to ward off the digestive action of the alimentary juices. The limb of a living frog is digested when introduced through a fistula into the stomach of a dog; and when the intestinal juice trickles from a fistula the neighboring skin, the snout, and the tongue of the animal soon become eaten away, owing to its licking the fluid, which rapidly digests these parts so as to destroy the skin and even expose the blood vessels.

We can, however, modify John Hunter's statement that the resisting power was associated with the life of the structures, by saying that it is not the property of an abstract "vital principle," but a special resisting power dependent upon the specific character of the vital processes of those textures which manufacture and are habitually exposed to the influence of the juices.