The changes involved in rendering food compounds soluble are intimately connected with a class of bodies known as ferments, to which brief reference has already been made, and it seems necessary before proceeding to a consideration of digestion as a process to learn something of the nature and functions of these agents, which are actively and essentially present in the digestive tract.

A ferment may be defined in a general way as something which causes fermentation; in other words, the decomposition of certain vegetable or animal compounds with which it comes in contact under favorable conditions. Ferments have been classified into two kinds, organized and unorganized. The so-called organized ferments are low, microscopic forms of vegetable life, generally single-celled plants. Those known as unorganized ferments are not living organisms, but are simply chemical compounds.

92. Organized Ferments

When milk is allowed to remain in a warm room for several hours, it becomes sour. An examination of it chemically shows that its sugar has largely or wholly disappeared and has been replaced by an acid. A study of the milk with the microscope, before and after souring, reveals the fact that there has been a marvelous increase in it of single-celled organisms or plants. The presence of this form of life is. regarded as the cause of the change of the sugar into lactic acid. We have here a so-called lactic acid ferment, which may typify the organized ferments known as bacteria. Numerous other fermentations of the same general kind are common to everyday experience. The changes in the cider barrel and the wine cask, the spoiling of canned fruits and vegetables, and the heating of hay and grain are illustrations of what is accomplished by these minute organisms.

Bacteria that cause disease and which multiply in the organs, and other tissues of the animal body, may also be properly called ferments, because in their growth new compounds, toxins1 perhaps, are formed which are as truly fermentative by-products as the carbonic acid and alcohol of cider and beer making. As this subject viewed on its pathogenic side is not important in this connection, we need to study organized ferments only so far as they relate to the preservation of foods and to changes in the alimentary canal. We shall be best equipped for controlling ferments and preventing their destructive action if we know what they are, and understand the general conditions under which they thrive. We should also know how, and to what extent, their action occasions harm.

93. Structure. Distribution

The organized ferments are classed in the vegetable kingdom. As a rule, each individual plant is a single cell, varying in shape and so minute as to be invisible to the unaided sight. It corresponds in its general structure to the cells which make up the tissues of the higher vegetable species, i.e., it consists of a cell-wall inside of which are protoplasm and other forms of living matter. These organisms are distributed everywhere, - in the air, in the soil, on surfaces of plants, and in the bodies of animals.

1 Poisonous albuminous bodies, produced by bacterial action; as, for instance, in typhoid fever, diphtheria, tetanus, and other diseases.

Whenever the right opportunity offers itself, they multiply and bring about all the results attendant upon their growth.