By metabolism is meant those physiologic processes brought about in living tissue by means of which the individual is able to form new tissue and reintegrate the old. Under this head comes the rejection of those substances that are unfit for use in the bodily economy.

In order that the functions of the body be carried on in a normal manner it is necessary that the amount taken up by the cells must balance the output. The metabolic equilibrium must be maintained. To carry on the work it is not sufficient that new material be taken in, more is required. These new substances must be assimilated by the body, broken down into various parts, and the waste portions excreted. Disturbances of any of these above factors, constituting metabolism, may give rise to diseased conditions of varying importance.

When the tissues are unable to carry on these molecular exchanges a pathologic condition exists. This may be either functional or structural, the latter generally being secondary. to the former.

Metabolism may be divided into two classes, according to whether simple substances are built up into complex, or the complex broken down into the simple. The building-up or constructive variety is called anabolic metabolism; the breaking down or destructive, catabolic metabolism.

By means of catabolism the "end products," those substances not required by the body, are formed, such as urea, water, etc. Anabolism is concerned in the rearranging of molecules so as to render them suitable for food.

A food is a substance that will form new or reintegrate old tissues. It may be either in excess or in diminution, or may vary in quality, the amount required depending upon the activity of the individual.

The assimilation of food depends upon the presence within the gastro-intestinal tract of certain digestive ferments, which may vary greatly in quantity.

The protein substances are acted upon by the pepsin in the stomach and the trypsin from the pancreas. Pepsin acts in an acid medium; trypsin in an alkaline. The necessary acid in the stomach is the hydrochloric. Changes from the normal amount of pepsin are unusual, but there may be great variations as far as the acid is concerned. It may be increased, hyperchlorhydria; diminished, hypochlorhydria; or absent, achlor-hydria. If absent or much diminished, the food, not being properly digested, will undergo fermentation. If there be any obstruction at the pylorus the stomach will tend to dilate.

The carbohydrates are acted upon first in the mouth by the salivary ferment, ptyalin; then in the intestines by amylopsin, a ferment derived from the pancreas.

The fats are acted upon by steapsin, a pancreatic ferment, and by the bile.

The condition of the individual depends upon the assimilation of the food, which may be abnormal in quantity or quality.

If the quantity taken up by the individual is diminished, either by lack in amount or by being deflected from its proper channels, certain pathologic conditions will result. These may be emaciation or starvation, in which case the body weight diminishes, the temperature falls, and the energies all fail. At first the reserved food is called upon. The circulating proteins are first used up, then the glycogen, and afterward the fats and the muscles. The heart and the central nervous system are the last structures to be involved. The organs become smaller, the excretions and secretions are gradually suspended. In the blood the leukocytes become much fewer, although the red cells appear in normal number. This is probably due to the loss of the blood-serum. Death takes place slowly, either from exhaustion, disorders of metabolism, or by terminal infections.

In marasmus, a term applied to babies and old people, the wasting away takes place more slowly than in starvation.

In it the trouble is very frequently not due to lack in quantity of food, but to improper assimilation.

If during the course of a definite disease these symptoms of slow starvation appear, the condition is called cachexia. In it there is a peculiar yellowish color of the skin and also a marked anemia. It is probably due to the formation of toxic substances, many resembling ferments, which produce injurious effects upon the normal tissues of the body.