Science has proved that for the most part the body does not use the food materials in their original form, but carries them through a series of transformations into substances more easily handled by the organism. Roughly speaking, we may say that the body carries the foodstuffs through practically four processes on the pathway through the body, namely, digestion, absorption, metabolism, elimination. ~Processes Included in Digestion.~--There are several processes concerned in this transformation of the food materials. Some are purely mechanical and have to do with the movement of the food mass through the digestive tract: others are of a chemical character and bring about distinct changes in the food materials themselves. These mechanical and chemical processes with the retarding and stimulating agents that influence them are called digestion. ~Absorption.~--After the food materials have undergone digestion, or simplification into more available substances, these substances are absorbed, that is, they are passed through the membranes lining the walls of the intestinal tract, and thence to the blood. ~Metabolism.~--The utilization of the transformed food materials and their final fate in the human body is included under the term Metabolism. ~Elimination.~--After the food materials have been utilized to the extent of the body's ability to handle them, their waste products are cast out of the organism by way of the skin, the lungs, the intestines and the kidneys. A brief description of these processes seems desirable here. ~Digestion.~--Mechanical digestion begins in the mouth, where through the action of the teeth, the tongue and the muscles of the jaw, the food material is ground up and liquefied to a certain extent and made ready for the chemical action which takes place, to a limited degree only, as a result of the salivary enzyme in the mouth. The eating of food causes a flow of saliva from the three pairs of large salivary glands, and from the numerous secretory cells situated in the membranes of the mouth. As a rule the food stays for too short a time in this organ for any appreciable amount of chemical action to take place, but the liquefaction of the food mass with the salivary juices which contain the ferment (ptyalin), prepares for its passage into the gastric organ, and allows the digestion of the starch (the only foodstuff affected by salivary digestant), to continue in that part of the stomach until its action is checked by the hydrochloric acid in the gastric juice. ~Arrangement of Food in the Stomach.~--To simplify the study of the gastric organs it may be well to think of the stomach as being divided into three regions, _i.e._, "the fundus, the middle region, and the pyloric end,"[52] each of which differs slightly from the other. After being swallowed, the food enters the region situated at the cardiac end, known as the fundus.