transforming some part of the food into suitable material for use in the body.  The saliva acts upon the starch of the food, changing it into sugar; the gastric juice digests albumin and other nitrogenous elements; the bile digests fat, and aids in the absorption of other food elements after they are digested; the pancreatic juice is not confined in its action to a single element, but digests starch, fats, and the albuminous elements after they have been acted upon by the gastric juice; the intestinal juice is capable of acting upon all digestible food elements.

[Illustration:  The Alimentary Canal, a. Esophagus; b. Stomach; c. Cardiac Orifice; d. Pylorus; e. Small Intestine; f. Bile Duct; g. Pancreatic Duct; h. Ascending Colon; i. Transverse Colon; j. Descending Colon; k. Rectum.]

THE DIGESTION OF A MOUTHFUL OF BREAD.—­A mouthful of bread represents all, or nearly all, the elements of nutrition.  Taking a mouthful of bread as a representative of food in general, it may be said that its digestion begins the moment that it enters the mouth, and continues the entire length of the alimentary canal, or until the digestible portion of the food has been completely digested and absorbed.  We quote the following brief description of the digestive process from Dr. J.H.  Kellogg’s Second Book in Physiology[A]:—­

[Footnote A:  Good Health Pub.  Co., Battle Creek, Mich.]

“Mastication.—­The first act of the digestive process is mastication, or chewing the food, the purpose of which is to crush the food and divide it into small particles, so that the various digestive fluids may easily and promptly come into contact with every part of it.

“Salivary Digestion.—­During the mastication of the food, the salivary glands are actively pouring out the saliva, which mingles with the food, and by softening it, aids in its division and prepares it for the action of the other digestive fluids.  It also acts upon the starch, converting a portion of it into grape-sugar.

“Stomach Digestion.—­After receiving the food, the stomach soon begins to pour out the gastric juices, which first makes its appearance in little drops, like beads of sweat upon the face when the perspiration starts.  As the quantity increases, the drops run together, trickle down the side of the stomach, and mingle with the food.  The muscular walls of the stomach contract upon the food, moving it about with a sort of crushing action, thoroughly mixing the gastric juice with the food.  During this process both the openings of the stomach are closed tightly.  The gastric juice softens the food, digests albumen, and coagulates milk.  The saliva continues its action upon starch for sometime after the food reaches the stomach.