CHAPTER III.
DIGESTION.
FOOD, before it can be utilized by the body, must undergo many mechanical and chemical changes to render it capable of digestion, absorption, and assimilation.
Digestion is the conversion of insoluble and indiffusible substances into soluble and diffusible substances capable of being absorbed. Absorption is the taking up of the digested food by the blood-vessels and lymphatics and conveying it to the blood, by which it is carried to every part of the body. Assimilation is the taking up by the different tissues from the blood such material as they need for growth and repair.
Digestion is carried on principally by ferments, and these act by contact. Food is taken into the mouth, masticated by the teeth, moistened by the saliva, and coated by the mucin in the saliva, which makes it easy to swallow. The saliva is an alkaline fluid secreted by three pairs of glands,—the parotid, submaxillary and sublingual. Ptyalin, which acts in an alkaline medium, is the ferment found in the saliva. It has the power of changing starch to maltose and dextrose, but has no effect on proteids or fats. The flow of saliva is continuous, but greatest during eating, about three pints being secreted every twenty-four hours.
Thorough mastication is very important, that the food may be finely divided before passing on into the alimentary canal. If not well masticated it is retained in the stomach for too long a time, thus favoring the development of bacteria, which give rise to acid fermentation.
