DIGESTION. The word digestion desig nates the physical and chemical changes which food substances must undergo before they can be absorbed into the blood and lymph. Only a few food substances can become part of the body without previous change. These are water, certain salts and grape sugar. The most import ant alimentary principles, namely, protein, car bohydrates and fats, cannot be absorbed in the form in which they are furnished by nature. The digestive process can be considered most conveniently under three headings: Secretion, Absorption and Peristalsis.
Secretion refers to the elaboration of com plex juices which contain, as a rule, digestive enzymes. One of the secretions, the gastric juice, contains free hydrochloric acid. Others, like the saliva, the pancreatic and the intestinal juices, are alkaline. The processes by which these digestive juices are formed and brought to the surface of the organ in which the diges tion takes place are complex and will require special description. When these secretions act on the food substances at the temperature of the body, the food substances are broken down into simpler compounds, the chemical mole cule becoming smaller and smaller. Some physiologists claim that the molecule of the food substance is reduced to its component atoms before it can be taken up by the living cells that line the digestive apparatus. If food substances are forced to enter the animal or ganism without being digested, for example, if egg albumen is injected into the blood circula tion, it is again excreted by the kidneys. Egg albumen is a foreign albumen and the object of the elaborate chemical process is to convert it into native albumen which chemically and physi ologically corresponds to that which enters the normal or native structure of the body.
Under Absorption are included the proc esses whereby the dissolved food substances, namely, the soluble end products of proteid, carbohydrate and fat digestion are taken up from the mucous membrane of the stomach and intestines and carried directly into the blood, or indirectly by means of the lymphatics. The
blood, in turn, carries these end products of di gestion to the cells constituting the organs and tissues in the body. The most important seat of absorption is the intestines. To a certain small extent absorption can take place from the mouth and the stomach.
By Peristalsis we mean the periodic con tractions and constrictions which the oesophagus, stomach and intestines undergo, and by means of which the contents of the digestive appara tus are moved from the oral end to the rectal end of the digestive tube. But these peristaltic movements are also essential to effect a proper mixing of the food with the digestive juices in the stomach and intestines. Peristalsis does not, therefore, always mean regular onward movement. At times, the food is churned back ward and forward in a part of the intestines not longer than 8 to 10 inches causing it to be thor oughly mixed with the secretion, and in the as cending colon there is an actual anti-peristalsis which compels the food that has entered the large intestine through the ileocecal val;e, after it has reached as far as the hepatic flexure of the colon, to return to the cecum. This may be repeated many times before the food is al lowed to pass into the transverse colon.
To the eye the peristaltic movement con sists of a constriction of the walls of the in testine which, beginning at a certain point, passes downward away from the stomach from segment to segment. The advancing area of con striction is preceded by an area of inhibition or relaxation. The peristaltic movement, there fore, consists of two acts—constriction of one small region, preceded by the inhibition of an other small region immediately in advance of it. These seem to follow in definite sequence and, when combined in an orderly manner, will facilitate the onward movement of the intes tinal contents. Bayliss and Starling were the ones first to describe this twofold character of the movement and they regarded it as a reflex which is controlled within the intestinal wall it self through intrinsic nerve ganglia and their afferent and efferent communications.