When the work of digestion is completed in the stomach the food is poured through the pyloric orifice into the small intestine where it undergoes further changes.
In negroes the small intestine is shorter and the large intestine longer than in the white man of similar build. There are also differences due to sex--extremes in males running from 15 ft. 6 inches to 31 feet, 10 inches and in females, from 18 feet 10 inches to 29 feet 4 inches. The tall thin type of person, with trunk of small circumference, has a shorter intestine than the heavy rugged type. There are three digestive juices which are poured into the intestine--bile, pancreatic juice and intestinal juice; all of these are alkaline in reaction.
The pancreatic juice is secreted by the pancreas and enters the intestine just below the union of the stomach and duodenum or upper portion of the small intestine. This juice, the secretion of which is excited by the action of the acid contents received from the stomach upon the walls of the intestine, is poured out about the time the contents of the stomach pass through he pyloric valve.
Pancreatic juice contains four enzymes. One of these known as diastase or amylase resembles ptylain and continues the work of digesting starches and sugars, converting these into a form of sugar known as monosaccharides. It is not destroyed by the acid contents of the stomach as is ptyalin. A second, known as trypsin, is a protein-splitting enzyme, but unlike pepsin, does not require the cooperation of an acid to accomplish its work. In fact, it is destroyed by a strong acid. By its action the peptones are converted into amino-acids. The third enzyme known as liapase causes fat to undergo cleavage forming fatty acids and glycerine. The fourth, chymosin, or pancreatic renin, coagulates milk.
Pavlov discovered that the pancreatic juice, as it leaves the pancreas, has no appreciable action upon proteins, but becomes rapidly active when a small quantity of the intestinal juice, which Pavlov has called enterokinase, which converts the inactive trypsinogen, from the pancreas, into active trypsin. Pavlov regarded enterokinase as itself an enzyme. Active trypsin in the pancreas and pancreatic duct might destroy these organs. Nature seems to have safeguarded the body by arranging that it cannot become active until it is in the presence of food in the intestine, where it is activated by the enzymic effect of the intestinal juice upon it.
Considering the pancreatic secretion, we find the same marvelous adaptation of the digestive properties to the class of food to be acted upon. Each kind of food calls for its own particular kind of juice. The character of these juices is often the direct opposite of that seen in the stomach. In the stomach the weakest juice is poured out upon milk and the strongest upon meat; in the duodenum the weakest juice is poured out on flesh and the strongest on milk. This statement, of course, has reference to the protein-splitting character of the juice.
With regard to the starch-splitting enzyme, this is present in greater quantity in the "bread juice" and in lesser quantity in "milk juice." The fat-splitting enzyme is very scarce in "bread juice" abundant in "milk juice," and intermediate in "flesh juice." "The work of the pancreas, like that of the gastric glands," to quote Pavlov, "is specialized both as regards the quantity and property of its juice, and the rate of its progress which the secretion takes for the different classes of food."
The second of the juices poured into the intestines is bile or gall. This is secreted by the liver and enters the intestine at about the point where the pancreatic juice enters. Its secretion goes on continuously but is accelerated after meals. It contains no enzyme and is, therefore, not a true digestive juice; but acts chiefly by producing a favorable environment for the action of the pancreatic enzymes. If it is prevented from entering the intestines, the ability to digest and absorb foods, particularly fats, is reduced. Bile increases the solubility of the fatty acids by emulsification, accelerates the action of the pancreatic liapase, stimulates intestinal activity, counteracts putrefaction, and assists in the union of water and oils.
Bile, secreted in the liver and conveyed by a duct to the duodenum, is not regarded as a true digestive juice because it contains no enzyme. But by alkalizing the acid bolus from the stomach, when this enters the duodenum, it provides a suitable environment for the operation of the pancreatic and intestinal enzymes. The hydrochloric acid, of the stomach, upon entering the intestine, acts as a powerful stimulus to the flow of pancreatic juice, intestinal juice and bile, but is antagonistic to the action of their enzymes. The bile counteracts the acid and produces a favorable medium for the action of these enzymes.
Bile is a powerful disinfectant and prevents putrefaction in the intestine. It also serves to prevent the formation of gas and helps to maintain the alkalinity of the intestine.
The third or intestinal juice is secreted in abundance by small glands in the walls of the small intestine. It contains an enzyme known as crepsin which cooperates with trypsin in the final stages of protein digestion. This juice also completes the preparation of carbohydrates for entrance into the blood.
Intestinal juice--succus-intericus--is elaborated by many microscopic glands embedded in the walls of the intestine. There are four kinds of glands which secrete intestinal juice--Crypts of Lieberkuhn, Brunner's glands, solitary glands and Pyer's patches or glands. The glands of Liebekuhn. secrete an intestinal juice containing several enzymes--erepsin (Proteolytic), lactase, invertase, (Amolytic), Maltase (Amolytic), and lactase which digests milk sugar. Brunner'sglands secrete a juice containing the enzyme, enterokinase, which, acting upon the trypsinogen of the pancreatic juice, as it enters the duodenum, converts this into the powerful protein-splitting enzyme, trypsin. Chymosis, which coagulates milk, is also contained in succus-interucus.
Having considered the processes of digestion let us now get some idea of the work accomplished by these. First, it is a refining process, breaking down the structure of the food and separating the nutritive portions from the waste or useless parts. Second, it splits up the large and complex molecules of food into smaller, less complex ones, in this way adding to the diffusibility of the food. Diffusibility is the capability of spreading and enabling substances to pass through ordinary membranes. Lastly, it standardizes our food. By this we mean it obliterates many of the characteristics of the various foods consumed and gives us, finally, practically the same set of products whatever the meal eaten. From the many strange and foreign compounds that are taken into the mouth, as food, are formed a few acceptible compounds.
When, finally, the work of digestion is completed in the intestine the carbohydrates have been reduced to a form of sugar known as monosaccharides, the fats have been converted into fatty acids and glycerol and the proteins have been reduced to amino-acids. Water and salts undergo no change. The waste portions of the food are separated from the usable portions and are sent on into the large intestine or colon to be expelled.
While fats, starches, sugars and proteins undergo several changes in the process of digestion, the mineral elements of our food are absorbed unchanged. They do not require to be digested.