This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
20. The time which is required to change food into chyme, varies according to the nature of the food. Animal food is digested sooner than vegetable. The average time required, is about three hours and a half. A good deal depends, also, on the degree of mastication it has undergone. If swallowed in large masses its solution must go on slowly.
21. As fast as the aliment is changed into chyme, it passes out of the stomach into the duodenum, and it generally stays in the stomach until it has undergone this change. Indigestible substances have thus been vomited up, more than a week after they were swallowed. All fluids which are swallowed, are supposed to be taken up by absorption.
22. If the par vagum, or nerve which goes from the brain to the stomach, be divided, digestion is impaired or suspended. The same happens under the influence of mental emotions, such as grief, anger, etc. Some physiologists have thought that this nerve presides over the secretion of the gastric juice; others, that it stimulates the muscular motions of the stomach; while a third class consider it to be the seat of sensation in the stomach, giving rise to hunger and thirst. But it is not yet fully settled what particular influence this nerve exerts over digestion.
23. The chyme, on passing from the stomach, is received into the duodenum. This, like the stomach, has a serous, muscular, and mucous coat, and has a mucous, as well as serous, secretion. In this portion of intestine, the chyme in its passage meets with the pancreatic and the biliary fluids; the irritating properties of the acid chyme, cause these fluids to be poured out in great abundance, as well as the other secretions ; and these are thoroughly mixed with the chyme by the contraction of the intestine. It now becomes of a yellowish, instead of a gray colour; its acid properties disappear, and large quantities of albumen are developed. This is supposed, by some, to be derived from the pancreatic fluid, which contains a large proportion of it.
24. As it becomes intimately mixed with the biliary and pancreatic secretions, the substance called chyle is produced. Though some say it is not to be found in the duodenum-but only the elements out of which it is formed. But albumen, which is the basis of chyle, exists abundantly in the duodenum ; and so also do particles of fibrin. By the microscope, globules also may be detected in the chyme, similar to those which are found in the chyle.
25. It would seem, then, that the great business of digestion is to change the food into albuminous matter, which forms the basis of the chyle as well as the blood ; and it is certain that no albumen is formed in the stomach; though the change which the food there undergoes, is an approach to the nature of albumen. As the bile is alkaline, it doubtless combines with, and neutralizes the acid properties of the chyme, which would precipitate the mucus of the bile, and leave it in a state of coagulation.
26. Chyle then is the fluid, which is taken up by the absorbent vessels, called lacteals, whether it exists ready formed in the chyme, or is manufactured out of it, by the action of these vessels themselves. It is usually of a milk white colour, but varies in appearance in different animals ; and according to the nature of the food. In animals that feed on flesh, it is opaque ; in such as live on vegetables, it is transparent ; in birds and fishes, thin, serous, and clear like water. It is saltish, and somewhat sweet to the taste ; heavier than water but less so than blood. It coagulates on standing, like blood, and separates into three portions ; a fluid, a coagulum, and a fatty substance. The fluid portion is chiefly of albumen and coagulates like the serum of the blood by heat, acids, and alcohol; the coagulum consists of fibrin, and a coloring matter, which is white.
27. Whatever the food may consist of, physiologists are now pretty well agreed, that the chyle will always be composed of fibrin, albumen, a fat matter, muriate of soda, and phosphate of lime, though in variable proportions. Food that contains much azote, such as that of animals, it is supposed will form chyle, which contains a greater portion of fibrin than that of vegetables, as azote is one of the chief elements of fibrin. Dr. Marcet states that chyle produced from vegetable aliments contains three times as much carbon as that formed out of animal substances. It is certain that chyle from animal food is milky ; and that from vegetables transparent. As both however are composed of the same essential elements, we may be assured, that man can live on either animal or vegetable diet, as it may be most convenient. As the most important part of digestion seems to be completed in the small intestines, we shall not follow the process any farther.
28. A few years since, a man in the United States army received a gun shot wound, which made a hole in his stomach ; and which left an opening into the cavity of that organ, so that the whole process of digestion could be seen, as it was going on. Dr. Beaumont, a physician in the army, availed himself of this singular case, and performed many curious experiments which occupied him for several years. The following are the most important inferences, which he drew from his experiments.
29. " Animal and farinaceous aliments are more easy of digestion than vegetables. Digestion is facilitated by minuteness of division and tenderness of fibre; hence the importance of thoroughly chewing the food. The ultimate principles of aliment are always the same, from whatever kind of food they may be obtained, whether vegetable or animal. The quantity of food generally taken into the stomach, is greater than the system requires. Solid food is easier of digestion than fluid. Stimulating condiments, such as spices, are hurtful to a healthy stomach.
30. The continued use of ardent spirits always produces diseases of the stomach. Hunger is the effect of the distension of the vessels, which secrete the gastric juice. The temperature of the stomach is 100 degrees of Fahrenheit. The gastric juice dissolves the food, and alters its properties. It also coagulates, or renders solid, albumen, and afterwards dissolves it.
31. The gastric juice is a clear and transparent fluid ; a little saltish, and somewhat sour to the taste. When pure, it suffers no change by keeping. Gentle exercise assists the digestion of the food. Water, ardent spirits, and most other fluids are not affected by the gastric juice, but disappear from the stomach soon after they are received."
What constitute the nutritive functions ? What the digestive apparatus ? What the mouth ? How many salivary glands are there ? How many teeth are there in an adult ? How divided ? When do they begin to appear ? When are they shed ? What is the outside of the tooth called ? How is the tongue useful in mastication ? Describe the esophagus-the stomach-the intestines-the liver-the pancreas-hunger and thirst. Describe the process of digestion. What is the gastric juice ? Chyme ? Chyle ? What is rennet-what used for ? Is digestion a chemical process ? How long does it generally take to convert food into chyme ? What influence has the par vagum nerve on digestion ? Where do the pancreatic and biliary fluids enter the intestines ? their use ? Is the nature of chyle changed by the kind of food ? Of what is it always composed ? What are the principle results of Beaumont's experiments in relation to digestion ?