This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
A mouthful of solid food is broken up by the teeth and rolled about the mouth by the tongue until it is thoroughly mixed with saliva and made into a soft pasty mass. The muscles of the cheeks keep this from getting between them and the gums.† The mass is finally sent on from the mouth to the stomach by the process of deglutition, which occurs in three stages. The first stage includes the passage from the mouth into the pharynx. The food being collected into a heap on the tongue, the tip of that organ is placed against the front of the hard palate, and then the rest of the tongue is raised from before back, so as to compress the food mass between it and the palate and drive it through the fauces. This much of the act of swallowing is voluntary, or at least is under the control of the will, although it commonly takes place unconsciously. The second stage of deglutition is that in which the food passes through the pharynx; it is the most rapid part of its progress, since the pharynx has to be emptied quickly so as to clear the opening of the air-passages for breathing purposes. The food mass, passing back over the root of the tongue, pushes down the epiglottis ; at the same time the larynx (or voice-box at the top of the windpipe) is raised so as to meet the epiglottis, and thus the passage to the lungs is closed.* The soft palate is, at the same time, moved into a horizontal position, so as to separate the upper (or respiratory) portion of the pharynx, leading to the nose and the Eustachian tubes (see Fig. 41), from its lower portion, which ends inferiorly in the gullet.
What is the chief use of saliva? Under what circumstances does it change starch into sugar? In what portion of the digestive tract is this action of the saliva stopped? Why? How does the saliva promote digestion in the stomach? Why should food be thoroughly chewed before swallowing?
What is the technical term for swallowing? In how many stages does swallowing occur?
* Hence the efficacy of a little carbonate of soda or apollinaris water taken before meals, in some forms of dyspepsia.
† Persons with facial paralysis have from time to time to press out with the finger food which has collected outside the gums, where it can neither bo chewed nor swallowed.
Finally the isthmus of the fauces is closed as soon as the food has passed through, by the contraction of the muscles on its sides, and the elevation of the root of the tongue. All passages out of the pharynx except the gullet being thus blocked, when the pharyngeal muscles contract the food can only be squeezed into the aesophagus. The muscular movements concerned in this part of deglutition are all excited without the intervention of the will; food touching the mucous membrane of the pharynx produces quite involuntarily the proper action of the swallowing muscles.* Indeed, many persons after having got the mouth completely empty cannot perform the movements of the second stage of deglutition at all. On account of the involuntary nature of the contractions of the pharynx any food which has once entered it must be swallowed; the isthmus of the fauces forms a sort of Rubicon; food that has entered the pharynx must be swallowed, even although the swallower learned immediately that he was taking poison. The third stage of deglutition is that in which the food is passing along the gullet, and is comparatively slow. Even liquid substances do not fall or flow down this tube, but have their passage controlled by its muscular coats, which grip the successive portions swallowed and pass them on. Hence the possibility of performing the apparently wonderful feat of drinking a glass of water while standing upon the head, often exhibited by jugglers; people forgetting that one sees the same thing done every day by horses and other animals which drink with the pharyngeal end of the gullet lower than the stomach.
Describe the first stage. What is the second stage? Which stage is most rapid? Why? How is the passage to the lungs closed while food is passing through the pharynx? How is the passage to the nose blocked? Describe the processes of the second stage of deglutition.
* The raising of the larynx during swallowing can be readily felt by placing the finger on its large cartilage forming " Adam's apple" in the neck.
The food having entered the stomach is exposed to the action of the gastric juice, which is a thin colorless or pale yellow liquid of a strongly acid reaction. It contains, beside water and some salts and mucus, free hydrochloric acid (about .02 per cent), and a substance called pepsin, which in acid liquids has the power of converting ordinary albumens into closely allied bodies called peptones. It also dissolves solid proteids, changing them at the same time into peptones.
How are the movements of the second stage of deglutition excited? What is the third stage of deglutition? Is it fast or slow? How is it that jugglers can drink while standing on the head?
Describe the gastric juice.
* The process is what is known as a reflex action. See Chap. XX.
Ordinary albumens are typical examples of what are called "colloids;" that is to say, substances which do not readily pass through moist animal membranes; peptones are kinds of albumen which do readily pass through such membranes, and are, therefore, capable of absorption from the alimentary canal. (See Dialysis, p. 212).
In the stomach the onward progress of the food is stayed for some time. The pyloric sphincter remaining contracted closes the aperture leading into the intestine, and the irregularly disposed muscular layers of the stomach keep its semi-liquid contents in constant movement, by which all portions are thoroughly mixed with the secretion of its glands. In the stomach part of the albumen of the food is dissolved and turned into peptones. Certain mineral salts (as phosphate of lime, of which there is always some in bread), which are insoluble in water but soluble in dilute acids, are also dissolved in the stomach. On the other hand, the gastric juice has no action upon starch, nor does it digest oily substances. By the solution of the white fibrous connective tissues the disintegration of animal foods, commenced by the teeth, is carried much farther in the stomach; and the food-mass, mixed with much gastric secretion, becomes reduced to the consistency of a thick soup, usually of a grayish color. In this state it is called chyme.
Name its more important constituents. What powers does pepsin possess?
What are colloids? Give examples. How do peptones differ from other albumens ?
Does food pass on immediately from the stomach to the intestine? How is it kept back ? How is it mixed with the gastric juice? What happens to albuminous foods in the stomach? Name another substance dissolved in the stomach. Name foodstuffs which are not changed in the stomach. How are animal foods broken up in the stomach?