Usually the food stays too short a time in the mouth for the starch to be acted upon there to any great extent, and until recently it was supposed that salivary digestion must cease almost as soon as the food reaches the stomach, since the activity of ptyalin is quickly checked by even small amounts of free hydrochloric acid. It was supposed that the food mass must soon be mixed with the gastric juice under the influence of the "churning" of the stomach contents by the muscular contraction of the stomach walls, which was so interestingly described by Dr. Beaumont in the account of his classical researches already referred to (pages 70 -71). From the nature of the case Dr. Beaumont's observations were made entirely at one point in the stomach. Here he found during digestion a vigorous muscular churning and mixing of the food mass with the gastric juice. For a long time this was supposed to represent the state of the entire stomach contents. This view has now been abandoned as the result of a number of recent investigations, among which those of Cannon and of Grutzner are of especial interest.

When a small amount of an inert metallic compound such as bismuth subnitrate is mixed with the food, it becomes possible to photograph the food-mass within the body by means of the Roentgen rays. By the use of this method Cannon has carried out an extended series of observations upon the movements of the stomach and intestines during digestion,1 upon the results of which the statements concerning the mechanism of digestion in this chapter are chiefly based.

Cannon's observations, confirmed by those of other investigators, show that the vigorous muscular movements described by Beaumont, and which generally begin 20 to 30 minutes after the beginning of a meal, occur only in the middle and posterior, or pyloric, portion of the stomach, while the anterior portion, or fundus, which serves as a reservoir for the greater portion of the food, is not actively concerned in these movements and does not rapidly mix its contents with the gastric juice.

That there is no general circulation and mixing of the entire stomach contents during or immediately following a meal is further shown by the experiments of Grutzner, who fed rats with foods of different colors and on killing the animals and examining the stomach contents found that the portions which had been eaten successively were arranged in definite strata. The food which had been first eaten lay next to the walls of the stomach and filled the pyloric region, while the succeeding portions were arranged regularly in the interior in a concentric fashion (Fig. 5). In describing this result Howell says: "Such an arrangement of the food is more readily understood when one recalls that the stomach has never any empty space within; its cavity is only as large as its contents, so that the first portion of food eaten entirely fills it, and successive portions find the wall layer occupied and are therefore received into the interior."

Fig. 5. - Section of frozen stomach of rat during digestion to show the stratification of food given at different times. (Grutzner.) The food was given in three portions and colored differently. Reproduced from Howell's Textbook of Physiology, by permission of the W. B. Saunders Co.

1 These and other investigations are fully discussed in Cannon's Mechanical Factors in Digestion. See also Carlson's Control of Hunger in Health and Disease.

The character of the gastric juice secreted in different parts of the stomach varies considerably, especially as regards its acidity. In the middle region the secretion is rich in acid, while both in the cardiac region and at the extreme pyloric end, the "border cells" or "cover cells" (from which the secretion of the acid appears to take place) are few in number or entirely lacking, and the juice secreted in these regions may be neutral or, according to Howell, even slightly alkaline.

The nature and extent of the muscular movements also vary greatly in the different regions of the stomach. The peristaltic waves of muscular constriction which bring about the thorough mixing of the food with the gastric juice begin in the middle region and travel toward the pylorus. Over the pyloric part of the stomach when food is present constriction waves are continually coursing toward the pylorus. The food in this region is first pushed forward by the running wave and then by pressure of the stomach wall is returned through the ring of constriction. Thus the food in this portion of the stomach is thoroughly mixed with the gastric juice and is forced by an oscillating progress toward the pylorus.

The food in the cardiac end of the stomach is not moved by peristalsis, and so comes only slowly into contact with the gastric juice; and since the juice secreted here contains little if any free acid, a large part of the food mass remains for some time (variously estimated at from 30 minutes to 2 hours or more) in approximately the same neutral or faintly alkaline condition in which it was swallowed, and salivary digestion continues in this part of the stomach without interruption. Thus, if the food has been thoroughly chewed and well mixed with saliva before swallowing, much if not most of its starch may be converted into dextrin and maltose in the cardiac region of the stomach before the activity of the ptyalin is stopped by contact with the acid of the gastric juice.

The fundus, however, is not entirely inactive, but acts as a sort of elastic pouch which is distended by and slowly contracts upon the food mass, thus gradually tending to move the posterior portions and particularly the more fluid portion into the pyloric region. As digestion proceeds, the pylorus opens more frequently and the stomach tends to empty itself more and more freely, until finally the pylorus may open to allow the passage of particles which have not been acted upon by the gastric juice. Whether the stomach will thus completely empty itself of one meal before the eating of the next will depend of course upon the length of the interval and the amount and character of the food composing the meal. Small test meals may disappear in from 1 to 4 hours, but meals approximating one third of the day's food may not disappear entirely from the stomach during 6 or 7 hours.