CHAPTER II. Preliminary Remarks on Physiology and Chemistry. (Part 2)

The Gastric Juice. The gastric juice in the pure state is a clear, almost colourless, fluid, which is acid in reaction and possesses a specific gravity of 1002 to 1003. It has a stale, sour taste. Its solid constituents equal only 0.56 per cent. The amount secreted has been variously estimated; thus Beaumont found it to equal about 180 grammes, and Griinewald 1580. According to the more recent investigations, the latter is more nearly correct than the former. We must thank the genial Russian investigator, J. P. Pawlow,* and his pupils for anumber of new and fundamental facts concerning the physiology of gastric secretion. The methods of Pawlow are distinguished from those of former investigators, especially of Heidenhain, by the fact that he was the first to be able to study conveniently the gastric secretion in the dog after the introduction of certain foods, and then determine the influence of the so-called sham-feeding upon the secretion of the gastric juice. Pawlow was thus likewise able to isolate the pancreatic juice and to observe the cor- relation between gastric and pancreatic seerction. Tn order to study the se- cretion of the gastric juice and its characteristics during the act of digestion, Pawlow constructed a small blind pouch out. of a portion of the stomach, which was separated from that organ itself by a septum of mucous mem- brane, and which communicated externally by means of a fistula. The in- nervation of the blind sac remained wholly intact, so that upon the intro- duction of food into the stomach, the blind pouch portrayed a true and accurate picture of the secretory activity of the stomach without interfer- ence from the food. ‘The following conclusions were drawn: ‘The gastric glands do not begin to seerete until food is introduced ; the amount of the gastric juice is propor- P. Pawlow, The Work of The Digestive Glands, Translation, London and Philadelphia, 1902. % Sticker, Wechselbezichungen zwischen Speichel und Magensaft, Volkmann's Sammlungen klinischer Vortriige, No. 297. % Biernacki, Die Bedeutung der Mundverdauung und des Mundspeichels fiir die ‘Thiitigkeit des gesunden und kranken Magens, Zeitschr. f. klin. Medicin, 1892, Ba. 21, Heft 1 u. 2. 26 A. Schuld, Inaug.-Dissert., Leiden, 1892; Maly’s Jahresb. fiir Thierchemie, 1893, S. 257. GASTRIC JUICE. 23 tionate to that of the food. The secreting stomach is constantly acid in reaction, and the more rapidly the gastric juice appears the greater is the amount of acid secreted. The total percentage of acidity is furthermore dependent, aside from the amount of gastric juice present, upon neutralisa- tion with mucus, eic. The activity of the gastric glands, after the ingestion of single components of food (like meat, bread, milk), proceeds with an ex- traordinary exactness, both with reference to the amount secreted and the ferments contained in the juice; this process may be recorded in the form of a distinct curve. Pawlow proved, furthermore, that the gastric juice which is secreted after bread-diet possesses the greatest digestive power ; next in digestive strength comes the gastric juice following meat-diet, whereas the least digestive strength is possessed by the gastric secretion which follows the ingestion of milk-diet. The total acidity is highest after meat-diet and lowest after bread-diet. With reference to the amount of weight, the greatest quantity of gastric juice is required by meat, and the smallest quantity by milk (with reference to equivalents in nitrogen, more is needed by bread, and less by meat). Each variety of food calls forth every hour a definite amount of secretion and characteristic changes in the nature of the gastric juice. In case of bread-diet, the average amount of juice secreted every hour is one and a half times less than after milk or meat; the total amount seereted, however, is greater than in case of milk, because the digestion is prolonged over a longer period of time. A pause exists between the introduction of the food and the beginning of secretion, called the latent period, which lasts from four and a half to ten minutes. With reference to the work of the pancreas, it was found that in case of “bread-juice” amylolytic ferment mainly was excreted, and that in “milk-juice” the fat- splitting ferment predominated, and that the largest percentage of proteid- splitting ferment occurs in “milk-juice,” it being less in meat- and bread- juice. In prolonged nourishment with meat the proteolytic ferment in- creases progressively, while the amylolytic ferment gradually lessens in amount, and vice versa in case of a bread-diet. If a dog in whom an cesophagotomy has been performed, is fed (sham. feeding), an abundant amount of gastric juice will appear at the gastric fistula in five minutes. There can be no doubt, therefore, but that nervous influence plays a réle in this phenomenon. After the vagus has been severed (under proper experimental precautions), sham-fecding has no longer any influence upon the secretion of the gastric juice. Tt follows, there~ fore, that the vagus is a secretory nerve to the gastric glands, and hence it is possible to excite secretion by stimulation of that nerve. Secretion does not set in until after the lapse of a latent period (fifteen to sixty minutes), which may be explained by the probable simultaneous stimulation of inhib- itory fibers. The vagus is also the secretory nerve of the pancreas. It also 24 PHYSIOLOGY AND CHEMISTRY. supplies that organ with inhibitory influences by the activity of inhibitory fibres, which directly affect its secretion. ‘Ihe sympathetic system also car- ries vasomotor constrictor as well as secretory fibres to the pancreas. ‘An abundant secretion of saliva is effected through psychical influence, as well as by the introduction of various substances into the mouth. It is of interest to know that in order to stimulate salivary secretion, the food must be in the dry state. A large quantity of gastric juice is secreted when “sham-feeding” is instituted, as well as when the animal is tempted with the sight of meat; this phenomenon is attributable to the cager longing for food and the fecling of contentment when the desire is satisfied (“appelite- juice”). On the other hand, chemical and mechanical stimulation of the oral and mucous membranes fails to excite reflexly the secretory nerves of the stomach. The appetite is the main and strongest stimulant of the secretory nerves of the stomach. 'The psychic secretion is the normal commencement of the secretory activity of the stomach in the majority of cases. When dogs were fed with about 100 grammes of meat at intervals of one and a half hours, it was observed that the act of ingestion was accompanied every time by an increase in the digestive power and the quantity of the gastric juice. If meat be given per os to dogs which possess, besides the fistula of the small resected stomach, a second fistula of the larger stomach, it will soon pass out through the latter: the gastric juice will then be found to be identical, both in regard to the quality as well as quantity, with that of sham-feeding. No secretion follows the direct introduction of bread or boiled cgg-albumen through the fistula into the larger. stomach, although it’ does occur after the introduction of meat. The juice is, however, delayed in appearance (fifteen to forty-five minutes, instead of six to ten minutes) ; it is present in very minute quantities during the first hour (3 to 5 cubic centimetres, instead of 12 to 15 cubic centimetres), and possesses a very low digestive power. If about 100 grammes of meat are introduced directly into the stomachs of two dogs each, one of which is subjected to active sham-fecding, it will be found at the end of one and a half hours that the weight of the meat will have been reduced 30 grammes in the dog subjected to sham-feeding, and only 6 grammes in the other dog. The normal secretory curve represents a summation curve which is composed of the curve of the appetite-juice and that of the juice occurring after the direct introduction of meat. ‘The stomach can be stimulated directly only specifically, and not mechanically. The glands may, however, become active by the mechanical action of the food upon the.gastrie walls, in com- bination with the effect of desire for food. If food is introduced into the stomach of a dog by means of a tube, or, better, through a fistula, the following phenomena will be noted: Water is GASTRIC JUICE. 25 a chemic stimulant of gastric secretion, but only weakly so. ‘Lhe ash con- stituents of meat, as well as sodium chloride, soda, and hydrochloric acid, are wholly inefficient; indeed, soda may have an inhibitory action, Fluid egg-albumen, pure peptone, and the products of digestion which were obtained of the action of strong and pure gastric juice upon raw fibrine, do not excite seeretion. Meat-broths, meat-juices, and solutions of meat- extracts are energetic stimulants of the gastric secretion, Starch, grape- sugar, and cane-sugar possess no influence. On the other hand, a combi- nation of starch-paste and meat-extract doubles the quantity of the secre tion. Fats do not stimulate seeretion. After the stomach has once seereted juice which possesses the power to digest food, products are formed in the digestion of bread and egg foods which increase the digestive strength of the juice. The high digestive power claimed for “bread-juice” is truly remarkable, and is accounted for by the amalgamation of the albumen with the starch in the bread. Fat, when introduced into the stomach with other articles of food, limits secretion. When fat is directly introduced into the stomach it inhibits, both quantitatively and qualitatively, the juice secreted during “sham-feeding.” The whole sceretory activity of the stomach is therefore nervous and (excepting the psychical secretion) reflex in origin. Gastric secretion cannot be stimulated reflexly by way of the rectum, although such an effect may be obtained through the small intestines. Hence we see that in the large majority of cases gastric digestion begins with an active central (automatic) stimulation of the secretory and trophic fibres of the glands of the stomach (“appetite-juice”). After an interval of more or less time following the ingestion of food, reflex stimula- tion begins in the stomach, while the automatic, psychical stimulation gradually disappears. When meat is introduced into the stomach, the cen- tre for the scerctory fibres is energetically stimulated (copious secretion) reflexly through the end-apparatus in the stomach; when bread is ingested, the trophic fibres are markedly stimulated through the end-apparatus (high digestive power). We note, with reference to normal stimulation of the innervation apparatus of the pancreas, that acids possess the greatest power for stimu. lation of that organ. Hydrochloric, phosphoric, citric, lactic, and acetic acids are all equally effective in this respect, and when they are introduced into the stomachi, panereatic juice is secreted in from two to two and a half minutes. Pepper, mustard, cte., are ineffectual. Solutions of sugar, pep tone, and albumen, when introduced into the stomach, do not affect the secretion of the pancreatic juice unless they possess an acid reaction; neu- tralisation of the latter causes inhibition of that secretion, Pancreatic secretion becomes active only when the acids pass from the stomach into the duodenum, from which point the pancreas is stimulated reflexly. If en- 26 PHYSIOLOGY AND CHEMISTRY. trance into the duodenum is impossible, no pancreatic secretion can be excited by way of the stomach, nor is it possible to stimulate that gland by way of the rectum. Accordingly, the secretion of the pancreatic juice always follows the entrance of the acid chyme into the duodenum. ‘The panereatie juice, which flows upon the appearance of acid solutions in the duodenum, possesses a maximal percentage of inorganic constituents and only an insignificant amount of organic material. ‘The pancreatic juice itself, and also its ash, are highly alkaline. Starch does not excite the secretion of pancreatie juice, but it causes an increase in the amylolytic ferment which that secretion contains. Fat is a true stimulant of pan- creatie secretion, and increases its fat-splitting ferment. 'The secretion of the pancreatic juice does not cease during sleep. Although psychical stimu- lation of panereatic secretion may exist, yet it plays a very unimportant part. Water is an independent stimulant of the pancreatic juice. ‘The meat-extracts act only by means of the water they contain. Solutions of the alkalies, including both the salts and metals, possess an inhibitory influ- ence upon the process; likewise solutions of sugar. The gastric juice of the fundus is characterised by a powerful mineral acid—hydrochloric acid —as well as three enzymes. 1. Hydrochloric Acid. The percentage of hydrochloric acid in human gastrie juice varies from 0.1 to 0.22 per cent., but in dogs it may reach 0.3 per cent. or even higher.* Schmidt (1847) was the first to determine analytically the presence of hydrochloric acid in the gastric juice. After determining the total amount of bases present (potassium, sodium, calcium, magnesium, iron oxide, and ammonia) and the quantity of chlorine, it was found, after saturation of the bases with hydrochloric acid, that a certain amount of chlorine remained unaccounted for. This represented from 2.5 to 5 grammes of hydrochloric acid to a litre. ‘The same result was obtained by means of titration with lime and baryta-water. ‘The above-mentioned percentage of hydrochloric acid is not reached until at the height of digestion; at the commencement and end of digestion this percentage is markedly diminished. Hydrochloric acid acts in six different ways, all of which are of great importance to the normal process of digestion. 1. Hydrochloric acid in the stomach is antizymotic and antiseptic, inas- much as it prevents abnormal fermentation and destroys pathogenic organ- Schoumow-Simanovski (Archiv ftir experimentelle Pathologie und Pharma- kologie, 1894, Bd. 33) found an acid degree of 4.6 to 5.8 per mille in pure, un- mixed gastrie juice of dogs.