My lord the stomach! He is not the only, nor is he the chief, agent in digestion; but with him rests the decision as to whether the food offered to the alimentary tract is suitable in quality and quantity. He is offended if it be not offered with all the circumstance and ceremony which becomes his rank. As an intimation that he is about to receive food, he accepts the news from the mouth that its nerve-endings are subject to mechanical stimulation. But the chewing of indiarubber would produce a like effect. The stomach, therefore, confers with the organs of taste and smell. If their report is favourable, he argues that the substance which the teeth are crushing will justify an outflow of gastric juice. He responds most generously when prolonged mastication assures him that he may trust to receiving the food in a sufficiently subdivided state. At our peril we neglect to propitiate my lord. Not always debonair when treated with consideration, he is morose or petulant when slighted. Never content with lip-service, he exacts the labour of teeth and tongue and palate. The tribute we offer may be of the best—savoury, wholesome, well cooked, well chewed—but if it be not tendered with some degree of love, if thoughts are concentrated on other things, if no attention is devoted to the meal, if no sense of liking accompanies our offering, my lord the stomach on his part affords the viands an indifferent reception. In consulting our own tastes we are to a large extent consulting the needs of the stomach. Ravenous and excessive feeding is not an exhibition of taste; it is a return to the instinct of the savage, who was never sure that he would get his full share, and was afraid to trust that another meal would be obtainable when nature declared it due. Some degree of epicureanism is favourable to digestion. The flow of gastric juice in the stomach occurs reflexly in response to the emotion of appetite, to stimulation of the nerves of taste and smell, to the obscure sensations which accompany the activity of the muscles of mastication.
The gastric juice secreted in a day amounts probably to about 8 or 9 pints. To this we must add, when considering the quantity of fluid which passes through the stomach, the saliva, which certainly reaches as much as 2 pints, and the beverages taken with food.
Gastric juice collected in the manner described above is a clear, colourless, inodorous fluid. It is very acid, and so powerfully peptic as to digest its own weight of coagulated white of egg. Its solid constituents amount to 0·5 per cent. They consist of the two ferments pepsin and rennin, with traces of proteins and mucin, and various inorganic salts. Its acidity is due to free hydrochloric acid to the amount of 0·2 per cent. This acid is more or less in combination with the pepsin. In pure gastric juice hydrochloric acid is the only acid present; but when mixed with food the juice contains other acids also, especially lactic.
When food first reaches the stomach, the alkaline saliva which accompanies it neutralizes the acidity of the gastric juice. For some time, probably about half an hour, the conversion of starch into sugar is still carried on by the ptyalin of the saliva, owing chiefly to the difficulty which the gastric juice encounters in permeating the masses of masticated food. The Bacillus acidi lactici is always present in the stomach. It converts some of the sugar into lactic acid; of this a small quantity is further changed into butyric and acetic acids, with the formation of carbonic acid and hydrogen gas. After a while the lactic acid is absorbed, and hydrochloric acid alone remains.
The secretion by the gastric glands of so powerful a mineral acid as hydrochloric has always aroused interest. How is it possible for the gland-cells to produce it without injury to them selves, or for the stomach to contain it without self-digestion? Many chemical and physical theories have been advanced in the belief that they rendered the process of its production less difficult to understand. All such theories are, however, inadequate to explain the secretion as a discontinuous process, which occurs only as a response to demand. That the source of the acid is the sodic chloride which the gland-cells take from the blood does not need assertion, but we cannot picture the process by which this exceedingly stable compound is decomposed otherwise than on the assumption that weaker acids, or, rather, acid salts, are also absorbed by the cells, and that, in accordance with the laws which govern the composition of salts in solution, an exchange of acids occurs. If sodic chloride and any acid salt—acid phosphate of sodium, for example—are in solution in water, the salts do not retain their form as we know them when isolated by crystallization. The mixture contains “free” hydrochloric as well as “free” phosphoric acid. It may be assumed that within secreting cells a similar exchange of acids takes place. By a process which we term “vital,” the acids are kept apart, and the hydrochloric acid is extruded by the cells. In the present state of knowledge this vital action is mysterious; but it is no more mysterious than the isolation of pepsin, or any other metabolic event which occurs within a cell.
The proteolytic ferment pepsin is active only in an acid medium. Yet apart from its digestive function as an ally of pepsin, hydrochloric acid by itself also exerts a valuable disintegrating action on certain constituents of the food. Possibly the most important results of the presence of free hydrochloric acid in the great chamber into which food is first received are due to its disinfective property. It destroys all the putrefactive germs which accompany the food, and many germs which, if introduced into the blood, would give rise to disease. It also destroys the germs which multiply in the stomach towards the end of each interval between two meals. When withdrawn from the body, gastric juice will keep an indefinite time, if evaporation of the acid be prevented.
Pancreas.—In structure the pancreas presents a marked resemblance to the salivary glands. Probably this resemblance is merely superficial. Minute examination reveals points, apparently of great morphological importance, in which they differ. In the gland-tubes of the salivary glands, and, indeed, in all glands with the exception of the pancreas, secreting cells project into the lumen. The secreting cells of the pancreas are invested internally by a layer of flattened scales (intra-acinar cells). They lie, therefore, between the basement membrane which invests them externally and this second layer of flattened cells which separates them from the lumen of the tube. At a very early date in embryonic life the gland-cells of the pancreas are filled with highly refracting granules. As this occurs long before any digestive action is called for, it may be taken as indicating that the pancreas has functions which other glands—the salivary, for example—do not possess. These granules do not, however, appear in all parts of the tubes. Certain portions of the tubes remain undeveloped—fail, that is to say, to acquire a secreting function—even in adult life. Such patches of cells, not disposed in gland-tubes, are known as islands of Langerhans. When the pancreas is over-stimulated by artificial means, leading to its extreme exhaustion, large portions of its glandular substance return to this primitive condition. The gland-cells not only discharge their stores of granules, but they lose the greater part of their cell-protoplasm. It would seem that, in their effort to meet the demand for ferments, they use up their own cell-substance in their manufacture. Having exhausted their coal, they stoke the furnace with the looms and furniture of the mill. It may be that other glands would do the same if it were possible to stimulate them as strongly as the pancreas can be stimulated. The result is probably due to the extreme susceptibility of the pancreas to the action of secretin, a substance made in the intestine. Secretin can be isolated and injected into the blood. We shall refer again to this chemical stimulation of the pancreas when tracing the progress of food through the alimentary canal.
The secretion of the pancreas is a clear, colourless, alkaline liquid of syrupy consistence. The quantity of juice secreted is relatively small, but the organic substances which it contains are in a concentrated form. They constitute as much as 10 per cent. of the pancreatic juice. Proteins are present, if the juice be fresh. If it has stood for any length of time, they are found as peptones. The digestive ferments of pancreatic juice are the most powerful which are secreted into the alimentary canal.
Bile.—In its most important functions the liver has no relation to digestion. It is a storehouse of absorbed food. This organ will therefore be treated in a separate chapter. The bile which the liver secretes into the alimentary canal has no chemical action on any of the constituents of food, with the exception of a feeble tendency to digest starch. Yet it is in some degree accessory to digestion. Poured into the second portion of the duodenum through an orifice common to the liver and the pancreas, it mingles with the semi-digested food, or “chyme,” which, about two hours after a meal, passes through the pyloric valve. Gastric digestion has converted the greater part of the proteid constituents of the food into peptones or intermediate stages. The proteoses or propeptones—a name is needed for the intermediate products of proteid digestion which does not commit us to any theory as to their chemical constitution—are quickly peptonized by the pancreatic juice. But portions of the proteins have escaped the action of gastric juice, or have at most been affected by its acid only; these are precipitated by the bile-salts on the mucous membrane of the small intestine, which is raised into projecting flanges for the purpose of delaying the passage of the chyme, in order that it may be thoroughly submitted to the digestive action of pancreatic juice. Bile-salts also favour the digestion of fat, and its passage through the intestinal wall. The action of bile-salts in spreading fats is well known to artists. Ox-gall is smeared upon glass when it is desired to apply oil-paints to its surface. When mixed with oil, it causes its emulsification, or breaking up into microscopic globules. In the absence of bile, but little fat passes into the lymph-vessels which convey digested food from the intestine to the thoracic duct, and so to the great veins of the neck. Its action is mechanical. It favours the digestion of fats by rendering them easily amenable to hydrolysis by pancreatic juice.
Bile as secreted by the liver is a clear, limpid fluid of low specific gravity; but during its stay in the gall-bladder it is concentrated by absorption of water, and mucin is added to it. It contains “bile-salts” of complex constitution. These salts favour the solution of certain by-products of cell-metabolism, cholesterin and lecithin; substances which are formed in many cells, both in animals and plants. Cholesterin occurs most abundantly in nerve-tissue and in blood-corpuscles. Lecithin also is a by-product of the metabolism of nerve-tissue. Protoplasm appears to be incapable of oxidizing these substances, as it does other products of metabolism. Other substances of equally complex constitution are reduced to urea if they contain nitrogen; to water and carbonic acid if nitrogen be absent. Cholesterin and lecithin have to be eliminated without further change. Some of the cholesterin is excreted by the sebaceous glands of the skin. It is the chief constituent of “lanoline” prepared from sheep’s wool; an unguent which owes its valuable properties to the resistance which cholesterin offers to cell action, and therefore to the action of living ferments. Bacteria cannot turn it rancid. The sebaceous glands have the power of directing metabolism into a channel in which cholesterin is the chief product, but apparently all cells make it in small quantity. The bile-salts carry cholesterin and lecithin into the alimentary canal, from which they are not reabsorbed. Some of the bile-salts are lost to the body, but the remainder re-enter the circulation, and recommence their work as vehicles for these inoxidizable and insoluble substances. In the gall-bladder cholesterin is apt to separate out from the bile in the form of gall-stones; but whether this is due to an excess of cholesterin in the bile, or to an abnormal, inflammatory condition of the lining membrane of the gall-bladder, is still an open question.