There can be little doubt that the conversion of proteid into peptone is normally brought about by the pepsin, which acts as a ferment, in some way or other facilitating a process which without it is extremely difficult to accomplish. Proteids may, however, give rise to peptone without the presence of any pepsin, if they be treated with strong acids, alkalies, boiling under high pressure, putrefactive and other fermentative actions. This, together with the analogy suggested by the chemical details of the amylolytic action of saliva, which one may say depends on a molecule of water being taken up, suggests that the change of proteid into peptone is also hydrolytic, the peptones being simply an extremely hydrated form of proteid.*
So far we have found that the action of the gastric juice affects proteids alone.. Its action on other constituents of food varies. Gelatinous material is dissolved by the gastric digestion and rendered incapable of forming a jelly; its conversion into peptone has, however, not been established. The connective tissue of meat and adipose tissue is therefore soon removed, and the muscle fibres fall asunder, the sarcolemma is dissolved, and the muscle substance converted into true peptone. The delicate sheets of elastic tissue, such as basement membranes and those of small vessels, are dissolved, but larger masses of yellow elastic tissue are not affected by the gastric digestion. The horny part of the epidermis, hairs, etc., are quite unaltered, and also the mucus, which passes along the alimentary tract without change. Bone dissolves slowly, the animal part being attacked at the surface by the gastric juice and the acid slowly removing the salts.
* Though proteids will not diffuse through a dead animal membrane when distilled water is used, a fair amount of diffusion takes place if a suitable solution of common salt be employed instead of water. It must also be remembered that the gastric mucous membrane is a living, active structure, and that the fluid into which the albumins have to diffuse may be regarded as a salt solution. It is therefore quite probable that a considerable quantity of albumin may be absorbed as such. The fact that peptone cannot be found in any quantity in chyle or portal blood tends to prove that the albumin does pass through the stomach wall without being changed into peptone.
The action of the gastric juice on milk is peculiar. On reaching the stomach, milk is curdled by a special ferment formed in the gastric mucous membrane. This ferment, known as "Rennet," is made from the stomach of the calf, and used in the manufacture of cheese. The precipitation of the casein (alkali albumin), which gives rise to the curdling of the milk, is not brought about by the hydrochloric acid (although the acidity would be quite sufficient), because neutralized gastric juice has the same effect. It appears that a special ferment (not pepsin) which directly affects the casein and causes its coagulation, must exist. It is not due to common lactic ferment, for though lactic acid is produced, it is formed too slowly to account for the very rapid coagulation of milk which occurs in the stomach.
The gastric juice has little effect on vegetable food in general, though well-masticated bread may be very materially altered, owing to the action of the saliva on the starch continuing until the mass is broken up, and the gastric juice then dissolving the proteids (gluten). The greater part of the substance of bread, however, leaves the stomach in an imperfectly digested state.
In short, the amount of change which any given form of food will undergo in the stomach will depend on the amount and exposed condition of the proteid it contains.
In recapitulating the chief events of gastric digestion, it must be remembered that while the food is yet in the mouth the secretion of the gastric juice commences, and is greatly increased by the arrival of a bolus of food and a quantity of frothy alkaline saliva.
As the stomach is filled, more and more secretion is produced, and as some food is absorbed an additional stimulus is applied. Being kept in motion in a large quantity of liquid which dissolves the cases in which the food particles are contained, the bolus of food soon falls asunder and each of its ingredients is fully exposed to the action of the gastric juice. The acid reaction of the gastric fluid neutralizes the alkalinity of the saliva, so that the action of the ptyalin is hindered, and the starch granules float about quite unaffected by the pepsin or hydro-chloric acid. The heat of the stomach melts the fats, and the motion breaks up the oily fluid into smaller masses. They are then mingled with the general liquid, which becomes more and more turbid owing to the admixture of starch granules, fat globules, dissolved parapeptones, and minute particles of partially digested proteids. This dull-gray, turbid fluid is called chyme. The proteids (the class of food stuffs affected by the gastric digestion) are changed more or less rapidly according as their particles are small and uncovered, or large and massed together, so that they are more or less readily reached by the gastric juice, and also in proportion to the facility with which they form acid albumin. The chyme contains but little peptones, so we may conclude that, when formed, they are rapidly absorbed, as are also the soluble sugar and ordinary fluids taken with the food. The chyme begins to leave the pylorus soon after gastric digestion has begun, some passing into the duodenum in about half an hour. The materials which resist the gastric secretion, or are affected very slowly by it, are retained many hours in the stomach, and the pylorus may refuse exit to such materials for an indefinite time, so that after causing much uneasiness they are finally removed by vomiting. However, many solid masses, unchewed vegetables, etc., escape through the pylorus when it opens to let out the chyme.