54 ARTIFICIAL DIGESTION. But this interference takes place only in an indirect way, for the section of those nerves is attended with such a paralysis of the stomach that those movements which so well serve to mix up the food with the gastric juice, and expel it through the pyloric valve, are put an end to. Bidder and Schmidt, from an examination of four dogs with gastric Effect of section fistulæ, demonstrated that the section of the pneumogastric of the pneumo- nerves does not exert that influence on the secretion of the gastric nerves. gastric juice which had been formerly supposed, for both in quantity and composition it remained the same. Even in those cases in which both they and others have observed a diminution in its amount, the result ought, probably, to be referred to the shock given to the entire system by the severity of the operation. The acidulating material of the gastric juice is hydrochloric acid. Is it possible by artificial mixtures containing that substance to reduce food articles to a digested condition? This inquiry introduces a description of the experimental investigations which have been made in artificial digestion. When water acidulated with hydrochloric acid is kept in contact with Artificial di- albumen, no action is perceptible at ordinary temperatures in a gestion. moderate period of time. If the temperature is raised to about 150° a slow dissolution ensues, which becomes better marked as the heat rises toward 212°. But if to the weak hydrochloric acid thus made to act on albumen, pepsin is added, the solution takes place with rapidity at moderate temperatures. An ounce of water, mixed with twelve drops of hydrochloric acid to which one grain of pepsin has been added, will completely dissolve the white of an egg in two hours at a temperature of 100°. It acts in the same manner on cheese or flesh, these nitrogenized articles being converted into soluble non-coagulable bodies. The acid does not enter into chemical combination with the dissolving organic matter. It may be recovered from the solution by resorting to proper processes. When striated muscular tissue is submitted to artificial digestion, it is first divided into its constituent fasciculi, and the transverse gestion of mus- striæ then disappear, the sarcolemma being destroyed. The course of the action seems to be the same in natural digestion. In the focal matter, shreds of muscular fasciculi still bearing their striation may be discerned. These, having by chance escaped solution during their sojourn in the stomach, have passed through the whole length of the digestive tube unchanged. Artificial di cular tissue. Pepsin-the substance resorted to in these experiments-may be obtained by macerating the mucous membrane of the stomach Pepsin, preparation and for a short time in lukewarm water. This water, along with properties of. a part of the pepsin, removes various impurities; it may there FUNCTIONS OF PEPSIN. 55 fore be cast away; the maceration being then continued with a fresh portion of cold water, and this being submitted to filtration, and subsequently evaporated at a low temperature to dryness, yields the pepsin as a gummy mass. From its solutions pepsin may be precipitated by corrosive sublimate or acetate of lead, and it may be separated from those combinations by sulphureted hydrogen. Wasmann availed himself of this fact to obtain it in a pure state. From this it would appear that it contains less carbon and more nitrothan the members of the protein group. gen un Pepsin re A weak acid therefore possesses at a high temperature the power of bringing into a state of solution the various nitrogenized food matters, and at lower degrees fails of that property; but in places a high the presence of pepsin the solvent powers are assumed temperature. der the latter circumstances, and therefore it is said of this substance that it replaces a high temperature. By its aid, hydrochloric or lactic acids present in the stomach reduce the food to a uniform pulpy mass -the chyme. Of all acids, these, however, alone are capable of forming digestive fluids. Formerly it was supposed that the act of digestion was simply mechanical, the food being ground down to chyme by the mo- Reaumur's extions of the stomach. Reaumur's experiments showed the periments with error of this supposition. He took small hollow silver balls, silver balls. perforated with holes, and, having filled them with meat, caused them to be swallowed by a dog. When they had remained in the animal's stomach a suitable length of time, they were withdrawn by a thread which had been previously attached to them. Now if the stomach acted by a triturating or grinding power, the material within the ball would be entirely protected, but if by a solvent power exerted by the gastric juice, the digestion should at most be only delayed. Accordingly, it was found that this was what actually took place, digestion being fully, though more slowly accomplished, the action commencing on the outside of the material, and gradually reaching its centre. If the balls were kept in the stomach long enough, they came out quite empty at last. Chief object of stomach diges The idea that there is something more than a simple solution of the food effected in the stomach, that some mysterious change is impressed upon it by the vitality of that organ, may there fore be abandoned. It does not appear that there is any es tion is the solu tion of the food. 56 NUTRITIVE MATTER IS DISSOLVED. sential difference between natural digestion and the artificial imitation of it, either as respects the order of action or the final result. Moreover, the anatomical consideration that the food is yet outside the body, though it is inside the stomach, should be sufficient to remove all errors of that kind. A living surface, such as the skin, never exerts any chemical action at a distance; and the lining membrane of the stomach, both as regards its physiological origin and its anatomical relation, is nothing more than a reflected continuation of the skin. The act of digestion is completed long before the nutrient material is taken up by the lacteals and veins, and thrown into the torrent of the circulation. But then, and not till then, is the food fairly in the interior of the body. The lacteals and veins can not exert their absorbent action on a substance presented to them unless it is dissolved in water. If not absolutely dissolved, at least it must be in that condition of minute subdivision which we see in emulsions. Though it has been stated that insoluble substances, such as charcoal, can find their way into the circulation in the solid state, there does not appear to be a sufficient weight of evidence to support such an improbability. In the economy of plants, it is In plants, all a general rule that nothing can have access to the interior of nutrient mate- their system except it be dissolved in water. All the varisolution in wa- ous gases and saline substances they require are obtained in a state of solution; the former are introduced, for the most part, through the leaves, the latter through the roots. The object aimed at in the construction of the digestive apparatus of the animal mechanism is absolutely the same. Plants use as their food inorganic matter only; the chief materials on which they depend, such as the salts of ammonia and carbonic acid, are abundantly soluble in water. The ascending sap rial must be in ter. obtains the former from decaying organic residues in the ground; the atmosphere presents the latter unceasingly to the leaves; and since the economy of many plants requires earthy salts, as silicates and phosphates, which are of sparing solubility in water, the difficulty arising from that want of solubility is avoided by the introduction of an immense quantity of water, which, after bringing into the plant the needful amount of mineral material, is evaporated off at the leaves. But the food of animals is essentially organic, and this, before it can be received into their blood, must be brought into the dissolved state. It must be submitted to a preparatory operation or series of operations. However complicated these The operations or the mechanism which accomplishes them may be, the end on the food are aimed at is clear. The action begins by the cutting, tearing, al and mechan- and crushing movements of the teeth, which break down all the larger portions, and carry on the process as far as it is possible by mechanical means. The stomach then continues the subdivision by chemical agency, to the end that a condition of solution may be purely chemic ical. OBJECTS OF DIGESTION. 57 attained. Digestion is not, therefore, to vitalize the food, as the ancients supposed, nor to communicate to it any new or obscure properties; it is for the purpose of comminuting, subdividing, dissolving, or bringing it into that minutely suspended state that it can without difficulty submit to the absorbing action of the lacteals and veins. There is a complete analogy between this operation and the artificial processes to which the chemist resorts in his laboratory for the solution of various bodies. He, too, uses mechanical implements—the mortar and pestle to grind, the hammer to crush, the rasp to abrade. When these have carried the subdivision sufficiently far, he resorts to acids or other solvents, and thus breaks down the compactness of the hardest minerals, and brings them into the dissolved state. The animal world presents us with a thousand illustrations of the principles here set forth, mechanical contrivances curiously arranged. For instance, birds, whose plan of organization is such as to meet the case of locomotion through the air, could not have the anterior part of their bodies loaded with teeth, accompanied as they must. have been with a powerful muscular apparatus. Such a mechanism would have rendered the animal top-heavy, and would have been totally inconsistent with flying. But, to avoid this difficulty, that which might truly be regarded as the mouth is lodged in the interior of the body, nearer the centre of gravity. It is the gizzard. Instinct teaches the bird to swallow small angular stones, and the food, rasped between powerful muscular surfaces, is soon brought into a fit condition for the action of the stomach. The chemist, too, puts fragments of glass or of quartz into the mortar in which he is conducting the reduction of a tough or resisting substance. The first object of digestion is, therefore, the subdivision of the food. The operation begins in the mouth by a resort to mechanical implements, and when these have carried the process as far as they can, the stomach continues the duty. In its cavity, when in full activity, the temperature is 100°; a periodically increasing and relaxing motion of revolution is kept up, gastric juice exudes in definite quantity, the hydrochloric and lactic acids exert their action, and in the course of three or four hours a complete reduction is accomplished. ach for differ Allusion has been made to the probability that different portions of the mucous membrane of the stomach discharge functions Regional diviswhich are wholly distinct, one portion being devoted to the ion of the stomelaboration of pepsin, another to the secretion of hydrochlo- ent functions. ric acid, another to the preparation of a special mucus. This view derives considerable support from many facts in comparative physiology. In those cases in which the food approaches, in its mechanical and chemical condition, to the form which it is destined to assume as a part of the body of the animal receiving it, the stomach is simple in construction, 58 rangement in different ani mals. DIGESTION IN INSECTS AND BIRDS. Fig. 8. and is little more than a mere dilatation of the alimentary canal. Thus, in the insect digestive tract shown in Fig. 8, h But Digestive tract of a carnivorous beetle. Even in these cases of minute organization, the mucous structure remains the same as in larger animals of the same mode Fig. 9. of life. The photographic representation in Fig. 9 displays the same reticulated appearance in the stomach of the carnivorous beetle as has been described in the case of that of man; and undoubtedly, with similarity of structure there is similarity in the manner of action. A regional division of the digestive apparatus is also presented in the case of many birds, as Mucous membrane of the stomach of a carnivo- is shown in the photo rous beetle magnified 50 diameters. Digestive com birds. graphic representation, Fig. 10, in which we have the digestive tract of the common fowl, a being the oesophagus leading partments of into the insalivating pouch or crop, b, which empties into the stomach, c, f and this into the gizzard, d. In the stomach, which is relatively small, the digesting material is mingled with the gastric juice before being submitted to the action of the gizzard. From the gizzard it is passed into the small intestine ff. In the figure, e is the liver, g, g, the cœca, and h the cloaca. Fig. 10. Digestive tract of the common fowl. |