At the end of twenty-four hours, the three portions were examined. That contained in the warm juice was completely dissolved, and presented the usual appearances. The portions contained in the cold juice and in the cold water very much resembled each other, and exhibited no appearance whatever of chyme. They were macerated or softened, but not digested. These experiments, and others of a similar nature, shew clearly that a temperature equal to ordinary blood-heat is requisite for chymification.
To make sure that it was the low temperature alone which prevented the occurrence of digestion in the experiment detailed, Dr Beaumont now placed the vial containing the meat which had been exposed without effect for twenty hours to the action of the cold gastric juice on a water-bath at the ordinary blood-heat. In a very short time “digestion commenced, and advanced regularly as in the other parcels.” The same results were always obtained from a repetition of these experiments, so that they may be held as perfectly conclusive in establishing the essentiality of heat to the digestive process. Common observation, indeed, establishes this truth. Dr Kitchener, for example, after stating that “a certain degree of heat is absolutely necessary to excite and support a regular process of digestion,” remarks, that, “when the circulation is languid and the food difficult of solution in aged persons and invalids, even external heat will considerably assist concoction, and the application of the calefacient concave (stomach warmer) will enable the digestive organs to overcome refractory materials, and convert them into laudable chyle.”[30]
Thirdly.—The necessity of gentle and continued agitation for the accomplishment of digestion, is so obvious from the preceding exposition, that it requires no direct experiments to establish it. When portions of meat were suspended in the stomach, by a string so short as to prevent them from being fully subjected to the motion already described as always going on during digestion, the action of the gastric juice was confined almost entirely to their surface, and a longer time was consequently required for their solution than when they were left at liberty. In like manner, when meat out of the stomach was placed in a vial containing gastric juice, its solution was uniformly accelerated by gentle agitation, which acted simply by removing the coating of chyme as it formed on the surface, and thus affording to the gastric fluid an easier access to the undigested portions below. Accordingly, when in one of Dr Beaumont’s experiments two ounces of unmasticated roasted beef were introduced through the external aperture into the stomach, and held by a string, only one-half of it was digested in four hours, evidently from the want of mastication confining the action of the gastric juice to the surface of the mass, and because the string prevented it from following the regular motions of the stomach.
Having now made the reader sufficiently acquainted with the agents concerned in, and the conditions essential to the performance of digestion out of the body, we have next to exhibit the same agents and the same conditions in their ordinary operation in the living being, and to describe the beautiful arrangements by which they are respectively and unerringly regulated.
It has been already shewn, that, in endowing us with appetite, Nature has intended both to insure by its means a timely provision for the wants of the system, and to guard against our eating more than enough to supply them. We have also seen that, within certain limits, the quantity of gastric juice secreted bears a direct relation to the quantity of food consumed; that when the food exceeds considerably the real necessities of the system, a part of it remains undigested, because the stomach is unable to secrete a sufficiency of fluid for the solution of the whole; and that, as a necessary consequence, indigestion follows. This being the case, we may expect to find all the arrangements of Nature made with a view to prevent us from hastily filling the stomach to repletion, without being fully warned beforehand of the error we are committing. And such accordingly is the fact.
Considered in this light, the processes of mastication insalivation, and deglutition, are not only useful in preparing the morsel for the future action of the gastric juice, but, by transmitting the food to the stomach in small portions at a time, likewise serve the important secondary purpose of preventing its too rapid or excessive distention. To this good end, indeed, the stomach itself contributes, as has been distinctly shewn by Dr Beaumont. In the natural state of that organ, a regular and gentle contraction of its whole fibres and cavity follows the introduction of each individual morsel, and it is not till the relaxation consequent on that contraction takes place, that another is willingly admitted. This arrangement was more than suspected by other physiologists, but it remained for the American experimenter to demonstrate its existence and purposes. It is true that, during a hurried repast for example, food may be rapidly introduced into the stomach by an active effort of the will, but it is precisely in such circumstances that we are apt to eat too much, and that indigestion follows; because, from no time being allowed for the secretion of the requisite quantity of gastric juice, and its proper mixture with each portion of the aliment, the stomach is placed in an unnatural situation, and its nerves cannot receive the same impression of “enough eaten,” which is designed by Nature to arise only from the one being duly proportioned to and mixed with the other. The advantage of the natural arrangement is therefore confirmed rather than refuted by what may at first appear an exception.
When Dr Beaumont depressed the valve in St Martin’s stomach, and introduced a few spoonfuls of soup at the orifice, he observed the rugæ or folds of the mucous membrane to close gently upon it, gradually diffusing it through the gastric cavity, and completely preventing the entrance of a second quantity till this diffusion was effected—when relaxation again took place, and admitted of a farther supply. When solid food was introduced in the same way, either in larger pieces or finely divided, the same gentle contraction and grasping motion were excited, and continued from fifty to eighty seconds, so as to prevent more from being introduced without considerable force till the contraction was at an end. When St Martin was so placed as to admit of the cardia or upper orifice of the stomach being brought into view, and was then made to swallow a morsel of food in the natural way, a similar contraction of the stomach, and closing of its fibres upon the bolus, was invariably observed to take place; and till this was over, a second morsel could not be received without a considerable effort. And accordingly, when, either from haste or hunger, we disregard the order of Nature, and hurriedly gulp down food without due mastication, and without allowing time for the regular contraction of the stomach, we necessarily expose ourselves to the risk both of overloading it, and of ultimately impairing its digestive power.
Such being the provision made for insuring the gradual admission of food into the stomach, the next requisite is its proper admixture with the gastric juice.