DIETETICS, the science or philosophy of diets ; that which teaches us to adapt particular foods to particular organs of digestion, or to particular states of the same organs ; so that the greatest pos sible portion of nutriment may be ex tracted from a given quantity of nutritive matter ; or a sufficient portion may be obtained with the least possible quantity of organic action and exhaustion. In this sense, the science of dietetics em braces a knowledge as well of the organs and economy of digestion, as of the sub stances to be digested ; and under this division we shall treat of it in the sketch before us.
The organs of digestion differ exceed ing in different classes of animals ; but in all, even in zoophytes and infusory worms, there is one which answers the purpose of a stomach, the most important of all the digestive organs. In the more perfect animals, the salivary glands, the pancreas, and the liver, are all said to concur with the stomach, and, perhaps, the smaller intestines, in the process of digestion ; and according to Cruikshank, about. a pint of gastric, or stomach secre tion, half a pint of saliva, half a pint of pancreatic juice, and twenty ounces of bile, are poured into the human stomach in the period of every twenty four hours: while the same process is aided by a considerable quantity of solvent fluid, of a different kind, secreted through the whole length of the internal surfaces of the intestines. Yet as some doubts havol been entertained as to the. relative con') tributions of these different viscera ; and as in different classes of animals they vary in every possible mode of deficiency, till at length, in the lowest orders, we find nothing but the stomach itself left to maintain the entire economy; more espe cially as we cannot at present enter into the question of the relative importance of the rest ; we shall confine our observa tions almost exclusively to the stomach; and shall only glance at the collatitious, • viscera, as we may perceive it absolutely; necessary.
If we look back into antiquity, we shall find that the earliest opinion on the cause of digestion was that of putridity. It was by this process that both Hippo crates and Empedocics supposed the food when taken into the stomach, to be- , duced to a proper state for the support of ; the animal system. Galen and his dis-1 ciples conceived an idea, that it was brought about by heat. Van Helmont; whose wild conjectures can only be ac counted for by the spirit and enthusiasm of alchemy which raged in his time, at tributed digestion to the vital energy of the soul, which resided, as he thought, in the stomach.
Grew and Santarelli were of opinion, that the spirits which are poured forth from the nerves of the stomach served for the concoction of the food. Boer haave, who has in reality only attempted to reconcile the variety of opinions that had been proposed before him, supposes there are twaprincipal agents in this vital function, viz. the different fluids that
are collected the stomach, and the me• chanical action of that organ ; the secon dary agents, according to him, are heat, air, the nervous fluid, the remains of the food, and an incipient fermentation, op posing it in the extensive sense in which it was considered before him. With re spect to the gastric fluid, his ideas ap pear to be indeterminate and unsettled : he, however, conceived that its action on the food was merely as a simple diluent, like water, when heated to the same tem perature. He had no suspicion of its being a solvent, or that it was capable of, acting upon the more tenacious and hard substances that were taken in as food. According to Pringle and Macbride, di gestion is carried on by a complete fer mentative process. The food, divided by mastication, and penetrated by the saliva, begins, as soon as it enters the stomach, to be agitated by that intestine motion which always accompanies fermentation : this motion is excited by the warmth of that viscus, by the old remnants of the food, by the gastric fluid, and more par ticularly by the saliva, which is above all adapted to produce and promote this process. They supposed that the first effect of this intestine commotion is to raise the solid parts of the aliment to the surface of the gastric liquor, where they will be for some time sustained by the air bubbles, which, on their ceasing, must fall down again, and be thoroughly incor porated with the fluids of the stomach. This mixture is rendered still more com plete by the peristaltic motion, the alter nate pressure of the diaphragm and abdo minal muscles, and the continual pulsa tion of the adjacent large vessels : in this state the food passes into the small intes tines, where the fermentative motion pro duces still greater changes by the assis tance of the bile and pancreatic juices ; it is then converted into chyle. Accord ing to the opinion of Haller, the gastric juice is more or less acid in different ani mals ; its action on the food very much resembles that of water, in which a little salt has been dissolved, which, from ex perience, is known to possess a very great resolvent power ; and the conse quence is, that an incipient fermentation takes place, which reduces the aliments to a pultaceous mass. In animals that feed on seeds, this process is assisted by trituration. These, with many other fan ciful opinions, took place in their turn, when Cheselden by chance happened to conjecture right, viz. flat digestion was performed by some unknown menstruum. This conjecture was confirmed by Reau mur and Spallanzani, who have proved the menstruum to be the gastric juice, by a number of experiments, a general view of which it will be necessary to give.