Disregarding, however, for the present what may appear only exceptions to the general rule, we must inquire into the nature of the sub stance which is found, under ordinary circum stances, in the proper digestive stomach, after it has experienced the full operation of the gastric juice. Although many observations have been made upon the pultaceous mass which is thus produced, our information re specting it is not very precise ; we are told little more than that the texture, odour, and flavour of the food employed are no longer perceptible, and it is said to have slightly acid properties, or rather to be disposed to pass into the acetous fermentation. As we remarked above, the change which the food undergoes is to be regarded as the result of chemical action, where not merely the mechanical texture and the physical properties of the substance are changed, but where it has acquired new chemi cal relations.
This conclusion is deduced from a number of very interesting experiments, which were performed successively by Reaumur, Stevens, and Spallanzani, and which consisted in insert ing different kinds of alimentary matter into perforated tubes or balls, or inclosing them in pieces of porous cloth. These were introduced into the stomach, and after some time were re moved from it and examined, when it was found that the inclosed substances had under gone more or less completely the process of chymification, while the enclosing body was not acted upon, thus -proving decisively that the effect was not produced by a mere mecha nical operation.* The results of these experi ments have been confirmed by some remark able facts, which bear still more directly upon the point under investigation, where certain in dividuals have had preternatural openings made into the stomach, either from accident or dis ease, while the functions of the part appear to have been but little, if at all, impaired. By this means the operation that is going forwards in this organ may be minutely watched in all its various stages, and we are enabled to ob serve the change which the food undergoes from the time that it enters the stomach until it passes from the pylorus, and to compare the changes which the different kinds of food ex perience during the progress of the whole mass.
A case of this kind is related by Circaud, where an individual lived many years with a fistulous opening into the stomach ;t but a much more remarkable case of the same de scription has been lately communicated by Dr. Beaumont. The individual in question was wounded, early in life, by a shot in the epigas tric region, which perforated the stomach. After some time the wounded part healed, with the exception of an aperture two and a half inches in diameter, which communicated with the stomach. Ile lived many years in this state, in perfect health and vigour, so as to be capable of following a laborious occupation, while the fistulous opening still remained. Under these circumstances he was made the subject of experiment by Dr. Beaumont, who for the space of eight years continued his ob servations, with great assiduity and minuteness, on the action of the stomach both in its ordi nary state, and when subjected to different con ditions, for the immediate purpose of the expe riment. We may remark generally, that the results of the experiments confirm those of Spallanzani in their most essential particulars, and at the same time enable us to decide upon some points which were left imperfect by that naturalisit Among the more important points respecting the formation of chyme, which appear to be confirmed by the experiments of Dr. Beau mont, are the following; that the different kinds of aliment all require to undergo the same process, by means of the gastric fluid, in order to be reduced into chyme ; that the rapidity of the process differs considerably according to the delicacy of their natural tex ture or the degree of their mechanical division; that the saliva is of no specific use in the con version of aliment into chyme; that animal substances are more easily converted into chyme than vegetables ; and that oily sub stances, although they contain a large quantity of nutriment, are comparatively difficult of digestion.* We must next inquire into the physical and chemical properties of the gastric juice, the fluid secreted from the interior of the stomach, by which the change in the aliment, that we have been describing, is produced. Since the
publication of Iteaumur's experiments, about the middle of the last century, the geneml opinion among physiologists and chemists has been, that the gastric juice possesses specific properties, which enable it to dissolve or com bine with the aliment ; and many experiments have been performed for the purpose of ascer taining the chemical nature of the secretion, so as to account for the powerful action which it appears to possess over such a great variety of substances. Besides the more general ac count which we have of the gastric juice by Boerhaave, Haller, and Reaumurif it was made the subject of an elaborate series of expe riments by Spallanzani;$ it was also analyzed by Scopoli§ and by Carminati,11 and has been lately examined by Dr. Prout,t and by AIM. Tiedemann and Gmelin.** The result is, upon the whole, rather unsatisfactory, or at least it may be said, that nothing has been detected in the fluid, which seems to account for or explain the powerful action which it exercises on the alimentary substances subjected to its influence.ff All that we learn is, that the gastric juice contains certain saline substances in small quantity, more especially the muriate of soda, in common with the other animal fluids, but that it does not differ essentially, , in its chemical properties, from saliva, or from the secretions of mucous membranes gene rally. Dr. Prout indeed informs us, that a quantity of muriatic acid is always present in the stomach during digestion at but as there does not seem to be any decisive evidence of its appearance previously to the introduction of the food into the stomach, we ought probably rather to consider it as developed by the pro cess of digestion, than as entering into the constitution of the gastric juice; nor indeed, if it were so, are we able to explain the mode in which it operates in converting aliment into chyme.§§ This apparent difficulty in account ing for the mode in which chyme is formed by the gastric juice, and,the supposed inadequacy of the agent to this purpose, has led to many singular theoretical opinions, which will be noticed in a subsequent part of this article.* But in whatever way, or upon whatever principle we may explain the action ofthe gastric fluid upon the aliment, we are irre sistibly led to the conclusion, that it is the physical agent which produces the effect, not only from those cases, where in consequence of a preternatural opening into the stomach we are able to observe the actual phenomena of digestion, but still more so, by the expe riments on what has been termed artificial di gestion, especially those of Spallanzani and Beaumont, where the gastric juice has been procured, and applied out of the stomach, and where the process of chymification has proceeded, as nearly resembling that in the stomach itself as might reasonably be ex pected, considering the unavoidable imper fection of the experiment. This imperfection respects both the mode of obtaining the gastric juice itself, and the mode of applying it to the aliment. We reduce the action of the stomach into somewhat of an unnatural con dition in order to procure the secretion, and in the application of it we are deprived of the contractile motion of the organ ; yet, not withstanding these unavoidable circumstances, the substances were reduced to a state very considerably resembling that of chyme. That this change was not produced by a mere me chanical action is proved by the circumstance, that the change in the substances operated on bore no proportion to the hardness of their texture or other physical properties. Thus we find that the gastric fluid acts upon dense membrane, and in some cases, even upon bone, while there are other substances, of a very delicate texture, which are not affected by it. This kind of selection of certain sub stances in preference to others bears so close an analogy to the operation of chemical affinity, that we ought not to refuse our assent to the idea of their belonging to the same class of actions, although it occurs under circum stances where we might not have expected to find it.