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The chemistry of digestion is a subject full of interest, and one that has received considerable elucidation from the researches of later years. The food begins to undergo change as soon as it is brought into contact with the secretion from the salivary glands, and passės through the process of mastication, although this is especially the case with starchy and farinaceous substances, which are converted into sugar. The saliva is naturally alkaline in its reaction, and contains a substance to which the name of ptyalin has been applied; this chemical substance resembles diastace. It induces a rearrangement of the elements of the starch, so that saccharine principle is produced; the saliva also contains sulphocyanides and a large quantity of saline material. Thus, thorough mastication serves a double purpose, not only to break down the solid portions of food, so that they may more readily undergo solution by the gastric juice, but to incorporate the salivary fluid, so that it also may be brought into contact and exert its energy upon the starchy elements. This metamorphosis
is extremely rapid, and it continues throughout the whole of the masticatory movements, during deglutition, and till the food reaches the stomach; and even then, it is not checked, although more important reactions take place by means of the gastric juice. In the duodenum and small intestine the same action persists as was commenced in the mouth, but with less energy. It is of great importance, therefore, that mastication should be thoroughly and efficiently executed, and the defect in this process is the first step towards the abnormal fermentative changes in the form of dyspepsia, which we have now before us.
In the stomach a different set of glands, peculiar to itself, secretes an acid fluid, known familiarly as the gastric juice ; this secretion contains a nitrogenous substance, pepsin, and an acid, variously regarded by chemists as hydrochloric or as lactic acid, besides saline materials and water. The pepsin acts upon fibrinous substances, causing their solution, without itself entering into combination or becoming decomposed; the fluid that is formed, although fibrinous or albuminous, possesses different properties from a mere solution of a protein compound, for it is not coagulable by heat; the term peptone has been applied to it, and the whole solution has been designated chyme.
The chemical solution of the nitrogenous portions of food by the pepsin is limited, but it is greatly increased by the presence of the acid of the gastric juice, and by the heat of the stomach and churning movements of the muscular walls. The food rotates along the lesser, and then from right to left backwards by the greater curvature; as the chymous fluid is formed, it passes by the pyloric valve into the duodenum, when it is mixed with the bile, and with the pancreatic secretion before it reaches the small intestine, and is absorbed into the system by the capillary veins and the lacteal processes.
This process of solution or digestion requires, according to the nature of the food, from two to four or five hours; and many substances that are taken cannot be dissolved at all, but pass in their crude state into the intestine ; and although the solvent powers exerted by the gastric juice are especially upon the protein compounds,—the nitrogenous elements,—still the changes upon the starchy portions commenced in the mouth are not entirely checked; the oleaginous elements are more thoroughly divided, and the saline and aqueous constituents are diffused completely through the semi-fluid aliment. In a normal state the solution should be completed after a certain time, and as one part after another passes into the duodenum or becomes absorbed, the stomach is left in a quiescent state. But, far different are the conditions ; found in dyspepsia of the kind we are considering; when, either from improper food, or from insufficient secretion of gastric juice, or because the fluid when formed is unable to pass from the stomach, fermentative changes take place.
The gaseous exhalations into the stomach consist of nitrogen and carburetted hydrogens, or fermentation takes place accompanied with the evolution of carbonic acid, or butyric acid is formed, or lastly, putrefactive changes arise, and sulphuretted hydrogen is produced; these several forms of fermentation arise from different causes, as we have before mentioned in speaking of distension of the stomach as a sign of gastric disease ; they produce diverse symptoms, and are amenable to various forms of treatment.
We have already enumerated several sources of gaseous distension of the stomach, and shall not again enter upon their full consideration. We have stated, 1st, that gas may be swallowed; or, 2ndly, produced by the decomposition of food in the stomach ; 3rdly, that it may be evolved from changes in the mucous secretions themselves; Athly, transuded from the blood; 5thly, regurgitated from the duodenum, or from a fistulous communication with the colon, or from some adjoining abscess; 6thly, gas of an offensive kind is produced by the decomposition of a growth in the stomach, as cancerous tumour.
The first form of gaseous distension that we have to notice, is quite independent of food, or rather is frequently produced by the want of it. The gas consists of nitrogen mixed with carburetted hydrogens and some carbonic acid; it is often suddenly evolved, and is especially connected with an exhausted state of the
nervous system ; it is often peculiarly marked in hyste rical patients, and in others it may be induced by mental excitement or depression, and abstinence from food is a very common cause of it.
This form of dyspepsia is of an atonic character; the want of power being due either to general loss of strength, or to deficient supply of nourishment. It is marked by a sense of sinking at the stornach, distension, and pain as that distension increases; the pain often commences in the back, and seems to pass round the body or through it to the scrobiculus cordis; headache is often present, and sometimes faintness ; if the pain become severe, the pulse assumes a compressive character, the tongue is not necessarily changed; eructation takes place, and with that relief, the pain subsides; or the gas passes downwards, and the pain then moves from the stomach, and entirely ceases as the gas is evaciated. This form of dyspepsia sometimes produces extreme prostration and collapse, and the severity is so great, that perforation of intestine is simulated. At the commencement of this state, if a small quantity of nourishment can be taken, the symptoms may be checked; stimulants may be given, as wine and brandy with nourishment; but if the distension have become severe, then antispasmodics are necessary, chloric ether, ether, camphor, ammonia, and when pain has supervened, opium should be given. We have known severe collapse quickly relieved by opiates freely administered;