Digestion is the solution of the food in the stomach for purposes of nutrition. Nitrogenous materials, egg, meat, muscle are digested principally in the stomach, but not entirely so, because the particles of albuminous food which pass from the stomach into the upper portion of the small intestine come into contact with the pancreatic fluid, which is the digestive agent for fats and starches, but which possesses also powerful digestive properties for nitrogenous or albuminous substances and even in a greater degree than the gastric juice of the stomach, which has a strong acid reaction, while the pancreatic juice has an alkali reaction; here is a very interesting illustration how similar digestive processes are accomplished under opposite chemical conditions. The intelligent reader must not fail to observe, from what has been said, that there are two distinctly different digestive processes, namely one going on in the stomach and another equally as important taking place in the small intestines.
Pepsin is the active principle of the gastric juice, held in solution in a clear colorless liquid, principally water; it has a sour taste and a peculiar characteristic sour smell. The length of time required by the gastric fluid to dissolve the food depends greatly upon the minuteness of the division of the solid substances to be acted upon, as well as upon the quantity and quality of the peptic fluid.
The pepsin changes the physical properties of nitrogenous substances so as to make them soluble in water in any proportion and when the albuminoids have acquired this property they are termed peptones; peptones are simply nitrogenous food which has been modified to fit it for absorption and nutrition of the body.
In the tissues of the body there are continual changes going on, termed in technical language tissue metamorphosis; the waste products that are thus formed constitute a group of highly nitrogenized substances, which in a healthy condition of the system are eliminated by the kidneys. These nitrogenized products are urea, urate of soda and uric acid; the accumulation of any of these substances in the blood gives rise to disease; the former is the cause of uræmia and uræmic convulsions, and the latter have been detected in the blood and exudations in cases of gouty and rheumatic disease. The characteristic gouty deposit is urate of soda, due to an excess of nitrogenous elements of the blood. An excess of uric acid constitutes a disease which is first recognized by a reddish crystalline sediment in the urine; the term lithiasis has been employed to denote this peculiarity. These lithates are always to be found in the urine of high livers and are due to an excessive consumption of nitrogenous food. The great English authority, Dr. Murchison, looks upon the excessive production of uric acid, or lithic acid, as it is sometimes called, as due to one of two causes and sometimes to both, namely to the excessive consumption of nitrogenous food, or to an inability of the liver to perform its duty, which among other things is to dispose of the nitrogenous waste products. From this point of view lithates or brick dust deposits in the urine are no sign that the kidneys are deranged, but quite the reverse may be true when the kidneys are overburdened in eliminating this excessive waste, for they are performing extra duty, which excess in living or a sluggish liver imposes upon them, and this may excite inflammation in the tissues of the kidneys and develop into what is known as Bright’s disease.
This very interesting exposition of the deposit of lithates or gravel in the urine will naturally suggest that when the urine is overloaded with these nitrogenous products nitrogenous food like eggs, cheese, beef, etc., should not be eaten for awhile, and that a vegetable diet should be principally relied upon, thus giving the liver a vacation; the diet should be supplemented with plenty of clean fresh water, and no liquors of any sort should be taken by those whose liver is affected.
Starch, sugar and fat are composed of carbon, hydrogen and oxygen; starches and sugars are termed carbo-hydrates; because the hydrogen and oxygen is always present in them in equal atomic weights so as to represent water; while in the fats the oxygen is considerably less. This is exemplified by comparing the chemical equivalents of starch and cane sugar with those of fats:
| Starch | C12H10O10 | Oleine | C94H87O15 | |
| Cane sugar | C12H11O11 | Margarine | C76H75O12 |
When starch is boiled with diluted nitric, sulphuric or muriatic acid for thirty-six to forty hours, it becomes colorless and thin like water, and is converted into a species of sugar. A similar process takes place when starch is taken as food, the diastase of the saliva, and the pancreatic and intestinal juices change starch into glucose, in which form it is ready for nutrition.
Cane and other sugars introduced into the system as ingredients of fruits and vegetables are not absorbed as such, but undergo a process of digestion which converts them into grape sugar or glucose; after that they are suitable for nutrition and absorption into the blood, but not before.
The digestion of starches and sugars has been incidentally referred to as not taking place in the stomach but in the upper portion of the small intestine, through the agency of the pancreatic juice which has the peculiar property of converting farinaceous and saccharine matters into grape sugar. But this transformation into grape sugar is not the final product of digestion. In the normal process of intestinal absorption the grape sugar is taken up by the portal capillary vessels and carried to the liver, where under the influence of this organ it is changed into liver sugar or glycogen, and it is as glycogen that it again enters the circulation, to disappear in the lungs. But if the liver from disease or other causes fails to perform this task, the glucose passes through the organ unaltered, and as such again enters the circulation where it acts as an irritant and finally is eliminated by the kidneys in the urine giving rise to the affection known as diabetes.