Mal-digestion in the bowels is very common. The food may be prepared in the mouth and stomach too imperfectly for its complete digestion in the bowels. All foods are liable to undergo putrefaction in the bowels when the previous part of digestion has not been properly performed. But protein foods (meat, fowl, fish, milk, etc.) are especially liable to undergo putrefaction after they leave the stomach unless they are promptly digested. A peculiar substance called tryptophan is split off from the proteins, and from it are formed indolactic acid and indican, which are absorbed and give rise to indicanuria and symptoms of auto-intoxication. A delay in the completion of digestion in the bowels may be due to some derangement of the other alimentary organs, but especially the liver or pancreas. The patient is usually treated for the consequences. In the first instance these are the ordinary symptoms of indigestion, constipation, or diarrhea, headache, biliousness, giddiness, nervousness, mental depression or melancholia, neuralgia, neurasthenia, anaemia, chlorosis, etc. After a time, however, various organs not directly connected with the stomach and bowels begin to suffer. The heart and blood-vessels become affected, there being weakness of the muscular portion of the circulatory apparatus, causing a low-pressure pulse (hypotonus) combined with symptoms of neurasthenia; but later on there is a gradual thickening of the fibrous and other tissues in the peripheral arteries and capillaries, which causes the blood pressure to be increased (hypertonus), and in consequence the heart becomes hypertrophied, the vessels sclerosed, and the kidneys undergo those changes which are found in Bright's disease. Careful observation has shown that these changes are due to toxaemia (auto-intoxication), arising from intestinal indigestion, especially of the proteins in the food. Moreover, it has been shown that the toxaemia is least when the patient is on a vegetarian diet, somewhat greater on a milk diet, much greater on a meat diet, and greatest of all when the diet consists of eggs. It is not to be supposed, however, that indigestion or mal-digestion of fat and carbohydrates does not occur. When the pancreas or liver is diseased the digestion of fats is much delayed; they become split into fatty acids, which may or may not be of a normal type, and a considerable portion of the unchanged fat passes out in the faeces, and thus much valuable nutriment is lost to the body. The digestion of carbohydrates in the intestinal canal may be defective owing to a deficiency of the pancreatic juice or succus entericus, and any delay in their digestion gives an opportunity for intestinal bacteria to ferment the sugars, cause the generation of gases, and transform starches into irritating acids.

The fact that auto-intoxication (self-poisoning) does not constantly occur in normal individuals is due to several factors. Under ordinary conditions the number of bacteria in the intestines is kept within moderate limits. The liver when in a healthy state has the power of destroying or modifying some of the poisons absorbed from the alimentary canal and eliminating others. Some of the poisons produced are antagonistic to each other. Various poisons arising from the decomposition of food in the bowels are broken down into harmless substances. Cholin and neurin are recognized as agents which produce serious consequences in general paralysis and other nervous diseases. They originate from degenerated nerve-cells. These poisons are also produced in the alimentary canal in consequence of the decomposition of proteins; under normal conditions neurin is transformed into cholin, and cholin is broken down to ammonia, carbonic acid, and marsh-gas, which are comparatively innocuous. In abnormal conditions such poisons may be absorbed into the blood and carried all over the body.

The products of intestinal indigestion, therefore, are (a) gases and acids, (b) toxins. The first group give rise to flatulence, irritability, and other signs of indigestion ; the second to toxaemia or the symptoms and consequences of auto-intoxication. Flatulence in the Bowels. - This is one of the most prominent symptoms of intestinal indigestion. The most important cause of it is the fermentation of carbohydrates with the formation of organic acids, and evolution of gases such as carbonic acid, hydrogen, nitrogen, marsh-gas, and sulphuretted hydrogen. Carbonic-acid gas is formed in large quantities from all kinds of carbohydrate foods, such as bread, sugar, puddings, etc. This gas is very diffusible; it passes readily through the coats of the alimentary canal into the blood, and is exhaled by the lungs; it is probable that a considerable amount of carbonic-acid gas is got rid of in this manner, but not all of it. It is a peculiar circumstance that a reverse current of carbonic-acid gas may occur; when the amount of this gas in the venous blood exceeds that in the bowels - e.g., during mental excitement - a violent attack of "wind" may occur so rapidly that it is impossible for it to arise from the food. In such a case it is believed the gas comes from the blood. Of the opposite character are hydrogen and nitrogen gases; these do not really diffuse into the blood. On the contrary, they are absorbed more slowly than any other gas generated from the food, and accumulate in the bowels until they are discharged. According to Ruge, who made numerous experiments, hydrogen and nitrogen are produced in greatest abundance when the food consists wholly of milk. These facts explain why some people have flatulence after milk more often than after other foods. Marsh-gas arises in the bowels from the mal-digestion of meat, beans, peas, lentils. It is more rapidly absorbed into the blood than nitrogen or hydrogen, but only one-third as rapidly as carbonic acid. The amount produced from peas and beans is sometimes very great, and it accumulates in the bowels and causes discomfort until it is absorbed or expelled. Sulphuretted hydrogen is generated from various foods. The amount detected in the gases of the alimentary canal is very small, but unfortunately it is absorbed more rapidly than any other gas and is very poisonous. Owing to the rapidity of absorption it is possible for the amount produced in a definite time to be underestimated in the analyses which have been recorded. The effects observed after inhalation of this gas are headache, vertigo (dizziness), nausea, and a sense of oppression in the chest and abdomen, followed by death when the subject is long exposed to it. It is probable that some of the symptoms of intestinal indigestion are due to these gases, the relative absorbability of which is as follows : nitrogen 1, hydrogen 2, oxygen 28, marsh-gas 39, carbonic-acid 'gas 100, and sulphuretted hydrogen 300.