This section is from the book "Materia Medica And Therapeutics: An Introduction to the National Treatment of Disease", by John Mitchell Bruce. Also available from Amazon: The pharmacology and therapeutics of the materia medica.
As the chyme passes along the small intestine, the chyle and other soluble constituents are absorbed, and what remains is moved onward into the great intestine, where it forms the bulk of the faeces. Along the whole route, fluid is passing in both directions between the intestinal contents and the blood-from the bowel into the vessels, and from the vessels into the bowel. The consistency of the faeces will, therefore, depend upon the activity of absorption, the activity of excretion, and, manifestly, the rate of transit. The more active the absorption, the less active the secretion, and the slower the rate of transit, so much the firmer will be the faeces; whilst liquidity of the faeces will be the result of imperfect absorption, excessive excretion, or rapid transmission. We are accustomed to speak of the one extreme as constipation, and of the other as diarrhoea.
Absorption from the bowel is carried on by the lacteal and portal systems. The great bulk of the water and salts enter the portal system, by a process of diffusion or osmosis. The activity of this process varies greatly-with the amount of water, salts, and proteids in the bowel, as compared with the blood plasma; with the chemical nature of these salts; with the rate of the circulation through the veins-that is, with the state of the liver; and with the condition of the membranes through which the fluids pass.
Excretion is so active in the small intestine that the faeces are as liquid at the ileo-ccecal valve as in the duodenum, i.e. the effect of absorption as regards water is entirely neutralised. The watery excretions, along with a small quantity of solids and gases, are separated partly by osmosis from the vessels, partly by the glands, the latter furnishing the succus entericus. The activity of the glands is doubtless dependent upon many influences connected with their vessels and nerves, and with the quality of the blood, which are still imperfectly understood.
The transit of the contents of bowels is effected by peristalsis. The muscular coat is innervated by the vagus and splanchnics, the former increasing peristalsis, the latter tending to restrain or inhibit it, just as the vagus inhibits the heart. Whilst the intestine is connected by these means with the cord and brain, its movements are chiefly automatic and determined by Auerbach's and Meissner's plexuses. The state of tension of the wall, the internal pressure of faeces and gas, is the ordinary stimulus of this mechanism; but the nerves or muscles, or both, are also stimulated by the bile; and may be either excited or depressed by many substances introduced through the blood, as we shall see under the next head, as well as (inversely) by the amount of blood supplied to them. In defaecation the will comes to the assistance of the automatic intestinal movements, and effects evacuation of the bowels.
General effects of evacuation of the bowels.- The effects of evacuation of the bowels are by no means purely local. On the contrary, the whole system is influenced by this act, to no great extent, it is true, under normal circumstances, but very markedly when it amounts to actual purgation. When the bowels are very freely moved, a certain amount of water is directly or indirectly removed from the circulation. Bile is swept out of the bowel, and the liver indirectly stimulated. Certain solids and gases excreted by the intestinal wall, that is, truly excrementitious substances, are thrown out of the system. The circulation in the abdomen is disturbed: the vessels are relieved from the pressure of the faeces; the blood flows more freely from the arteries through the portal system and liver; whilst the volume of blood in the portal system and liver is temporarily reduced by the watery excretion. The heart and vessels generally are thus in turn relieved; the blood pressure in the systemic arteries falls; the cerebral circulation is especially depressed on account of its position, so that faintness may be the result; the respiratory movements become easier; the activity of the venous circulation is increased; and the temperature falls. Amongst the abdominal vessels, the circulation through the renal artery and vein is increased, and with it the volume of urine secreted, diuresis being more readily induced after purgation, unless the quantity of water drained off by the bowel have been excessive.
The means of acting physiologically upon the intestine which are at our command are of a much more artificial kind than any we have yet encountered, and introduce us to a large number of medicinal substances.
1. Food. The influence of the food is felt in the bowels, and affords us a ready means of acting upon them. Many kinds of food increase the action of the bowels, notably coarse, indigestible articles of diet, such as the husk of cereals made into "brown bread" and "whole-meal"; green vegetables; oils; fruits, fresh or preserved, which contain abundant salts and sugars; soups, broths, and other preparations of meat; eggs; ale and beer; tea and coffee, when properly prepared; and water taken at bed-time, or in the early morning before breakfast. On the contrary, cold articles of food, milk, spirits, red wines, and tea and coffee made strong and badly, are constipating in their effects. Perfect digestion in the mouth, stomach, and duodenum, is one of the most powerful means of preserving or restoring the natural action of the bowels.
We now pass from these natural means of acting upon the bowels, to others of a distinctly medicinal character.
2. Measures which act upon the intestinal Blood-vessels : Drastics; Astringents; Constringents.-A number of substances disturb transudation by acting upon the blood-vessels in the intestinal walls.