2. Considering the whole lymphatic system as a set of branching tubes passing from the extravascular regions just outside the small arteries and veins and capillaries, to the large venous trunks, it is obvious that the mean pressure of the blood in the subclavian at the junction with the jugular is the cause of the movement, etc., assisted perhaps by the respiratory movements, and other causes, as osmosis, etc.
The average quantity of solid fecal matter evacuated by the human adult in twenty-four hours is about five ounces; an uncertain proportion of which consists simply of the undigested or chemically modified residue of the food, and the remainder of certain matters which are excreted in the intestinal canal.
Gases contained in the stomach and intestines. The sources of the gases contained in the stomach and bowels may be enumerated:
1. Air introduced in the act of swallowing either food or saliva.
2. Gas developed by the decomposition of alimentary matter, or of the secretions and excretions mingled with it in the stomach and intestines.
3. It is probable that a certain mutual interchange occurs between the gases contained in the alimentary canal, and those present in the blood of the gastric and intestinal blood-vessels.
The movement of the intestines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intestinal coats. The contractions, which may commence at any point of the intestine, extend in a wavelike manner along the tube. This is due to the involuntary longitudinal and circular muscular fibers contracting successively from above downwards and from behind forwards, etc. The movements take place slowly, and in health are commonly unperceived by the mind, but they are perceptible when they are accelerated under the influence of any irritation.