This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Human bile when quite fresh is a golden brown liquid. It is alkaline, and besides coloring matters, mineral salts and water, contains the sodium salts of two nitrogenized acids, taurocholic and glycocholic, the former predominating in human bile.
Bile has no digestive action upon starch or albumens. It does not break up fats, but to a limited extent emulsifies them when shaken up with them outside the body, though far less perfectly than the pancreatic secretion. It is even doubtful if this action is exerted in the intestines at all. In many animals, as in man, the bile and pancreatic ducts open together into the duodenum, so that on killing the creature during digestion and finding emulsified fats in the chyle it is impossible to say whether or no the bile had a share in the work. In rabbits, however, the pancreatic duct opens into the intestine about a foot farther from the stomach than the bile-duct; and it is found that if a rabbit be killed after being fed with fatty food, no milky chyle is found above the point where the pancreatic duct opens. In the rabbit, therefore, the bile alone does not emulsify fats ; and since bile is much the same in rabbits and other mammals, it probably does not emulsify fats by its reaction in any mammals. The inertness of bile in digestion has caused it to be doubted whether it is of any use, and whether it should not be regarded merely as an excretion, poured into the alimentary canal to be got rid of. But there are many reasons against such a view. In the first place, the entry of the bile into the upper end of the small intestine, where it has to traverse a course of more than twenty feet before getting out of the body, makes it probable that bile has some function to fulfill in the intestine. One use is no doubt to assist by its alkalinity in overcoming the acidity of the chyme, and so to allow the pancreatic secretion to act upon proteids. Constipation is also apt to occur in cases where the bile-duct is temporarily stopped, so that the bile probably helps to excite the contrac- tions of the muscular coats of the intestines; and it is said that when the bile secretion is deficient putrefactive changes are extremely apt to occur in the intestinal contents. Apart from such secondary actions, however, the bile probably has some influence in promoting the absorption of fats. If one end of a very narrow glass tube moistened with water be dipped in oil the latter will not rise in it, or but a short way; but if the tube be moistened with bile instead of water the oil will ascend higher. Again, oil passes through a plug of porous clay kept moist with bile, under a much lower pressure than through one wet with water. Hence bile by moistening the cells lining the intestine may facilitate the passage into the villi of oily substances. At any rate, experiment shows that if the bile be prevented from entering the intestine of a dog the animal eats an enormous amount of food compared with that amount which it needed previously; and that of this food a great proportion of the fatty part passes out of the alimentary canal unabsorbed. There is no doubt therefore that the bile somehow aids in the absorption of fats.
Describe fresh human bile. What is its reaction? Name its chief constituents.
Name foods on which bile has no influence? How does it act upon fats when shaken with them? Give a reason for doubting if it emulsifies fats in the intestine.
Give reasons for believing that bile is not a mere excretion. How does bile aid the digestive power of the pancreas? Point out other uses of bile. Describe experiments which tend to prove that bile helps in promoting the absorption of fatty matters from the intestine.