Bile is a bitter, viscid, greenish-yellow fluid, the taste and general appearance of which are familiar to most people, and the office of which in the animal economy must be one of no small moment, if it be justly chargeable with even a tenth part of the catalogue of human ills which are laid to its account. Its secretion seems to go on continuously; but the quantity produced depends much on the amount of venous blood which is circulating through the liver at the time. Hence, in health, it is always greatest soon after a meal; because, as we have already seen, the supply of blood in both the arteries and the veins connected with digestion is then at its maximum.
But as the secretion of the bile is constantly going on, and its presence in the duodenum is required only when the latter contains chyme, a contrivance becomes requisite for storing it up in the interval, to be ready for use when wanted. This is effected by the small shut sac or bag G, named the gall (or bile) bladder, which adheres to the lower surface of the liver, and is always most full after a fast of some duration. The bile contained in the gall-bladder is generally more viscid, dark, and bitter, than that which proceeds directly from the liver—apparently from the absorption of its more fluid parts. It is in this bag that gall-stones, as they are called, are sometimes formed, and it is their passage through the narrow tube in which it terminates that causes the acute pain so often complained of in that affliction.
From the liver the bile is conducted into the duodenum through a membranous tube, of about the diameter of a quill, and which is called the biliary or hepatic duct. In its course a similar pipe, called the cystic duct (from κυστις, kystis, a bladder), from the gall-bladder, unites with it into one trunk, like the two limbs of the letter Y; and this trunk enters the duodenum by an orifice common to it and the pancreatic duct.
In the healthy state bile is to be found only in the duodenum, and not in the stomach; although one would suppose the contrary from the familiar way in which we speak of the stomach being oppressed with bile, and of our being “very bilious.” When vomiting is excited, either artificially or by illness, we, no doubt, often bring up plenty of bile. Sometimes this bile has been existing in the stomach, and causing nausea; but very often it is brought into the stomach solely by the inverted action which constitutes vomiting, and was consequently placed there by the very remedy which is supposed to have cleared it away. The process of vomiting is accompanied by an inverted action of the intestinal canal whereby it propels its contents upwards instead of downwards, and thus the bile is, as it were, forced up from the duodenum into the stomach, instead of being propelled downwards and expelled in the usual way. Hence, in sea-sickness for example, the first fits of retching generally bring up nothing but food or mucus—the real contents of the stomach,—and it is only after continued straining that bitter bile makes its appearance. In the healthy state, fat or oily food often causes the presence of bile in the stomach, as if its aid were necessary there for the accomplishment of digestion.
The pancreas, or sweetbread, PP, is a large oblong gland, which lies across the spine, and secretes a fluid almost identical with the saliva. Its duct, as stated above, enters the duodenum along with the biliary duct, so that the two fluids meet at their entrance, which takes place at the first curvature of the intestine, at the distance of about one-third of its whole length from the stomach.
The bile and pancreatic juice thus poured out together are both requisite for the formation of chyle, and apparently modify the action of each other. The bile being somewhat of an unctuous nature, and the pancreatic juice somewhat alkaline, their union forms a kind of saponaceous compound, which is less irritating and more easily incorporated with the chyme than pure bile.
In proportion as the chyme is formed and expelled from the stomach, it is received into the duodenum, where it probably undergoes some farther change even before arriving at the entrance of the biliary and pancreatic ducts. From the numerous folds or wrinkles which line the inner surface of the duodenum and impede the motion of its contents, and also from the intestine itself being more fixed down, and less subjected to the influence of the movements of respiration, the progress of the chyme along its surface is very slow. Every particle of chyme is thus allowed to receive its share of the bile and pancreatic juice as it proceeds on its course, and time is afforded for the requisite changes taking place.
By simply stating that, in the duodenum, the chyme becomes mixed with the two fluids just mentioned, and that the result is its separation into a fluid, milky, and nutritive portion, named chyle, which is absorbed by the lacteals, and a darker yellow-coloured thickish mass which remains in the bowel, we communicate to the reader almost every thing that is known on the subject. Theories and conjectures could be added, but scarcely any facts of a very positive nature.
The remainder of the small intestine, namely, the jejunum (from jejunus, fasting or hungry, because it is generally empty) and the ileum (from ειλεω, eilio, I twist or turn about), marked RSSSS in the cut on p. [155], are merely continuations of the duodenum, and have precisely the same number of coats and the same general structure. Their inner surface presents the same kind of folds or rugæ, whereby the extent of the mucous membrane is increased, and ample space given for the ramifications of bloodvessels and the origin of the absorbents by which the chyle is sucked up and carried into the system. Exhalation or exudation also goes on from their surface, and, in bowel-complaints, often becomes so excessive as in a few days to reduce the patient to the extremity of emaciation and weakness.