To secure full and natural action in the intestinal canal, several principal conditions are thus necessary, failure of any of which may impair their activity. The first condition is well-digested chyme and chyle; the second, a due quantity and quality of mucous and vascular secretions from the villous coat; the third, full contractile power in the muscular fibres of the intestine, and free action of the abdominal and respiratory muscles; and the last a due nervous sensibility to receive impressions and communicate the necessary stimulus. And hence, when the bowels act imperfectly, it is of importance to ascertain to what cause the inability is to be ascribed, that an appropriate treatment may be devised.
Such are the general structure and uses of the intestinal canal; but there are modifications in its individual portions on which it may be right to offer a few additional remarks. We shall begin with the duodenum.
The duodenum (from duodeni, twelve, being considered equal in length to twelve finger-breadths) commences at the pyloric orifice of the stomach, from which it crosses over under the lower surface of the liver, towards the right side; it then descends in front of the right kidney, and there forming a second curve, it proceeds again towards the left, and a little beyond the spinal column terminates in the jejunum. It thus describes a course like the letter C, and has its convexity turned towards the right, while the pancreas or sweetbread (PP) lies in the space enclosed by its concavity. To enable the reader to form some notion of the relative position of these parts, I have introduced a wood-cut on the next page, shewing the situations and appearance of the different organs after the intestines, as in the figure on page [155], have been removed from the body. The letters LLLL point out the inferior surface of the liver, a little raised from its natural position to shew the gall-bladder G and the pancreas PP, round the right end of which the duodenum is curved. S indicates the spleen, with a vacant space over it, in which the stomach lies. The kidneys, KK, lie one on each side of the spine; and the two pipes UU are the ureters which convey their secreted fluid to the bladder B. The letters AA indicate the great artery (the aorta), through which the nutritive blood descends to supply the bowels and lower parts of the body; and VV mark the corresponding vein (the cava), by which the dark blood returns from the extremities towards the heart. R is the beginning of the rectum or straight gut seen at YY in the cut on page [155].
The duodenum, being thus in the immediate vicinity of the spine, is fixed firmly down in its position by the connecting membrane, and is not left to float loosely in the cavity of the abdomen or belly. Had it not been tied down in this way, it would not only have acted by its weight as a continual drag upon the stomach and disturbed its functions, but likewise have been constantly altering its own relation to the pancreatic and hepatic (or liver) ducts, and thereby affecting the flow of their respective fluids into its cavity, by which the chylification would have often been interrupted.
The duodenum is much smaller in diameter than the stomach, but larger than the jejunum or ileum; and its muscular coat is also thicker. From its size and the importance of the changes effected in it, it has been considered by some as a secondary stomach or ventriculus succenturiatus. Its mucous coat, which has a soft velvety feel, presents a greater multitude of the folds or plaits already described, and which have for their object to extend its surface and delay the passage of the chyme. Some notion of their appearance may be formed from the subjoined wood-cut. These folds or rugæ are called valvulæ conniventes or folding valves, and are inherent in the nature of the mucous coat, and not produced by mere folds of the whole thickness of the intestine, consequently they exist even when the latter is distended. They are comparatively few in number in the part of the duodenum near the stomach, and gradually multiply in the course downwards till they arrive at the maximum of development at its lower end and in the jejunum: they again diminish in the ileum, and disappear altogether in the large gut. The bloodvessels and nerves of the duodenum are extremely numerous, and indicate the importance of its functions.
The duodenum serves to receive the chyme as it issues from the stomach, and to prepare it for the farther changes which it is about to undergo. But as other organs, namely the liver and pancreas, are directly concerned along with the duodenum in effecting chylification, it will be proper to take a cursory view of them also before describing the rest of the intestine.
The liver (LLLL, p. [171]) is a very large glandular body lying under the short ribs of the right side, immediately below the diaphragm or midriff, to which it is attached by strong ligamentary bands, which sustain its weight and keep it in its place. Its office is to secrete the bile, but it differs in one important particular from every other secreting organ. Its secretion is derived, not, as in other instances, from the arterial or nutritive blood, but from the venous or exhausted blood, which is collected from all the abdominal organs, and transmitted through it for this purpose on its way back to the heart. From this peculiarity, it is legitimately enough inferred that the liver serves the double purpose of providing a fluid indispensable for chylification and the proper action of the bowels as organs of excretion, and also of separating from the venous blood useless or spent materials, which require to be thrown out of the system. The influence of the bile as a stimulant to the bowels is proved by the fact that costiveness ensues when it is deficient in quantity, and an opposite condition when the secretion is redundant, as during the heat of autumn.