SMALL INTESTINE.— ThC next portion of the alimentary canal is that which is included between the pyloric and ilio-cmcal valves, and is named, from its diameter, the small intestine tant metamorphosis in another class of organic sub stances. (See p. 362.) (Lat. Intestinum tenue, Fr Intestin grRe, Germ. Duenndarm).
The shape of all this portion is cylindrical. Its average length is about 20 feet ; its dia. meter about inches.* But apart from those varieties in its dimensions which it presents in different individuals, the yielding nature of the tube allows it to be narrowed by artificial extension. While, vice versa, it is just as easily shortened by dilatation. And it is very' difficult accurately to estimate those minor degrees of distention to which it is liable. Hence little stress can be laid upon the statement of Cruveilhier, that the small intestine tapers away from the duodenum to near its extrernity, where it suddenly dilates to enter the large intestine.
The small intestine occupies the cavity of the belly. Its commencement, at the pyloric extremity of the stomach, is placed in the right hypochondrium; its termination, in the mann that begins the large intestine, occu pies the right iliac fossa, to which this part of the intestinal canal is fixed. The few inches of bowel immediately above this extremity frequently occupy the pelvic cavity. But almost all the intervening portion is so free to move, that each particular point of its length may be found in any part of the ab domen or pelvis.
The duodenzon.—Tbat upper part of the small intestine which is directly continuous with the stomach,is distinguished, both from this organ, and from the lower part, by certain peculiari ties. And though these chiefly affect its exter nal anatomy', still we shall hereafter find that they are not unaccompanied by differences in the structure of its mucous membrane. Start ing from the constriction before alluded to, as marking the site of the pyloric valve, the in testinal tube forms a curve in the shape of a horse-shoe around the head of the pancreas ; receives the duct of this gland, as well as that of the liver ; and is closely fixed by perito neum to the posterior wall of the belly. This
fixed commencement of the small intestine has been named the duodenum, in consequence of its length being estimated at twelve finger bread ths (ScoSeicacticToxov, zwoelffingerdarm). It has also been called the ventriculus succentu riatus, or supplementary' stomach ; —a vague term, which was probably based upon an in accurate notion of its office.
Beginning at the pyloric constriction, the duodenum proceeds outwards, backwards, and a little upwards, to the under surface of the right lobe of the liver. It then turns down wards, and a little inwards, in front of the right kidney; so as often to impress a shallow fossa on the hepatic surface in front of that depression which corresponds to this organ. After a short perpendicular course, it finally makes a second bend, by which it regains a horizontal direction, and passes from right to left, and a little upward, in front of the vena cava, the aorta, the right erns of the diaphragm, and the vertebral column. It terminates opposite the left side of the body of the second lumbar vertebra, at a point which corresponds to the commencement of the mesentery. Here the intestine becomes free, and is named the jejunum.
The length of all this curve, when unfolded, is about ten inches. But for the sake of greater exactness, the three chief portions of the duodenum may be described separately.
The first portion is called the superior transverse or hepatic. It is much the shorter of the three, being scarcely two inches in length. It lies solely- in the right hypochon drium; and, near the neck of the gall-bladder, terminates by bending downwards to merge into the second portion. Like the stomach, it is invested by peritoneum on both surfaces. This membrane is derived from the gastric omenta previously described; the extreme right of the gastro-hepatic omentum being some times called the ligamentum hepatico-duodenale. The latter fold of serous membrane also forms the anterior boundary of the foramen of Wins low, or opening by which the general sac of the serous membrane communicates with the bag of the omentum; and it contains the hepatic duct and vessels.