When food is received into the stomach, the muscu lar fibres, at that part where they are most abundant, are thrown into contraction, so as to divide the cavity into two portions : a larger next the great end of the stomach, which is called by MI Home the cardiac por tion ; and a lesser portion, called the pyloric, as being next the pylorus. By this separation, the food is retain ed in the great end of the stomach, till it has been suffi ciently acted on by the juices secreted into that cavity, when the contraction ceasing, it is suffered to pass into the pyloric portion, where it first begins to be converted into chyle. See Phil. Trans. 1807, part n.
The glands situated within the mucous membrane of the stomach secrete a fluid that is plentifully poured into the cavity, and Iiirms the general solvent of the food. This is called the gastric juice. We know little of this fluid except from its effects ; for, though many attempts have been made by Spallauzani, Gosse and others, to collect and analyse it, it is probable that the liquor they obtained was a mixture of gastric juice with saliva and mucus. Indeed, it seems likely that it is never secreted in any great quantity except during digestion. From the great variety of substances which it is capable of dis solving, and from its action on the stomach after death, we know that it is a very powerful chemical agent in decomposing animal and vegetable bodies, though we cannot determine on what its activity depends. The information that has been acquired respecting it will be given in the article CHEMISTRY and PHYSIOLOGY.
We have said that the intestines commence from the pylorus of the stomach. These, though only a continua tion of the same canal, arc, for convenience, divided into several portions, distinguished by particular names. First, with respect to their size they are divided into small and large intestines, the former extending from the pylorus of the stomach to a large and sudden dilata tion of the canal. This is the most natural division, for the portions into which the small and the great intes tines are divided, are merely artificial.
The first portion of the small intestines is supposed to extend about twelve fingers-breadth from the pylorus, and is thence called duodenum. Its limits are more pre cisely ascertained than that of the other small intestines. The course of this part of the intestines from the pylo rus, is obliquely across the spine, a little downward, and towards the right side, where it is connected with all the neighbouring organs, in particular with the gall bladder, to all of which it is bound by the general in vesting membranes.* The duodenum is nearly cylin
drical, or at least a transverse section of it is circular ; it is of a larger diameter than any other part of the small intestines, though considerably less than the pylo ric portion of the stomach. This bowel is perforated inter.ally by a tube from the liver and the gall-bladder, to he afterwards described.
The rest of the small intestines have, by most anato mists, been divided into two portions, jejunum and ilium ; but, as this division is merely arbitrary, and not, as far as we can perceive, attended with any advantage, we shall follow Haller and Bichat in considering them as one, and we shall give them the appropriate name of small intestines, as they arc of less diameter than any other part of the alimentary canal. This portion is very long, and tills up the greatest part of the belly with its numerous windings or convolutions (Sec Plate XIX. Figs. 9. and 11, R, R, R); though it occupies chiefly the umbilical region of the belly, and the atlantal or upper part of the pelvis. The length of the small in testines has been estimated at four or five times that of the body to which they belong, though this must be ta ken with some limitation. They float loosely towards the fore part and sides of the belly, but are firmly con nected and confined towards the spine by their common membrane. Their peripheral or outer surface, is ex tremely smooth and equal, thus allowing the freest mo tion to the bowels. The small intestines terminate suddenly by a peculiar structure, which will be noticed immediately, about the middle of the right iliac region.
Where the small intestines terminate in the right iliac region, a sudden expansion takes place in the ali mentary canal, forming the commencement of the large intestines, which after crossing the belly on the fore part, and almost entirely surrounding the small intestines, ter minate in the anus. This expanded portion has also been divided by minute anatomists into three portions, called ex eunt, colon, and rectum. As the first of these, however, is merely the beginning of the colon, and is accordingly by some called the caput ceccum coli, the blind or close head of the colon, we shall consider the large intestines as divided into colon and rectum.