12. In strict language the alimentary canal includes the whole passage from the mouth to the anus, but the term is more usually employed to express only the stomach and intestinal tube. It may be represented as a long canal, commonly calculated as being five or six times the length of the adult, differently twisted upon itself, and of different dimensions in various parts of its extent. Anatomists describe it as composed of several distinct tunics, or coats, the existence of which may be traced throughout its whole extent, although their structure undergoes variation in the different divisions of the canal; but this will be better understood when we come to speak of its individual parts. The intestinal canal is susceptible of a peculiar motion, which arises from the successive or simultaneous contraction of its longitudinal or circular fibres, and has been differently denominated by authors; some have named it vermicular, others peristaltic. This contraction always takes place slowly, and in an irregular manner; it is, however, capable of being accelerated by the action of certain stimulants.

It does not seem to be sensibly controlled by the will; nor, indeed, does it appear to be dependent upon the nervous system, for it proceeds in the stomach after the section of the eighth pair of nerves, and it even continues, though the intestinal canal be entirely separated from the body: at the same time it appears, from the experiments related by Dr. W. Philip, that although these muscular fibres be independent of the nervous system, they may in every instance be influenced through it; a fact of very great pathological importance, since it follows that the muscular fibres of the canal may not only be affected by causes acting directly on them, but by such as act through the medium of their nerves. M. Majendie observes, that the peristaltic motion becomes more active by the weakness of animals, and even by their death; and that in some, by this cause, it become considerably accelerated. The object of this motion is to propel forward the contents of the canal, and to favour those changes which they are destined to undergo. The intestinal canal is never in a state of complete collapse, it always contains gas or vapour, which prevents its sides from coming into contact.

It has been stated that this canal is of different dimensions in various parts of its extent, and it is principally from this diversity of magnitude that anatomists have established those divisions which we have next to consider.

13. The stomach is a membranous bag, very much resembling in shape the pouch of a bag-pipe, or, more strictly speaking, a conoid bent upon itself. It is not easy to determine its exact capacity in the living body, , nor is it a fact of much practical importance: in various states of disease, we have reason to believe that it is considerably augmented in size. It has two orifices: the one termed the cardia, which is a termination of the tube we call the oesophagus; the other, which communicates with the small intestine, and to which the term pylorus has been given. The latter is raised up, being nearly, but not quite level with the former, so that its upper and lower surfaces form, as it were, two concentric circles, one on the upper side, which is called the small curvature, and one on the lower, which is termed the great curvature. The stomach is situated immediately below the diaphragm, the cardia being nearly opposite to the middle of the vertebrae. From thence it bulges out to the left side, the great curvature coming forward and downward: it then passes on to the right side, rising upwards, so that the pylorus is not much farther from the diaphragm than the cardia; when, therefore, a man is in an erect posture, substances must ascend to pass through the pylorus.

It is, however, evident that its situation, and relation with the neighbouring organs, will always suffer variations according to its degree of distention; the following observations will therefore deserve the attention of the pathologist: - in its flaccid state, it occupies the epigastrium and part of the left hypochondrium; whilst, when distended, it exchanges its flattened for a rounded form, and fills almost completely the left hypochondrium; the greater curvature descending towards the umbilicus, particularly on the left side; on account of the resistance opposed by the vertebral column, the posterior surface of the stomach cannot distend itself in that direction; this viscus is therefore wholly carried forward. The dilatation of the stomach produces very important changes in the abdomen: the total volume of the cavity augments; the belly juts out; the abdominal viscera are compressed with greater force; and the necessity of passing urine or faeces is frequently experienced. At the same time, the diaphragm is pressed towards the breast, and it descends with some difficulty; whence the respiratory motions are impeded. The stomach, although a single bag, must be considered as divisible into two distinct cavities, to which different offices are evidently assigned.

The left half has always larger dimensions than the right; and M. Majendie calls the one the splenic part, because it is supported on the spleen, and the other the pyloric part, since it is supported on the pylorus.

14. The stomach has been described as composed of several membranes, viz. the peritoneal, muscular, nervous, and villous coats. The nervous coat, however, of" Haller and the old anatomists, is now acknowledged to be nothing more than cellular membrane; and we might with equal propriety dismiss the two former from the number. The peritonscal covering, being common to all the contents of the abdomen, can scarcely be recognised as one of the proper coats of the stomach; while it has been very justly observed, that the muscular fibres, arranged between the peritonaeum and the villous membrane, cannot maintain the name of a coat with propriety, since the term signifies a containing membrane, whereas the muscular fibres owe their connexion with each other to interposed cellular membrane. One layer of these fibres runs longitudinally from the cardia to the pylorus; another takes a circular course, embracing, as it were, the stomach from one curvature to the other, and constituting what are called the transverse fibres. There remains, then, only the villous membrane; and this, in fact, is the only proper intestinal coat, or containing membrane of the aliment.