Now since, as we remarked ahove, a free peritoneal surface always indicates a layer of peritoneum, the lesser omentum having two free surfaces consists of two layers ; and its two surfaces being continuous around the vessels mentioned, its two layers are continuous in like manner. It, therefore, is a portion of perito neum doubled or folded upon itself, enclosing vessels and a duct in the extremity of the fold ; just as we saw was the case with the falciform lig,ament enclosing, in the extremity of its fold, the obliterated umbilical vein.
When a double peritoneal sheet passes across from one bowel to another, or from the parietes to a bowel, it is described as attached along the lines where it first lights upon or comes in con tact with such parts. Speaking in such a way, the lesser omentum is attached to the liver and stomach by the whole extent of its borders, except that small portion between the duode num and porta which is free : and in fact this border is said to be free only because that which it encloses is small ; if the gall-duct were an inch in diameter, the right border of the lesser omentum would be said to be attached to the gall-duct. Disregarding at present the last observation ; the line of attachment, then, of the lesser omentum is continuous all around except at its free border. Let us trace this line of attachment from the porta of the liver to the pyloric end of the stomach in the circuitous direction in which alone it can be done. From the porta, then, we trace this line along the posterior half of the antero-posterior fissure of the liver, inclining a little to the left of this fis sure so as to reach the cardiac end of the sto mach, and thence along the lesser curvature of the stomach to the pylorus.
The gastric attachment of the' lesser omen tum is placed transversely, whilst its he patic attachment runs antero-posteriorly, with only a moderate inclination from side to side, so that this otnentum has a kind of twist.
It seems as though the gall-duct in gaining the shortest route from the liver to the duode num had carried out the superjacent peritoneum from the cardia and lesser gastric curvature into a fold, as far out as the position of the straightest line from the porta to the pylorus ; that this fold would have projected in the middle line, but that the enlargement of the right lobe of the liver displaced its posterior part with the cardia to the left, whilst the duodenum being brought into adhesion with the posterior abdominal walls displaced its anterior part to the right, and that both displacements have resulted in an almost tmnsverse instead of an antero-poste rior horizontal direction. In many vertebrate
animals, especially those below the class INIam malia, the duodenum is not adherent to the posterior abdominal parietes, and the pylorus as well as the cardia is frequently in the middle line, whilst the two lobes of the liver are of pretty equal transverse extent ; in such cases the lesser omentum extends antero-posteriorly in the middle line, (figs. 490, 491,) and this, we consider, is its typical position. This point will be more fully considered when we come to a particular description of the omenta; at pre sent we are endeavouring to demonstrate the continuity, merely, of the peritoneum through out : it is the existence of the omenta, or rather their distorted position in the human subject, that renders this demonstration so difficult.
It is necessary at this stage of our description to study the peritoneal sheet, or bag, with two free surfaces, called the greater omentum. On making an incision, as above, through the abdo minal parietes, the liver and stomach are at once brought into view ; but the small intes tines are concealed by the great omentum co vering them in front. It is a membmniform apron, having plentiful reticulations of vessels, and often loaded with fat, especially near the vessels. Viewing it undisturbed it appears to be pendent from the greater curvature of the stomach, and to have a free inferior border touching, usually, the pelvic region ; but on lifting it up and looking at its posterior aspect, it is seen to be attached also to the transverse portion of the colon, which at once informs one that it is double. The fact of its being double, however, may be much more strikingly demon strated in the following manner. If a catheter he held in the foramen of Winslow, and air be blown through it, the great omentum (provided there be no abnormal breach of continuity or adhesion in it) will become inflated like a great bladder ; the inflation extending, not only downwards below the greater curve of the sto rnach, but to the left beyond its fundus, and also to the lesser omentum. The cavity so in flated is called the sac of the omentum, or the posterior cavity of the peritoneum, and the fo ramen of Winslow is the orifice that leads to this sac—the neck that connects together the ante rior and posterior cavities of the peritoneum, making them one.