3. Mesenteric Peritoneum, connecting these two, carrying the intestinal blood vessels and lymphatics and acting as a suspensory support to the alimentary canal.
The dorsal mesentery in fishes, amphibia and reptiles contains smooth muscular fibers derived from the mesoderm. These bands of smooth muscle fibers are also encountered, though less well developed, in the mesentery of birds and mammals. The so-called “suspensory muscle of the duodenum” belongs to this category. It consists of a few strands of unstriped muscular and fibrous tissue which passes from the præaortal tissue around the origin of the superior mesenteric artery and cœliac axis to the duodeno-jejunal angle. Fasciculi from this band may penetrate into the root of the mesentery (Gegenbaur).
Similar muscular fasciculi have been observed in the peritoneal folds of the ileo-cæcal junction (Luschka) and in the mesorectum—forming in the latter situation the recto-coccygeal muscles of Treitz, and in the female the recto-uterine muscles.
In its earlier stages the primitive common mesentery forms a membrane which carries the intestinal blood vessels between its two layers, surrounds the embryonic alimentary canal and attaches the same to the ventral aspect of the chorda dorsalis and aorta. This is the permanent condition in many of the lower vertebrates in which the intestinal tube is suspended by a simple dorsal mesentery, a condition which is repeated by the embryos of man and the higher vertebrates. From this primitive common mesentery are derived, by further development, displacement and adhesion, all the other mesenteries, omenta and peritoneal folds of the adult. The character and degree of these subsequent changes is determined by the increase in length and change in position of the intestine and the growth of large organs, like liver, spleen and pancreas. Many portions of the intestinal canal, at first suspended by the mesentery and freely movable within the abdominal cavity, become later, by secondary adhesion, firmly connected with adjacent portions of the tube or with the abdominal parietes.
In certain of the lower vertebrates (fishes) large sections of the intestine lie entirely free within the abdomen, their only connection with the parietes being afforded by the blood vessels. This condition depends upon absorption of the original mesentery. A similar process, though much more circumscribed, is observed in the omenta of many mammals, which appear perforated at several points.
| Fig. 33.—Schema of alimentary canal and accessory organs, derived from same. (After Bonnet.) | Fig. 34.—Reconstruction of alimentary canal of human embryo of 4.2 mm. × 24. (After His.) |
Derivatives of the Entodermal Intestinal Tube.—The entodermal epithelium is physiologically the characteristic element of the alimentary canal. Besides lining the entire internal surface of the tube it gives rise by budding and protrusion from the intestinal canal to a series of organs which from the mode of their development must be regarded as diverticular or derivatives of the alimentary canal (Figs. 33, 34, and 35). These organs, proceeding in order cephalo-caudad, are the following:
The salivary glands.
Thymus and thyroid.
The lungs.
Pancreas.
Liver.
The epithelium of all these structures is derived from the primitive entoderm of the intestinal tube, except the epithelium of the salivary glands, which, being derived from the stomadæal invagination, is ectodermal in character. We have previously noted the general history and appearance of the yolk-sac and its connection by means of the vitello-intestinal duct with the intestine. In contradistinction to the adult organs just noted the yolk-sac or umbilical vesicle is merely a temporary embryonal appendage to the alimentary canal. It also differs from them in the fact that it is not an extension or budding from the completed intestinal tube, like the liver and pancreas, but indicates, by the implantation of the duct ([Fig. 21]), the last point at which closure of the intestinal canal takes place, when after obliteration of the duct the separation of the intestine from the yolk-sac is completed.
The segment of the primitive alimentary canal cephalad of the attachment of the vitello-intestinal duct gives rise to the pharynx, œsophagus, stomach, proximal portion of small intestine proper and its derivatives, the liver and pancreas.