THE PERITONEUM 1245 



The peritoneal "cavity" is subdivided into a greater and a lesser cavity or sac, which 

 communicate through the foramen of Winslow (foramen epiploicum). The 

 greater sac is opened when the ventral abdominal wall is pierced; the lesser is 

 situated behind the stomach and adjoining structures, and may be regarded as a 

 diverticulum of the greater sac. 



The disposition of the peritoneum as it is applied to the parietes and also 

 reflected over the viscera contained in the abdominal cavity is best understood 

 after tracing the developmental (embryologic) history of the peritoneum and the 

 alimentary tract. 



Development of the Peritoneum and Alimentary Tract. 1 The segment- 

 ing ovum, at an early stage, becomes a blastodermic vesicle with two layers 

 of cells composing its wall, named, from without inward, ectoderm and ento- 

 derm. The cavity of the vesicle is filled with the nutritive yolk (vitellus}. Only 

 a part of the ovum is destined to form the embryo, the remainder being used 

 up in the formation of membranes and other appendages which are concerned 

 in protection and nutrition; the ovum may, therefore, be divided into its embryonic 

 area and extra-embryonic portion. In the embryonic area, in its caudal part, 

 appears the transitory primitive streak and groove produced by an axial thickening 

 of the ectoderm. From the sides of the primitive streak a third layer of cells, 



FIG. 961. Diagrammatic outline of a longitudinal vertical section of the chick on the fourth day. ep. Ecto- 

 derm, sm. Somatic mesoderm. hy. Kntoderm. rra. Visceral mesoderm. a/. Cephalic fold. pf. Caudal fold. 

 am. Cavity of true amnion. ys. Yolk sac. i. Intestine, s. Foregut. a. Future anus, still closed, m. The 

 mouth, me. The mesentery, al. The allantoic vesicle, pp. Space between inner and outer folds of amnion. 

 (From Quain's Anatomy, Allen Thomson.) 



the mesoderm, extends in all directions between ectoderm and entoderm, extending 

 alongside the neural tube and notochord. The extension of the mesoderm takes 

 place throughout the whole of the embryonic and extra-embryonic areas of the 

 ovum except in certain regions. One such area, devoid of mesoderm, is seen 

 immediately cephalad of the neural tube. This is named the buccopharyngeal 

 area, since it afterward forms the temporary septum between the primitive mouth 

 and primitive pharynx. This membrane extends from the head to the peri- 

 cardial area. A similar area devoid of mesoderm is seen immediately caudad of 

 the embryonic area, and is named the cloacal membrane, since it afterward forms 

 the temporary septum between the anal pit and the primitive hindgut. 



While the paraxial mesoderm extends along the neural tube and notochord 

 and in all directions, its lateral portion spilts into two concentric layers; the outer 

 or somatic layer becomes applied to the ectoderm, and with it forms the somato- 

 pleure or body wall ; the inner or splanchnic layer adheres to the entoderm, and 

 with it forms the splanchnopleure, from which the greater portion of the digestive 

 tract is formed. The space created between the layers of the mesoderm is 

 termed the ccelom or body cavity. A portion of this space is later enclosed within 

 the embryo, and is called the embryonic ccelom, while the portion of the ccelomic 







1 In the preparation of this chapter the editor has consulted several original treatises, notably G. S. Hunting- 

 ton's "Anatomy of the Human Peritoneum and Abdominal Cavity," 1903. 



