XL] 



THE ALIMENTARY SYSTEM. 



457 



1 6. The PERITONEUM is, as was mentioned at the begin- 

 ning of this lesson, a serous membrane lining the cavity of 

 the abdomen, and reflected, from its posterior wall, over the 

 viscera which the abdomen contains, and indeed more or 

 less completely embracing them, but nevertheless really 

 retaining them external to its proper cavity, as it is a com- 

 pletely closed sac. 



To understand fully this complex structure it will be well 

 to revert to what was said regarding embryonic development. 



It will be remembered that the laminae ventrales split, each 

 lamina thus forming an outer and an inner lamella ; that the 

 two outer lamellae meet ventrally and form the abdominal 

 wall of the body ; and that the two inner lamellae also meet 

 ventraliy and form the alimentary canal the space included 

 between the two pairs of lamellae (i.e. between the outer sur- 

 face of the alimentary canal and the inner surface of the 

 body-wall) constituting the pleuro-peritoneal cavity. 



Now, the whole of the surface of this cavity becomes modi- 

 fied to form a continuous serous membrane, and this mem- 



FIG. 388. DIAGRAM OF THE DEVE- 

 LOPMENT OF THE TRUNK AND ITS 

 SKELETON, as shown in a section 

 made at right angles to the trunk's 

 long axis. 



nc, neural canal ; ex, epaxial cartilages 

 ascending to surround it ; px, par- 

 axial cartilages descending in the 

 plate, or layer (I've), external to //, 

 the pieuro-peritoneal cavity; Ivi, in- 

 ternal plate of the split ventral 

 lamina. 



FIG. 389. DIAGRAM OF THE FURTHER 

 DEVELOPMENT OF THE TRUNK, as 

 shown in a section similar to the last. 



a, alimentary canal supported by a me- 

 sentery 2 formed of the dorsal portion 

 of the inner parts of the split wall of 

 the embryonic ventral laminae : e, 

 epaxial arch ; //, hypaxial arch de- 

 scending in the median line in the 

 root of the inner part of the split wall 

 of the ventral laminae ; J>, rib, bi- 

 furcating proximally and abutting 

 ventrally against the sternum, which 

 thus completes the paraxial arch ; HI, 

 peritoneum, bounding on all sides pi, 

 the pleuro-peritoneal space. 



brane is the peritoneum. But the root part of the inner pair 

 of lamellae i.e. that part which intervenes between the 

 alimentary canal and the axial skeleton diminishes and 



