ABDOMINAL WALL 399 



posterior wall or lamella is formed by the fusion of the posterior 

 layer of the aponeurosis of the internal oblique with the 

 aponeurosis of the transversus abdominis (Fig. 148). 



Superiorly, the rectus muscle rests directly upon the costal 

 cartilages, and the sheath is merely represented by the apo- 

 neurosis of the external oblique, which covers the muscle 

 anteriorly. Inferiorly, the posterior wall of the sheath is also 

 absent, and the rectus rests on the transversalis fascia. Here, 

 however, the anterior wall is formed by a blending of all three 

 aponeuroses (Fig. 150). 



The lower free margin of the posterior lamella of the 

 sheath can be easily defined by raising the rectus and working 

 with the handle of the knife. It frequently presents a sharp 

 lunated edge, the concavity of which is directed downwards to 

 the pubes. It is called the linea semicircularis (O.T. semilunar 

 fold of Douglas). The inferior epigastric artery will be 



External oblique 



Internal oblique 

 Transversus abdominis 

 Fascia transversalis 



FIG. 1 50. Transverse section through the Abdominal Wall a short distance 

 above the Pubes. 



observed to enter the sheath by passing upwards in front 

 of this free border (Fig. 149). 



The linea semicircularis of Douglas is often rendered in- 

 distinct by the presence of scattered tendinous bundles crossing 

 behind the lower part of the rectus. 



Linea Alba. The linea alba can now be studied to the 

 best advantage. It is a dense fibrous cord or band which 

 extends perpendicularly between the xiphoid process and 

 the symphysis pubis. It is formed by the union and decussa- 

 tion of the fibres composing the aponeuroses of the two 

 oblique muscles and the transverse muscles of opposite sides. 

 Above the umbilicus it is broad and band-like ; whilst below 

 this point it becomes narrow and linear. A close examination 

 will show that it is pierced by several small round openings, 

 for the transmission of blood-vessels, and from some of these 

 the dissector may even observe minute fatty masses protruding. 

 A little below its middle is the umbilicus, but the foramen, 

 of which this is the remains, is now completely closed ; 



