THE ABDOMINAL WALL 



223 



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

 neurosis of the external oblique, which covers the muscle 

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

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

 the anterior wall is formed by a blending of all three apo- 

 neuroses (Fig. 100). 



The lower free margin of the posterior wall of the 

 sheath can be defined with the handle of the knife after the 

 rectus is raised. It frequently presents a sharp lunated edge, 

 the concavity of which is directed downwards towards the 

 pubis. The curved edge is called the linea semicircularis 

 (O.T. semilunar fold of Douglas]. The inferior epigastric 

 artery enters the sheath by passing upwards in front of the 

 linea semicircularis (Fig. 99). 



The linea semicircularis is, however, often rendered in- 



External oblique 



--- Internal oblique 



Transversus abdominis 



Fascia transversalis 



i 



FIG. ioo. Transverse section through the Abdominal Wall a short distance 

 above the Pubes. 



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 

 that 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 observe minute fatty masses protruding. A 

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

 which the umbilicus is the remains, is completely closed at 

 birth ; indeed, in the adult the linea alba is stronger at that 

 point than elsewhere. 



Fascia Transversalis. The transversalis fascia is a thin 

 layer of fascia which is spread out upon the deep surface of the 



