THE ABDOMINAL WALL 223 



satisfactorily be carried quite to the linea alba as the medial 

 part of the aponeurosis is fused with that of the internal oblique 

 beneath it. 



Examine the following muscles, proceeding in a similar manner: 



(a) The internal oblique muscle (m. obliquus internus abdomi- 

 nis). Origin: the inguinal ligament, a second sheet of the 

 lumbodorsal fascia, and the posterior four ribs. Insertion: 

 the linea alba. The fibres pass downward and forward. 

 The ventral aponeurosis is much broader than that of the 

 external oblique. Near the mid-ventral line it is split into 

 dorsal and ventral leaves, containing between them the 

 thin rectus abdominis muscle. Along the line of cleavage, 

 which is known as the linea semilunaris, there is often 

 a small deposit of fat. 



(b) The rectus abdominis muscle. Origin: lateral border of 

 the sternum, including the xiphoid process; also the 

 ventral surfaces of the first to seventh costal cartilages. 

 Insertion : at the anterior end of the pubic symphysis. It 

 is a thin, strap-like muscle, enclosed between two sheets of 

 the aponeurosis of the internal oblique, and separated from 

 its fellow of the opposite side by the linea alba. 



The artery passing forward, for the most part in this muscle, is the 

 inferior epigastric, a branch of the external iliac (p. 255). It anas- 

 tomoses with the superior epigastric artery, a continuation of the 

 internal mammary (p. 326). It gives off the external spermatic 

 artery, a small vessel which perforates the abdominal wall and extends 

 backward, supplying the sac of the testis in the male and ending in the 

 female in the wall of the vulva. 



(c) The transverse muscle (m. transversus abdominis), the 

 deepest muscle of the abdominal wall. Origin: seven 

 I^^sterior ribs, the tips of the transverse processes of the 

 lumbar vertebrae by a thin aponeurosis (also termed the 

 middle layer of the lumbar fascia) , and the inguinal ligament. 

 Insertion : the linea alba, by ^n aponeurosis which fuses with 

 the weakly developed dorsal leaf of the aponeurosis of the 

 internal oblique to form the dorsal wall of the sheath of the 

 rectus abdominis. The fibres are directed downward and 

 slightly backward. 



