DISSECTION OF THE ABDOMEN. 301 



Directions.— The internal oblique covers the transversalis and rectus 

 abdominis muscles. Tlie outer edge of the last may be seen through 

 the thin tendon of the internal oblique, and through the same tendon 

 the posterior abdominal artery may be seen if well injected (Plate 40). 

 The circumflex iliac artery is on the deep surface of its muscular 

 portion. In order to see these connections of the muscle to the most 

 advantage, incise the muscle along the line of junction of the muscular 

 fan and the tendon. Eaise the muscular portion carefully, and hook it 

 back. Strip away entirely the aponeurotic tendon, using the scalpel 

 where the tendon is firmly adherent to the rectus abdominis. 



The Circumflex Iliac Artery. This is a branch of the external 

 iliac artery, and will be better seen in the dissection of the sublumbar 

 region (Plate 44). It has an anterior dividon whose branches are 

 distributed to the internal oblique' and transverse muscles in the flank, 

 and a posterior division which, after giving some twigs to the oblique 

 muscles, perforates them below the angle of the haunch, and descends 

 to the thigh. 



The Posterior Abdominal Artery (Plate 40). This is a branch of 

 the prepubic artery, beginning at the inner side of the internal abdominal 

 ring. It places itself on the abdominal aspect of the internal oblique 

 muscle, crosses behind and internal to the ring, and runs forwards to 

 enter the rectus abdominis, in which, about midway between the sternum 

 and the pubis, it anastomoses with the anterior abdominal artery. 



These arteries are accompanied by veins of the same names. 



The Pectus Abdominis (Plate 40). This muscle extends in the 

 form of a broad band from the sternum to the pubis, at the side of 

 the linea alba. To a large extent it separates the internal oblique and 

 transverse muscles, but beyond its outer border these muscles are in 

 contact in the flank and below the extremities of the ribs. The muscle 

 is widest about its centre, and it is crossed from side to side by a 

 number (about a dozen) of white lines — linece transversce, which are 

 caused by as many tendinous intersections of its muscular substance. 

 It arises from the lower face of the sternum, and from the five costal 

 cartilages behind the 4th. It is inserted into the anterior border of the 

 pubis by the prepubic tendon. 



Action. — Similar to that of the oblique muscles. 



Nerves (Plate 40). At the lower ends of the last ten intercostal 

 spaces, the intercostal nerves are prolonged beyond the rim of overlapping 

 cartilages to pass between the straight and transverse muscles, giving 

 fibres to both and also some perforating twigs to reach the skin. The 

 last dorsal nerve (behind the last rib) has a similar distribution. The 

 inferior primary branches of the 1st and 2nd lumbar nerves are similarly 

 prolonged after furnishing twigs to the oblique muscles in the flank. 



Directions. — Cut the rectus abdominis transversely about the umbilicus, 



