292 THE ANATOMY OP THE HORSE. 



(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. Raise 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 antenor division whose branches are 

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

 and a 2^osterior 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 prcpubic 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 Rectus 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 

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

 a number (about a dozen) of white lines — linece transversoi, 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 I'each 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 um- 

 bilicus, and reflect it forwards and backwards from the subjacent trans- 

 versalis. Look for the anterior abdominal artery on its deep face. 



The Anterior Abdominal Artery is one of the terminal branches 

 of the internal thoracic artery. It appears at the side of the ensi- 



