THORAX AND ABDOMEN OF THE HORSE 85 



independent muscle under the name of m. retractor costoi. It consists 

 of a thin triangular sheet of fibres, arising by aponeurosis from the 

 transverse processes of the first three or four lumbar vertebrae, and 

 inserted to the caudal border of the last rib. 



Dissection. — Make a longitudinal incision through the internal oblique 

 just lateral to the margin of the straight muscle. A second incision, 

 transverse in direction, should next be made through the fleshy part of 

 the muscle about midway between the coxal tuber and the last rib. In 

 making the second incision the dissector must avoid cutting certain 

 blood vessels and nerves that are to be examined in association with the 

 transverse abdominal muscle. The structures to be preserved are 

 branches of the deep circumflex iliac, lumbar and intercostal arteries 

 and veins, and Ijranches of the ilio-hypogastric, ilio-inguinal, lateral 

 cutaneous and intercostal nerves. 



Now reflect the combined aponeurotic tendons of the two oblique 

 muscles where they form an investment for the straight abdominal 

 muscle, turning the combined sheet towards the middle line until the 

 linea alba is reached. This reflection will be facilitated by a transverse 

 incision about midway between the pubis and the xiphoid process of 

 the sternum. 



In raising the aponeurotic sheet from the surface of the straight 

 muscle it will be found that some resistance is off'ered along transverse 

 lines (tendinous inscriptions) that cross the straight muscle. The 

 sheath and the tendinous inscriptions are closely adherent to each 

 other. 



M. RECTUS ABDOMINIS. — The straight abdominal muscle forms a 

 broad band running along the ventral aspect of the abdominal wall, and 

 separated from its fellow muscle by the linea alba. The muscle is 

 broadest as it lies over the cartilages of the ribs from the ninth to the 

 thirteenth. From this level it gradually narrows towards the pubic 

 region. 



The origin of the muscle is from the cartilages of the ribs from the 

 fourth to the ninth, and from the outer surface of the sternum. Its 

 insertion is by a dense and very stout tendon to the pubic tubercle and 

 the immediately adjoining part of the pecten of the pubis. The dis- 

 sector should notice that the so-called accessory ligament of the hip- 

 joint is continued from the tendon of insertion. 



Crossing the straight muscle transversely are some ten or a dozen 

 irregular tendinous inscriptions (iuscriptiones tendinea?) that doubtless 

 contribute to its strength. 



Dissection.— Cut across the straight abdominal muscle about the 

 middle of its length and raise the two portions from the dorsal layer of 

 the sheath that contains them. The vessels and nerves of the 

 abdominal wall are then exposed to examination. 



A. EPIGASTRICA CAUDALls. — The caudal epigastric artery is a branch 

 of the pudendo-epigastric trunk, and crosses the medial aspect of the 

 inguinal canal in order to reach the deep face of the rectus abdominis 



