Overview
It has a costal part and a lumbar part. Its fleshy part covers the lateral abdominal wall and the caudoventral thorax; ventrally it continues as a broad aponeurosis that reaches the linea alba. The model shows the fleshy part. In the horse, the outer surface of the muscle is reinforced by the tunica flava (tunica flava abdominis), a sheet of yellow elastic tissue that helps carry the weight of the abdominal viscera.
Caudally its aponeurosis splits into an abdominal tendon and a pelvic tendon; the superficial inguinal ring lies between them.
Origin
- Costal part: lateral surfaces of the caudal ribs, by slips that interdigitate with those of the serratus ventralis.
- Lumbar part: last rib and thoracolumbar fascia.
Insertion
Through its aponeurosis: linea alba, prepubic tendon, coxal tuber and inguinal ligament.
Action
Compresses the abdomen (expiration, defecation, urination, parturition) and flexes the trunk to the same side.
Innervation
Ventral branches of the caudal thoracic and cranial lumbar nerves.
Clinical relevance
- Inguinal hernia in stallions passes through the inguinal canal, whose outer opening lies in this aponeurosis. It can strangulate a loop of small intestine and cause acute colic that needs surgery.
- Colic surgery is almost always done through the ventral midline, but some standing procedures, such as laparoscopy or ovariectomy, go through the flank, across the oblique and transverse abdominal muscles.