M. obliquus externus abdominis, digitatio costalis XI
The external abdominal oblique is the most superficial of the flat abdominal muscles. Its fibers run caudoventrally. It originates from the lateral surfaces of the ribs (roughly ribs 4–18) via a series of costal digitations that interdigitate with the serratus ventralis thoracis, and inserts via a broad aponeurosis onto the linea alba, prepubic tendon, and tuber coxae. The aponeurosis splits into an abdominal tendon and a pelvic tendon; the gap between them forms the superficial inguinal ring (framed by the medial and lateral crura / pelvic tendons). In males the spermatic cord passes through this ring; in mares the round ligament does. The muscle is essential for abdominal press (defecation, urination, parturition, expiration) and for lateral flexion of the trunk. Superficial to the muscle lies the tunica flava abdominis (yellow abdominal tunic), a prominent elastic layer unique to large ungulates. This modified deep fascia provides passive elastic support to the heavy abdominal viscera, acting like a natural girdle. It is particularly well developed in horses and cattle. The many individual costal digitations are simply the sequential rib origins of the same muscle and do not require separate clinical consideration.
Origin: Lateral surface of rib 15
Insertion: Linea alba and prepubic tendon via aponeurosis
Action: Flexes the trunk laterally; compresses the abdomen; assists expiration
Innervation: Ventral branches of thoracic (T10–T18) and lumbar (L1–L3) spinal nerves
The superficial inguinal ring is a critical landmark in inguinal hernia and castration. Inguinal hernias can occur, especially in stallions and foals. The muscle and tunic are important for supporting visceral weight; disruption (e.g., prepubic tendon rupture) is serious. The muscle contributes to forced expiration.
## Overview Strain or tearing of the costal digitations of the external abdominal oblique muscle produces localized pain and swelling over the ribs or flank. Severe disruption can lead to abdominal w
Sudden abdominal swelling, especially after foaling or trauma, needs immediate veterinary attention (possible hernia or prepubic tendon rupture). In stallions or colts, scrotal or inguinal swelling should be evaluated promptly. The abdominal wall normally feels firm; soft or bulging areas warrant examination.
## Overview Strain or tearing of the costal digitations of the external abdominal oblique muscle produces localized pain and swelling over the ribs or flank. Severe disruption can lead to abdominal wall herniation. Trauma from kicks or sudden rotational effort is the usual cause. ## Causes Blunt trauma or acute overload. ## Diagnosis Palpation of pain and swelling, ultrasound of the body wall, and exclusion of rib fracture. ## Treatment Rest, anti-inflammatories, and support. Surgical repair if a hernia develops. ## Recovery and Prognosis Good for simple strains. Hernias require surgical consideration. ## Prevention Trauma avoidance. Signs include: localized swelling over the ribs or flank; pain on palpation; reluctance to bend laterally; possible hernia if severe
The superficial inguinal ring is a critical landmark in inguinal hernia and castration. Inguinal hernias can occur, especially in stallions and foals. The muscle and tunic are important for supporting visceral weight; disruption (e.g., prepubic tendon rupture) is serious. The muscle contributes to forced expiration.
Sudden abdominal swelling, especially after foaling or trauma, needs immediate veterinary attention (possible hernia or prepubic tendon rupture). In stallions or colts, scrotal or inguinal swelling should be evaluated promptly. The abdominal wall normally feels firm; soft or bulging areas warrant examination.
See the External Abdominal Oblique - Costal Digitation XI on our interactive 3D horse model. Open 3D Model