Musculus obliquus externus abdominis
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 surfaces of ribs 4-18
Insertion: Linea alba, prepubic tendon, and tuber coxae
Action: Compresses abdomen; supports viscera; lateral flexion; assists expiration
Innervation: Ventral branches of thoracic and lumbar 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 A heave line is the visible hypertrophy of the external abdominal oblique muscle that develops in horses with chronic severe equine asthma (formerly recurrent airway obstruction or heaves
## Overview External oblique strain is injury to the external abdominal oblique muscle, typically from kicks or falls. It causes flank pain, swelling, and reluctance to bend. Severe tears may produce
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 A heave line is the visible hypertrophy of the external abdominal oblique muscle that develops in horses with chronic severe equine asthma (formerly recurrent airway obstruction or heaves). It runs diagonally from the flank toward the elbow and reflects prolonged increased respiratory effort. The underlying disease is an allergic airway inflammation triggered by dust and mold. ## Causes Chronic exposure to airborne allergens causing bronchospasm and mucus accumulation. ## Diagnosis Clinical signs of chronic cough, nasal discharge, and increased respiratory effort, plus the heave line in advanced cases. Endoscopy and bronchoalveolar lavage support the diagnosis. ## Treatment Environmental management (soak hay, maximize turnout, reduce dust) is the cornerstone. Corticosteroids and bronchodilators control inflammation and spasm. ## Recovery and Prognosis The heave line may persist. Clinical signs can be well controlled with strict management, but the condition is usually lifelong. ## Prevention Minimize dust and mold exposure, especially in stabled horses. Signs include: visible diagonal muscle ridge on the flank; labored breathing with abdominal effort; chronic cough; nasal discharge; exercise intolerance
## Overview External oblique strain is injury to the external abdominal oblique muscle, typically from kicks or falls. It causes flank pain, swelling, and reluctance to bend. Severe tears may produce ventral edema or progress to hernia. ## Causes Direct trauma. ## Diagnosis Clinical palpation and ultrasound. ## Treatment Conservative management with rest and anti-inflammatories. Surgery for hernias. ## Recovery and Prognosis Good for uncomplicated strains. ## Prevention Safe group housing and fencing. Signs include: localized flank swelling; pain on palpation; reluctance to bend; possible ventral edema
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 on our interactive 3D horse model. Open 3D Model