M. obliquus externus abdominis, digitatio costalis I
The most cranial costal digitation of the external abdominal oblique, originating from the lateral surface of the 4th or 5th rib. The external abdominal oblique is the most superficial of the flat abdominal muscles, with fibers running caudoventrally. Its costal digitations interdigitate with the serratus ventralis thoracis.
Origin: Lateral surface of rib 5
Insertion: Linea alba and prepubic tendon via aponeurosis; tuber coxae
Action: Flexes the trunk laterally; compresses the abdomen; assists expiration; supports abdominal viscera
Innervation: Ventral branches of thoracic (T10–T18) and lumbar (L1–L3) spinal nerves
Strain or tearing of individual costal digitations of the external abdominal oblique, often from trauma (kicks) or sudden rotational movements. Common site for abdominal wall herniation in horses.
Strain or tearing of individual costal digitations of the external abdominal oblique, often from trauma (kicks) or sudden rotational movements. Common site for abdominal wall herniation in horses. Signs include: Localized swelling over the affected rib; pain on palpation; reluctance to bend laterally; may develop into a hernia if multiple digitations are disrupted.
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