Overview
Veterinary nomenclature distinguishes two groups, the superficial pectorals (mm. pectorales superficiales) and the deep pectoral (m. pectoralis profundus). In the horse the superficial pectorals have two parts:
- Descending pectoral (m. pectoralis descendens): cranial, from the manubrium to the arm. It forms the bulge of the chest seen between the forelimbs.
- Transverse pectoral (m. pectoralis transversus): from the sternum to the antebrachial fascia.
The deep pectoral (ascending pectoral) is much larger and spreads over the ventral thorax as far as the abdominal wall. The horse also has a prominent subclavius (m. subclavius), which climbs along the cranial border of the supraspinatus. Together with the serratus ventralis, the pectorals form the thoracic sling, which suspends the trunk between the thoracic limbs.
Between the descending pectoral and the brachiocephalicus lies the lateral pectoral groove, in which the cephalic vein runs up, visible under the skin.
Origin
- Descending pectoral: manubrium of the sternum.
- Transverse pectoral: ventral surface of the sternum.
- Deep pectoral: ventral surface of the sternum, costal cartilages and fascia of the ventral abdominal wall.
Insertion
- Descending pectoral: crest of the humerus, together with the cleidobrachialis.
- Transverse pectoral: medial antebrachial fascia.
- Deep pectoral: lesser and greater tubercles of the humerus.
Action
Adduct the thoracic limb and support the trunk between the limbs. The deep pectoral also draws the limb caudally and, with the limb planted, pulls the trunk cranially.
Innervation
Cranial and caudal pectoral nerves (nn. pectorales) from the brachial plexus.
Clinical relevance
- The pectoral mass is one of the horse's intramuscular injection sites; local reactions can make limb movement painful.
- Infection with Corynebacterium pseudotuberculosis causes large abscesses in the pectoral region ("pigeon fever"), mainly in dry areas and in autumn.