Overview
It has a cervical part (pars cervicalis), very extensive in the horse, and a thoracic part (pars thoracica). Thick tendinous sheets run through it and let it bear weight passively. The horse has no clavicle, so the thoracic limb does not articulate with the trunk: it is attached by a synsarcosis, of which the serratus ventralis is the main component. Together with the pectorals it forms the thoracic sling, from which the trunk hangs between the two scapulae.
Origin
- Cervical part: transverse processes of the caudal cervical vertebrae.
- Thoracic part: first ribs, by slips that interdigitate with those of the external abdominal oblique.
Insertion
Serrated face (facies serrata) on the dorsal part of the medial surface of the scapula, and the scapular cartilage.
Action
Supports the trunk and moves the scapula. The cervical part draws the dorsal angle of the scapula cranially and the thoracic part caudally. With the limb fixed, the thoracic part assists inspiration.
Innervation
Ventral branches of the cervical nerves (cervical part) and long thoracic nerve (n. thoracicus longus) for the thoracic part.
Clinical relevance
- Its tendinous sheets make the thoracic sling the proximal part of the stay apparatus: the trunk is suspended without continuous muscle contraction while the horse stands.
- Intramuscular injections in the neck are given in a triangle bounded by the nuchal ligament (dorsal), the cervical vertebrae (ventral) and the cranial border of the scapula (caudal). The needle enters the cervical part of this muscle and its neighbours; too high and it reaches the nuchal ligament, too low and it reaches the vertebrae.