Overview
The horse has no clavicle. Instead, cranial to the shoulder joint, the muscle is crossed by a clavicular intersection (intersectio clavicularis), a faint tendinous sheet that divides it into two muscles:
- Cleidobrachialis (m. cleidobrachialis): from the clavicular intersection to the arm.
- Cleidocephalicus (m. cleidocephalicus): from the clavicular intersection to the head. In the horse it has only a mastoid part (cleidomastoideus); there is no cleido-occipitalis.
In the equine neck the brachiocephalicus is partly fused with the omotransversarius, which lies deep to it.
Origin and insertion
- The cleidobrachialis runs from the clavicular intersection to the crest of the humerus, distal to the deltoid tuberosity, and to the brachial fascia.
- The cleidomastoideus runs from the clavicular intersection to the mastoid process of the temporal bone.
Action
It is the main muscle that advances the thoracic limb (shoulder extension). With the limb fixed, it draws the head and neck ventrally and laterally.
Innervation
Accessory nerve and ventral branches of the cervical nerves for the cleidomastoideus; axillary nerve for the cleidobrachialis.
Clinical relevance
- It forms the dorsal border of the jugular groove. The external jugular vein, in this groove, is the usual route for blood sampling, intravenous medication and catheter placement in the horse.
- Its action sets the length of the cranial phase of the forelimb stride. A shortened cranial phase can reflect pain in the neck or shoulder region, as well as distal lameness.