Overview
It lies on the lateral surface of the scapula and shoulder, caudal to the spine, and covers part of the infraspinatus. The horse has no acromion: the scapular spine ends distally without that prominence. The deltoideus therefore has no separate acromial part as in cattle; it arises almost entirely from an aponeurosis attached to the spine and the caudal border of the scapula.
Origin
Spine of the scapula, through an aponeurosis, and caudal border of the scapula.
Insertion
Deltoid tuberosity (tuberositas deltoidea) of the humerus, very prominent in the horse.
Action
Flexes the shoulder and abducts the arm.
Innervation
Axillary nerve (n. axillaris).