The deltoid is a thick triangular-shaped muscle of the shoulder girdle. It arises from two bones – the scapula and the clavicle, and extends to the humerus of the upper arm. According to the origin of its fibers, the deltoid muscle can be divided into three parts - acromial (lateral), spinal (posterior), and clavicular (anterior).
Origin: the acromial part originates from acromion of the scapula (laterally), the spinal part - from the spine of the scapula (posteriorly), and the clavicular part - from the acromial end of the clavicle (anteriorly).
Insertion: the muscle fibers merge together and insert into the deltoid tuberosity of the humerus, which is located in the middle of the lateral aspect of the shaft of the humerus.
Action: the deltoid enables movements of the upper limb, which are provided according to the activated part the muscle:
- Clavicular or anterior part – internal rotation of the upper arm;
- Spinal or posterior part – external rotation of the upper arm;
- Acromial or lateral part – abduction of the upper arm;
- Clavicular and spinal parts – adduction of the upper arm;
- Clavicular and acromial parts – flexion of the upper arm;
- Acromial and spinal parts – extension of the upper arm.
Innervation: axillary nerve, which originates from the anterior rami of spinal nerves C5 and C6.
Blood supply: deltoid and acromial branches of the thoracoacromial artery, subscapular artery, anterior and posterior circumflex humeral arteries (all these vessels arise from the axillary artery), and deltoid branch of the deep brachial artery (arises from the brachial artery).