Description
The deltoid muscle is a thick, triangular muscle located in the shoulder region, responsible for lifting the arm and facilitating a wide range of shoulder movements. It is named for its resemblance to the Greek letter delta (Δ) and is critical for the abduction, flexion, extension, and rotation of the arm.
Location
The deltoid muscle is situated in the upper arm and shoulder area, covering the shoulder joint and forming the rounded contour of the shoulder.
Development
The deltoid muscle develops from the dorsal muscle mass during embryogenesis, originating from mesenchymal cells that differentiate into myoblasts, which then fuse to form muscle fibres.
Function
The deltoid muscle consists of three distinct parts, each with specific functions:
- Anterior fibres: Flex and medially rotate the arm.
- Middle fibres: Abduct the arm.
- Posterior fibres: Extend and laterally rotate the arm.
Structure
The deltoid muscle comprises three distinct sets of fibres:
- Anterior (clavicular) part: Originates from the lateral third of the clavicle.
- Middle (acromial) part: Originates from the acromion process of the scapula.
- Posterior (spinal) part: Originates from the spine of the scapula.
These fibres converge to insert on the deltoid tuberosity of the humerus.
Arterial Supply
The deltoid muscle is primarily supplied by the posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery.
Venous Drainage
Venous drainage of the deltoid muscle corresponds with its arterial supply, draining into the axillary vein via the accompanying veins of the posterior circumflex humeral artery and the thoracoacromial vein.
Lymphatic Drainage
Lymphatic drainage from the deltoid muscle is directed towards the axillary lymph nodes.
Innervation
The deltoid muscle is innervated by the axillary nerve (C5-C6), which arises from the posterior cord of the brachial plexus.
Normal Variants
- Accessory bands: Occasionally, accessory muscular slips may be present.
- Variation in muscle insertion: Variations in the exact point of insertion on the humerus can occur.
References
Moore, K.L., Dalley, A.F. and Agur, A.M.R., 2013. Clinically Oriented Anatomy. 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Gray, H., Standring, S. and Ellis, H., 2005. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 39th ed. Edinburgh: Elsevier Churchill Livingstone.
Netter, F.H., 2014. Atlas of Human Anatomy. 6th ed. Philadelphia: Saunders.