deltoïde

Deltoïde is a term used in French to denote several concepts in anatomy and geometry, and it is derived from the triangular, delta-shaped form that the word evokes.

In anatomy, the deltoïde muscle, or deltoid, is a large, triangular shoulder muscle that gives the shoulder its rounded contour. It consists of three functional heads: anterior, middle, and posterior. The muscle originates from the lateral third of the clavicle, the acromion, and the spine of the scapula, and it inserts onto the deltoid tuberosity of the humerus. It is innervated mainly by the axillary nerve (C5–C6) and receives blood supply from branches of the posterior circumflex humeral artery and the deltoid branch of the thoracoacromial trunk. Its actions include abduction of the arm (primarily by the middle head), with the anterior fibers contributing to flexion and medial rotation and the posterior fibers assisting in extension and lateral rotation. The deltoïde plays a central role in shoulder stabilization and movement and is a common site for intramuscular injections.

In geometry, the deltoïde (often called the Steiner hypocycloid) refers to a three-cusped plane curve generated when a circle rolls inside a larger circle with three times its diameter. The resulting curve has three symmetrical cusps and resembles a rounded triangle. The deltoïde appears in various mathematical contexts, such as caustics of circles and envelope problems, and is notable for its simple, highly symmetric shape.

Etymology-wise, the term reflects the delta-like, triangular form associated with the curve and the muscle’s broad, triangular contour.