The neck is divisible into an anterior and a posterior triangle by the sternocleidomastoid muscle on the anterior and lateral aspects. Each of these triangles are composed of sub-triangles that are bounded or defined by structures that are both visible and palpable, and structures that are not palpable. The anterior triangle may be further subdivided into the submental (apex at the chin, hyoid bone as base, anterior bellies of digastric muscles as sides), carotid (posterior belly of digastric and stylohyoid as superior base, sternocleidomastoid as lateral side, superior belly of omohyoid as medial side), muscular (median line of the neck as base, anterior margin of sternocleidomastoid as inferior side, superior belly of omohyoid as superior side) and the digastric (anterior belly of digastric anteroinferiorly, posterior belly of digastric and stylohyoid as the base, mandible as lateral side, anterior belly of digastric as medial side) triangles, while the posterior triangle includes the occipital (posterior edge of sternocleidomastoid as anterior side, anterior edge of trapezius as posterior side and inferior belly of omohyoid as inferior base) and supraclavicular (omoclavicular) triangles as its sub-triangles.
The omoclavicular triangle, which is also called the subclavian triangle because the third part of the subclavian artery is located in it, is indicated on the surface of the neck by the supraclavicular fossa which is also the lowest portion of the posterior triangle. This triangle is clinically relevant, for example it is a common site for the palpation of pathologically enlarged lymph nodes, as well as a site where the pulsations of great veins may be seen. It contains the subclavian artery, the inferior part of the external jugular vein, and these vessels are separated within it by the investing layer of deep cervical fascia. The trunks of the brachial plexus may be felt above and behind the triangle.
The subclavian triangle is bounded by the clavicle inferiorly and the posterior border of sternocleidomastoid anteriorly and its superior side is formed by the inferior belly of the omohyoid muscle. The first digitation of serratus anterior muscle and the first rib forms the floor of the triangle. This is owing to the fact that the posterior end of the first rib may be felt as fullness in the posterior aspect of the fossa.