Triangles of the neck.  The neck is divided into two large triangles (anterior and posterior cervical triangles) by the sternocleidomastoid muscle. It arises from two heads (sternal and clavicular) inferiorly and takes an oblique course superiorly to insert into the mastoid process and lateral aspect of the superior nuchal line. These larger triangles are further subdivided into smaller triangles. Anteriorly, the muscular triangle or infrahyoid compartment is bounded by the body of the hyoid bone and midline, the superior belly of the omohyoid, and the lower anterior border of the sternocleidomastoid. It consists of the infrahyoid muscles and overlies the thyroid and parathyroid glands and visceral compartment. The submandibular triangle is bounded by the anterior and posterior digastric muscles and lower border of the mandible. It is largely filled by the submandibular gland. The hypoglossal nerve can be seen entering this triangle where it passes deep to the mylohyoid muscle to enter the floor of the mouth. The marginal mandibular branch of the facial nerve often descends into this triangle and crosses superficial to the facial vessels. Inferior to this nerve is the cervical branch of facial that descends into the neck to supply the platysma muscle. The only unpaired cervical triangle is the submental triangle bounded by the right and left anterior digastric muscles and the body of the hyoid. The posterior triangle is divided into unequal sized triangles by the inferior belly of omohyoid. Along the midpoint of the posterior border of the sternocleidomastoid is Erb's point where cutaneous branches of the cervical plexus emerge. One of these, the transverse cervical nerve, crosses the muscle to supply skin overlying the anterior triangle. (Image courtesy of AL Rhoton, Jr.)