The four pairs of suprahyoid muscles are related to the submental and submandibular triangles (Fig. 8.166). They pass in a superior direction from the hyoid bone to the skull or mandible and raise the hyoid, as occurs during swallowing.
The stylohyoid muscle arises from the base of the styloid process and passes anteroinferiorly to attach to the lateral area of the body of the hyoid bone (Fig. 8.167). During swallowing it pulls the hyoid bone posterosuperiorly and it is innervated by the facial nerve [VII].
The digastric muscle has two bellies connected by a tendon, which attaches to the body of the hyoid bone (Fig. 8.167):
The posterior belly arises from the mastoid notch on the medial side of the mastoid process of the temporal bone.
The anterior belly arises from the digastric fossa on the lower inside of the mandible.
The tendon between the two bellies, which is attached to the body of the hyoid bone, is the point of insertion of both bellies. Because of this arrangement, the muscle has multiple actions depending on which bone is fixed:
When the mandible is fixed, the digastric muscle raises the hyoid bone.
When the hyoid bone is fixed, the digastric muscle opens the mouth by lowering the mandible.
Innervation of the digastric muscle is from two different cranial nerves.
The innervation of the posterior belly of the digastric muscle is by the facial nerve [VII], whereas the anterior belly of the muscle is innervated by the mandibular division [V3] of the trigeminal nerve [V].
The mylohyoid muscle is superior to the anterior belly of the digastric and, with its partner from the opposite side, forms the floor of the mouth (Fig. 8.167). It originates from the mylohyoid line on the medial surface of the body of the mandible and inserts into the hyoid bone and also blends with the mylohyoid muscle from the opposite side.
This mylohyoid muscle supports and elevates the floor of the mouth and elevates the hyoid bone. It is innervated by the mandibular division [V3] of the trigeminal nerve [V].
The geniohyoid muscle is superior to the floor of the oral cavity and is not generally considered a muscle of the anterior triangle of the neck; however, it can be regarded as a suprahyoid muscle. It is the final muscle in the suprahyoid group (Fig. 8.167). A narrow muscle, it is superior to the medial part of each mylohyoid muscle. The muscles from each side are next to each other in the midline.
The geniohyoid arises from the inferior mental spine of the mandible and passes backward and downward to insert on the body of the hyoid bone.
It has two functions depending on which bone is fixed:
Fixation of the mandible elevates and pulls the hyoid bone forward.
Fixation of the hyoid bone pulls the mandible downward and inward.
The geniohyoid is innervated by a branch from the anterior ramus of C1 carried along the hypoglossal nerve [XII].
The four infrahyoid muscles are related to the muscular triangle (Fig. 8.166). They attach the hyoid bone to inferior structures and depress the hyoid bone. They also provide a stable point of attachment for the suprahyoid muscles. Because of their appearance, they are sometimes referred to as the “strap muscles.”
The sternohyoid muscle is a long, thin muscle originating from the posterior aspect of the sternoclavicular joint and adjacent manubrium of the sternum (Fig. 8.168). It ascends to insert onto the body of the hyoid bone. It depresses the hyoid bone and is innervated by the anterior rami of C1 to C3 through the ansa cervicalis.