The suprahyoid muscles are
| Digastricus | Mylohyoideus |
| Stylohyoid | Geniohyoideus |
The Digastric (fig. 590) consists of two fleshy bellies united by an intermediate rounded tendon. It lies below the body of the mandible, and extends, in a curved form, from the mastoid process to the chin. The posterior belly, longer than the anterior, arises from the mastoid notch of the temporal bone and passes downwards and forwards. The anterior belly arises from the digastric fossa on the base of the mandible close to the median plane; and passes downwards and backwards. The two bellies end in an intermediate tendon, which perforates the Stylohyoid muscle. It is held in connexion with the side of the body and the greater cornu of the hyoid bone by a fibrous loop, which is sometimes lined by a synovial sheath. An aponeurotic layer, sometimes named the suprahyoid aponeurosis, is given off from the tendon of the Digastric muscle, and is attached to the body and greater cornu of the hyoid bone.
Relations.-Its superficial surface is in relation with the Platysma, Sternocleidomastoid, part of the Splenius, Longissimus capitis, mastoid process, Stylohyoid and the parotid gland. The deep surface of the anterior belly lies on the Mylohyoid; that of the posterior belly on the Superior oblique, Rectus capitis lateralis, the transverse process of the atlas vertebra, the accessory nerve, internal jugular vein, occipital artery, hypoglossal nerve, the internal and external carotid, the facial and lingual arteries and the Hyoglossus muscle (fig. 591).
Nerve-supply.–The anterior belly of the Digastric is supplied by the mylohyoid branch of the inferior dental (inferior alveolar) nerve; the posterior belly by the facial nerve.
Actions.–When the anterior belly of the Digastric takes its fixed point below, it depresses the front of the mandible. When both bellies are in action from above they elevate the hyoid bone.
The Stylohyoid (figs, 591, 592) arises by a delicate little tendon from the posterior and lateral surfaces of the styloid process, near its base; and passing downwards and forwards, is inserted into the body of the hyoid bone, at its junction with the greater cornu, and just above the Omohyoid. It is perforated, near its insertion, by the tendon of the Digastric.
Nerve-supply.-The Stylohyoid is supplied by the facial nerve, Action.-The Stylohyoid draws the hyoid bone upwards and backwards. The stylohyoid ligament-In connection with the Stylohyoid muscle a ligamentous band, named the stylohyoid ligament, may be described. It is a fibrous cord, which is attached to the tip of the styloid process of the temporal bone and to the lesser cornu of the hyoid bone. It frequently contains a little piece of cartilage in its center, is often partially ossified, and in many animals forms a distinct bone, termed the epihyal. The Stylohyoid ligament represents a portion of the skeletal element of the second visceral arch.
Relations.-Its superficial or inferior surface is in relation with the Platysma, the anterior belly of the Digastric, the suprahvoid aponeurosis, the superficial part of the submandibular (submaxillary) gland, the facial and submental vessels, and the mvlohyoid vessels and nerve. Its deep or superior surface is in relation with the Geniohyoid, part of the Hyoglossus, and the Styloglossus muscles, the hypoglossal and lingual nerves, the submandibular ganglion, the sublingual gland, the deep portion of the submandibular gland and the submandibular duct, the lingual and sublingual vessels, and the bucca.l mucous membrane.
Actions.—Acting from below the Mylohyoid depresses the front of the mandible; acting from above it raises the hyoid bone and the floor of the mouth.
The Geniohyoid is a narrow muscle, situated above the medial part of the Mylohyoid. It arises from the inferior genial tubercles on the back of the symphysis menti, and runs backwards and slightly downwards, to be inserted into the anterior surface of the body of the hyoid bone; it is in contact with its fellow of the opposite side.
Nerve-supply. -The Geniohyoid muscle is supplied by the first cervical nerve through the hypoglossal nerve.
Actions,-When the Geniohyoid acts from the hyoid bone, it depresses the front of the mandible; when it acts from the mandible it raises and pulls forwards the hyoid bone.
The infrahyoid muscles are:
Sternohyoideus
Sternothyreoideus
Thyreohyoideus
Geniohyoideus
The Sternohyoid (figs. 590, 592), a thin, narrow muscle, arises from the posterior surface of the medial end of the clavicle, the posterior sternoclavicular ligament, and the upper and posterior part of the manubrium sterni. Passing upwards and medially, it is inserted into the lower border of the body of the hyoid bone. It sometimes presents, near its origin, a transverse tendinous intersection. Below, the Sternohyoid is separated from its fellow by a considerable interval; but the two muscles come into contact with each other in the middle of their course, and are contiguous above.
Nerve -supply,–The Sternohyoid muscle is supplied by branches from the loop (ansa hypoglossi) between the ramus descendens hypoglossi and the nervus descendens cervicalis.
Action.-The Sternohyoid depresses the hyoid bone.
The Sternothyroid (figs. 590, 592) is shorter and wider than the Sternohyoid, and lies under cover of it. It arises from the posterior surface of the manubrium sterni below the origin of the Sternohyoid, and from the edge of the cartilage of the first rib, and sometimes that of the second; it is inserted into the oblique line on the lamina of the thyroid cartilage. At the lower part of the neck this muscle is in contact with its fellow, but it diverges as it ascends; it is occasionally traversed by a transverse or oblique tendinous intersection. It is closely applied to the anterolateral aspect of the lobe of the thyroid gland.
Nerve-supply.-The Sternothyroid muscle is supplied by branches from the ansa hypoglossi.
Action.-The Sternothyroid draws the larynx downwards.
The thyrohyoid, a small, quadrilateral muscle, may be looked upon as an upward continuation of the Sternothyroid. It arises from the oblique line on the lamina of the thyroid cartilage, and is inserted into the lower border of the greater cornu of the hyoid bone.
Nerve-supply.–The Thyrohyoid muscle is supplied by a branch from the hypoglossal nerve. Like the nerve to the Geniohyoid, this nerve is ultimately derived from the first cervical nerve
Actions.-The Thyrohyoid depresses the hyoid bone, or raises the larynx.
The Omohyoid (figs. 590, 592) consists of two fleshy bellies united at an angle by an intermediate tendon. It arises from the upper border of the scapula near the suprascapular notch, and. occasionally from the suprascapular (superior transverse) ligament, its extent of attachment to the scapula varying from a few millimeters to 2.5 cm. From this origin, the inferior belly forms a flat, narrow fasciculus, which inclines forwards and slightly upwards across the lower part of the neck, being bound to the clavicle by a fibrous expansion, it then passes behind the Sternocleidomastoid and there ends in the intermediate tendon. The superior belly passes almost vertically upwards from this tendon, close to the lateral border of the Sternohyoid muscle, and is inserted into the lower border of the body of the hyoid bone, lateral to the insertion of the Sternohyoid. The intermediate tendon, which varies in length and form, usually lies on the internal jugular vein, opposite the arch of the cricoid cartilage. It is held in position by a process of the deep cervical fascia, which ensheathes it and is attached below to the clavicle and the first rib; it is by this fascial process that the angular form of the muscle is maintained.
Nerve-supply.-The superior belly of the Omohyoid is supplied by the ramus descendens hypoglossi; the inferior belly by a branch from the ansa hypoglossi.
Actions.-The Omohyoid muscle depresses the hyoid bone and carries it backwards and laterally. The Omohyoids are concerned also in prolonged inspiratory efforts; by rendering tense the lower part of the deep cervical fascia they lessen the inward suction of the soft parts, which would otherwise compress the great vessels and the apices of the lungs.
The inferior belly of the Omohyoid divides the posterior triangle of the neck into an upper or occipital and a lower or subclavian triangle, while its superior belly divides the anterior triangle into an upper or carotid and a lower or muscular triangle.
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