May 18, 2012
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Supra And Infrahyoid Muscles

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.

Figure 590

The Digastric divides the upper part of the anterior triangle of the neck into three triangles : (1) the digastric triangle, bounded above by the base of the mandible and a line continuing this from the angle of the mandible to the Sternocleidomastoid, below by the posterior belly of the Digastric and the Stylohyoid, in front by the anterior belly of the Digastric; (2) the carotid triangle, bounded above by the posterior belly of the Digastric and Stylohyoid, behind by the Sternocleidomastoid, below by the Omohyoid; (3) the submental triangle, bounded on each side by the anterior belly of the Digastric, and inferiorly by the body of 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.

Figure 591

The Mylohyoid (figs. 590, 592) is situated immediately above the anterior belly of the Digastric, and forms, with its fellow of the opposite side, a muscular floor for the cavity of the mouth. It forms a flat, triangular sheet, which arises from the whole length of the Sylohyoid line of the mandible. The posterior fibers pass medially and slightly downwards, to be inserted into the front of the body of the hyoid bone near its lower border. The middle and anterior fibers are inserted into a median fibrous raphe which stretches from the symphysis menti to the hyoid bone. This median raphe is sometimes wanting, if so, the two muscles are continuous.

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.

Figure 592

Nerve-supply.-The Mylohyoid muscle is supplied by the mylohyoid branch of the inferior dental (inferior alveolar) nerve.

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.