Digastricus.
Stylo-Hyoid.
Mylo-Hyoid.
Genio-Hyoid.

Dissection. To dissect these muscles, a block should be placed beneath the back of the neck, and the head drawn backwards, and retained in that position. On the removal of the deep fascia, the muscles are at once exposed.

The Digastric, so called from its consisting of two fleshy bellies united by an intermediate rounded tendon, is a small muscle, situated immediately beneath the side of the body of the lower jaw, and extending, in a curved form, from the side of the head to the symphysis of the jaw. The posterior belly, longer than the anterior, arises from the digastric groove on the inner side of the mastoid process of the temporal bone, and passes downwards, forwards, and inwards. The anterior belly, being reflected upwards and forwards, is inserted into a depression on the inner side of the lower border of the jaw, close to the symphysis. The tendon of this muscle perforates the Stylo-hyoid, and is held in connection with the side of the body of the hyoid bone by an aponeurotic loop, lined by a synovial membrane. A broad aponeurotic layer is given otf from the tendon of the digastric on each side, which is attached to the hyoid bone: this is termed the supra-hyoid aponeurosis. It forms a strong layer of fascia between the anterior portion of the two muscles, and forms a firm investment for the other muscles of the supra-hyoid region, which lie beneath it.

The Digastric muscle divides the anterior superior triangle of the neck into two smaller triangles; the upper, or sub-maxillary, being bounded above by the lower jaw; below, by the two bellies of the Digastric muscle: the lower, or superior carotid triangle, being bounded above by the posterior belly of the Digastric; behind, by the Sterno-mastoid; below, by the Omo-hyoid. (fig. 136).

Relations. By its superficial surface, with the Platysma., Sterno-mastoid, part of the Stylo-hyoid muscle, and the parotid and sub-maxillary glands. By its deep surface, its anterior beily lies on the Mylo-hyoid, the posterior belly lies on the Stylo-glossus, Stylo-pharyngeus, and Hyo-glossus muscles, the external carotid and its lingual and facial branches, the internal carotid, internal jugular vein, and hypoglossal nerve.

The Stylo-Hyoid is a small, slender muscle, lying in front of, and above, the posterior belly of the Digastric. It arises from the middle of the outer surface of the styloid process; and, passing downwards and forwards, is inserted into the body of the hyoid bone, just at its junction with the greater cornu, and immediately above the Omo-hyoid.

This muscle is perforated near its insertion by the tendon of the Digastric muscle.

Relations. By its superficial surface, with the Sterno-mastoid and Digastric muscles, the parotid and submaxillary glands. Its deep surface has the same relations as the posterior belly of the Digastric.

Dissection. The Digastric and Stylo-hyoid muscles should be removed, in order to expose the next muscle.

The Mylo-Hyoid is a flat triangular plane of muscular fibre, situated immediately beneath the anterior belly of the Digastric, and forming, with its fellow of the opposite side, a muscular floor for the cavity of the mouth. It arises from the whole length of the mylo-hyoid ridge, from the symphysis in front, to the last molar tooth behind. The posterior fibres pass obliquely forwards, to be inserted into the body of the os hyoides. The middle and anterior fibres are inserted into the median fibrous raphe, where they join at an angle with the fibres of the opposite muscle. This median raphe is sometimes wanting; the muscular fibres of the two sides are then directly continuous with one another.

Relations. By its superficial or inferior surface with the Platysma, the anterior belly of the Digastric, the supra-hyoid fascia, the submaxillary gland, and submental vessels. By its deep or superior surface', with the Genio-hyoid, part of the Hyo-glossus, and Stylo-glossus muscles, the lingual and gustatory nerves, the sublingual gland, and the buccal mucous membrane. Wharton's duct curves around its posterior border in its passage to the mouth.

Dissection. The Mylo-hyoid should now be removed, in order to expose the muscles which lie beneath; this is effected by detaching it from its attachments to the hyoid bone and jaw, and separating it by a vertical incision from its fellow of the opposite side.

The Genio-Hyoid is a narrow slender muscle, situated immediately beneath the inner border of the preceding. It arises from the inferior genial tubercle on the inner side of the symphysis of the lower jaw, and descends downwards and backwards, to be inserted into the anterior surface of the body of the os hyoides. This muscle lies in close contact with its fellow of the opposite side, and increases slightly in breadth as it descends.

Relations. It lies between the Mylo-hyoid and the Genio-hyo-glossus muhcles.

Nerves. The Digastric is supplied, its anterior belly, by the mylo-hyoid branch of the inferior dental; its posterior belly, by the facial and glosso-pharyngeal; the Stylo-hyoid, by the facial and glosso-pharyngeal; the Mylo-hyoid, by the mylo-hyoid branch of the inferior dental; the Genio-hyoid, by the lingual.

Actions. This group of muscles performs two very important actions. They raise the hyoid bone, and with it the base of the tongue, during the act of deglutition; or, when the hyoid bone is fixed by its depressors and those of the larynx, they depress the lower jaw. During the first act of deglutition, when the mass is being driven from the mouth into the pharynx, the hyoid bone, and with it the tongue, is carried upwards and forwards by the anterior belly of the Digastric, the Mylo-hyoid, and Genio-hyoid muscles. In the second act, when the mass is passing, the direct elevation of the hyoid bone takes place by the combined action of all the muscles; and after the food has passed, the hyoid bone is carried upwards and backwards by the posterior belly of the Digastric and Stylo-hyoid muscles, which assists in preventing the return of the morsel into the cavity of the mouth.

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