These consist of four muscles arranged in three layers : the first containing the digastric and stylo-hyoid, the second the mylo-hyoid, and the third the genio-hyoid.
The digastric, named from its two bellies, is composed of two flattened and somewhat spindle-shaped bellies united by a tendon.
- Of posterior belly, the digastric fossa of the temporal bone;
- of anterior belly, the lower border of the body of the mandible just internal to the symphysis.
By its central tendon, which is attached to the outer part of thi lower border of the body of the hyoid bone and the adjacent part of the great cornu.
The posterior belly at its origin consists of short tendinous fibers which soon form a laterally compressed muscle. This passes downwards, forward and inwards, and converges upon a laterally flattened rounded tendon about half an inch above the tip of the great cornu of the hyoid bone.
The anterior belly arises by short tendinous fibers, and forms a muscle flattened from before backwards and from above downwards, which is shorter and smaller than the posterior belly. Its fibers converge as they pass backwards and slightl; downwards and outwards, to end in the flattened tendon a quarter of an inch above the junction of the body and great cornu of the hyoid bone. The central tendon is bound down to its insertion upon the hyoid bone, chiefly by a fibrous expansion given off from its lower border, and to a very small degree also by the oblique arch over it formed by the division of the stylo-hyoid muscle. Often ; part of this aponeurosis passes inwards across the middle line and, uniting mth it fellow, forms a membranous covering to the deeper structures, and unites th inner borders of the two anterior bellies.
The posterior belly, which is really a distinct muscle, belonging to a deeper stratum of the muscular planes, receives a special branch from th facial nerve which enters the upper part of its deep surface. Perhaps this may be due to the fact that this part of the muscle assists in swallowing ; for the facia nerve, by means of the Vidian and the relations which through the lesser superficial petrosal nerve it has with the otic ganglion, may be considered to take part in the function, as well as in the expression of emotions.
The anterior belly receives at the outer part of its deep surface the termina filaments of the mylo-hyoid twig from the inferior dental branch of the inferio maxillary.
- The posterior belly draws upwards and backwards the hyoid bone as in the elevation of the larynx in the second part of deglutition;
- the anterior belly, acting from above, draws upwards and forwards the hyoid bone ; and, acting;
- in the opposite direction, it assists in the depression of the lower jaw and in opening the mouth. Although a comparatively weak muscle, it acts with con siderable power in this movement, for it is inserted at the end of the lever of the second order formed by the mandible, while the resistance which it has to overcome is exerted by muscles acting much nearer to the fulcrum. Moreover, its directioi downwards and backwards is at a considerable angle with the line of the lever, viz that which joins the prominence of the chin to a point a little above the inferio dental foramen.
- If the mandible be fixed and both bellies act, the hyoid bone will be drawn directly upwards. By this action the muscle is of great importance for the elevation of the tongue, which rests upon the upper surface of the hyoid one. It will therefore help in the first part of deglutition, in which the back of he tongue is pressed against the hard palate.
The posterior belly hes beneath the mastoid process, the sterno-mastoid, splenius, and trachelo-mastoid muscles, the facial vein, and the parotid gland; in front lies the stylo-hyoid muscle; deeply, the middle constrictor of the harynx, the hyo-glossus, the external and internal carotid arteries, and some of the branches of the external carotid, the internal jugular vein, the hypoglossal and uperior laryngeal nerves.
The tendon lies beneath the deep cervical fascia, platysma myoides, and part of the stylo-hyoid muscle. Above is the submaxillary gland. On its deep surface is the rest of the stylo-hyoid muscle, the mylo-hyoid, the hyo-glossus, and the hypo-glossal nerve.
The anterior belly is covered by the deep cervical fascia and platysma myoides, and it lies upon the mylo-hyoid muscle.
A second posterior belly may arise in front of the angle of the mandible ; lips may also arise from the styloid process, or the pharynx. The anterior belly may be absent ; it may be partly or entirely united with that of the opposite side, or may send a slip to the median raphe of the mylo-hyoid.
The stylo-hyoid, named from its attachments, is a slender fusiform muscle with a bifurcated lower extremity.
The back and outer surface of the styloid process of the temporal bone rear its base.
The lower border of the body of the hyoid bone at the point of union with the great cornu.
Arising by a short tendon, its fibers soon become fleshy and pass downwards and forwards. Just before its insertion they divide into two bundles, which form an obliquely directed arch bridging over the tendon of the digastric muscle.
A special branch of the facial nerve, which enters its deep surface from behind.
The same as that of the posterior belly of the digastric, viz. to draw the hyoid bone backwards and upwards.
Superficially, the parotid gland and deep cervical fascia ; in front, the submaxillary gland ; behind, the posterior belly of the digastric ; deeply, the middle constrictor and hyo-glossus and the external carotid artery.
The stylo-hyoid may arise in part from the cartilage of the external auditory meatus. It may be absent, or its insertion may be undivided in which case it may pass to the inner or outer side of the digastric tendon. A second stylo-hyoid may run beneath the hyo-glossus to the lesser cornu of the hyoid bone.
The mylo-hyoid, named from its attachment to the lower jaw and to the hyoid bone, is a triangular and somewhat curved sheet.
The whole length of the mylo-hyoid ridge on the inner surface of the body of the mandible.
- The lower edge of the anterior surface of the body of the hyoid bone;
- a median raphe extending from the middle of the lower border of the body of the hyoid bone to the back of the symphysis of the mandible immediately below the genial tubercles.
Arising by fleshy and short tendinous fibers intermingled, the muscle passes inwards and slightly downwards to its insertion by short tendinous fibers into the median raphe and hyoid bone. Sometimes the fleshy fibers are here and there continuous with those of the other side. Each of them is somewhat arched, so that the whole sheet has a slight downward convexity. The muscles of the two sides together form a curved diaphragm which stretches across the angle contained between the two portions of the mandible.
From the third division of the fifth cranial nerve, by the mylo-hyoid branch of the inferior dental which enters the under surface of the muscle bv several filaments.
- To raise the tongue, the floor of the mouth, and the hyoid bone, as in mastication and the first part of swallowing. By its elevation of the hyoid bone it will also exert some influence on the larynx and lower part of the pharynx. These actions will be most efficiently carried out when the jaw is closed.
- Acting from below, it will help in the depression of the lower jaw and in opening the mouth.
Upon its under surface lie the superficial portion of the submaxillary gland, the submental artery, and the anterior belly of the digastric muscle.
Above: it is in contact with the genio-hyoid and hyo-glossus, the sublingual gland, the deep portion of the submaxillary gland, and the hypoglossal nerve.
Variations: The mylo-hyoid may be closely connected with the anterior belly of the digastric. Openings are sometimes found in the muscular sheet, containing lobules of the submaxillary gland.
The genio-hyoid, named from its attachment to the chin and the hyoid bone, is somewhat fusiform, but flattened from above downwards below and from side to side above.
The lower genial tubercle.
The upper and anterior surface of the body of the hyoid bone.
Arising by a short tendon, its fibers pass backwards and slightly downwards, close to those of the fellow muscle of the other side. Near the hyoid bone they spread out laterally, and occupy nearly the whole of the upper and anterior surface of the body, sometimes even a small portion of the greater cornu.
The hypoglossal nerve, which sends filaments to the deep surface.
- To raise and draw forwards the hyoid bone;
- to draw down the mandible. In its direction and action it is closely related to the anterior belly of the digastric.
Superficially, the mylo-hyoid muscle ; deeply, the genio-hyo glossus ; on its median surface, with its fellow.
The genio-hyoid may be double, or it may form one muscle with its fellow of the opposite side.
These consist of four muscles arranged in three layers : the first containing the digastric and stylo-hyoid, the second the mylo-hyoid, and the third the genio- hyoid.