internally joins a similar process from the opposite side to form with it a tendinous expansion, (often assisted by a few fleshy fibres from the anterior belly of the digastric,) which reaches from the os hyoides as far as the jaw, and contributes to support the floor of the mouth.
The relations of this muscle are complicated and important: the convexity of its curve is the upper limit of the anterior triangle of the neck; its concavity bounds a space, the area of which extends within the jaw to the myloid ridge, containing various parts, and named from the muscle the digastric space; its posterior belly crosses the external and internal carotid, the facial, lingual, and occipital branches of the former, the internal jugular vein, the three divisions of the eighth, the ninth, and the sympathetic nerve, the side of the pharynx, the trachelo-mastoid and styloid muscles, and the hyo-glossus. The sterno-mastoid and splenius cover its origin; the portio dura emerges at its anterior edge, along which the posterior aural artery runs, and round which the posterior part of the parotid gland is folded. Its anterior belly and tendon support the submaxillary gland, are covered by fascia and platysma, and correspond to the mylo-hyoid muscle, which is covered and strengthened by the aponeurotic expansion derived from the digastric.
The action of this muscle varies according to the fixity of the jaw; when the mouth is firmly closed, the contraction of the two bellies will draw the hyoid bone vertically upward, and communicate to the pharynx the movement of elevation, which adapts it for receiving the masticated food. A firm closure of the jaw, a contraction of the digastric muscles, and consequent shortening of the pharynx, (indicated by rising of the pomum Adami,) are well known acts in the process of deglutition. When the hyoid bone is fixed by its depressors (and perhaps in some degree retracted by the joint actions of the posterior belly of the digastric and of the omo-hyoid), the anterior belly, both passively as a reflected cord, and actively, in virtue of its muscular fibres, depresses the lower jaw and opens the mouth. Simultaneously, too, with its act of raising the pharynx, this muscle must tighten, by its posterior belly, the mesial aponeurotic expansion, which joins it to its fellow; and, by so doing, must assist the mylo-hyoid in raising the floor and reducing the capacity of the mouth. It fulfils, therefore, important uses in the mechanism of deglutition.
The stylo-hyoid muscle is an accessory to the posterior belly of the digastric, and arises from the outer surface of the styloid process, about midway from its base, by a small round tendon, which soon swells into an elongated body. This lies along the posterior belly of the digastric, parallel to its anterior edge, and, when it reaches the os hyoides, is inserted into the outer surface of that bone, at the union of its body and cornu, by short aponeurotic fibres. It usually divides, just previously to its insertion, to give passage to the tendon of the digastric. The portio dura of the seventh pair emerges between its origin and that of the digastric: in other respects its relations so entirely agree with those of the descending belly of that muscle, as do likewise its uses, that no particular description of these is necessary.
The mylo-hyoid muscles are so mutually dependent that they might almost be described as a single muscle. They arise on either side from the oblique or myloid ridge on the buccal surface of the lower jaw in its whole extent, i.e. from opposite the last molar tooth to the neighbourhood of the symphysis. The fleshy fibres, that succeed the short aponeurosis of origin, proceed parallelly toward the median line, and are inserted into a raphe, which reaches from the symphysis of the jaw to the body of the hyoid bone, and likewise into the upper border of the body of that bone. The anterior fibres are short; those which succeed progressively increase in length, and the posterior, which are fixed to the hyoid bone, are of all the longest. Each muscle is, therefore, triangular, having an outer edge by which it rises from the jaw, an inner edge of union with its fellow, and a posterior edge, which is seen to extend, in the digastric space, from the posterior extremity of the myloid ridge to the upper edge of the body of the hyoid bone, close to its cornu. The under surface of the muscle corresponds to the submaxillary gland and to the insertion of the digastric; its upper surface sustains the tongue and floor of the mouth,—from the mucous membrane of which it is separated by Wharton's duct, the sub-lingual gland, and gustatory nerve; it also corresponds to the hyoglossus, genio-hyoideus, and genio-hyo-glossus, and to the termination of the lingual artery and nerve. The duct of the sub-maxillary gland winds round its posterior edge, in proceeding to open beside the frænum linguæ. The habitual state of this muscle is one in which it is rendered, with its fellow, convex downward by pressure of the superincumbent parts; and so its surfaces cannot strictly be said to face upward and downward, but with a modification of these directions respectively inward and outward. Thus the two muscles furnish a concave floor to the mouth, and it is only in their contraction, which accordingly diminishes the cavity, that this becomes strictly horizontal. Their action, especially when assisted by other muscles, is to propel the masticated food by lessening the capacity of the mouth.
The hyo-glossus is a thin quadrilateral plane of parallel muscular fibres, having the attachments which its name indicates. It rises from the entire length of the great cornu and adjoining part of the body of the os hyoides, on their upper surface, and ascends to be inserted into the side of the tongue. From beneath its anterior thicker edge the lingual artery emerges; its posterior thin border receives the insertion of the stylo-glossus; its deep surface corresponds to the genio-hyo-glossus and lingualis, from the former of which it is partly separated by the lingual artery; its external face is separated from the mylo-hyoid muscle by the lingual and gustatory nerves and duct of the submaxillary gland.
