from their fellows on the opposite side only by the mesial raphe of the cervical fascia,—covering the trachea, thyroid body, with a portion of the larynx, and overlapping the sheath of the carotid vessels. They are isolated from each other, and from the other muscles of their neighbourhood, by processes of the cervical aponeurosis. The sterno-hyoid arises just within the thorax from the deep surface of the manubrium sterni, from the cartilage of the first rib, and from the ligament of the sterno-clavicular joint, and is separated from that of the opposite side by nearly the whole breadth of the sternum. As it ascends, it more nearly approaches its fellow, and the two are inserted side by side into the under surface of the body of the os hyoides, in close connexion, by their outer edges, with the omo-hyoid muscles, which are inserted beside them. The sterno-hyoid lies in its whole length on the sterno-thyroid muscle and its prolongation the thyro-hyoid, and these separate it from immediate contact with the important organs to which it is related.
The sterno-thyroid is broader and rises lower within the chest,—from the cartilage of the second rib, and from the adjoining surface of the sternum, on which it extends almost to the median line: its fibres ascend nearly vertically, and terminate at an oblique fibrous arch on the ala of the thyroid cartilage, and at the tubercles, to which this arch is attached; hence a muscle of similar volume is prolonged, (which may be described as rising from the oblique cord and from its points of attachment, but which, in direction, size, and form, accurately continues the sterno-thyroid,) and, after a course of an inch and a half, is inserted into the body and part of the cornu of the os hyoides, beneath the omo-hyoid and sterno-hyoid, and superficially to the thyro-hyoid membrane. To this is given the name of thyro-hyoid.
The stemo-thyroid and thyro-hyoid are covered throughout by the sterno-hyoid and in part by the sterno-mastoid and omo-hyoid muscles. The sterno-thyroid corresponds by its inner edge to the inferior thyroid vein,—by its outer edge receives the terminal branch of the descendens noni, by its deep surface covers the thyroid body and many of its vessels, the trachea and part of the larynx, and the sheath of the carotid vessels: by its origin it enters into the mediastinum, covers the great arterial trunks springing from the arch of the aorta and the brachio-cephalic veins. From these parts it is separated by the remains of the thymus gland. The thyro-hyoid muscle covers the superior laryngeal nerve and artery as they pierce the wall of the larynx. These muscles are fleshy in their whole extent, with exception of the short tendinous fibres, by which they have their origin and insertion: the sterno-thyroid has frequently a transverse tendinous intersection in some part of its course.
The omo-hyoid is a slender but long bi-ventral muscle, obliquely extending from the superior costa of the scapula to the os hyoides. It arises by short tendinous fibres at the root of the coracoid process, from the ligament which crosses the coracoid notch, and from the adjoining part of the costa, directs itself with a slight ascent towards the median line, and, in emerging from behind the clavicle, frequently derives a few fibres from its posterior edge. It contracts to a flattened tendon as it passes beneath the sterno-mastoid, and abruptly changes its direction from a nearly horizontal to a vertical course, by undergoing a trochlear reflexion in a loop of the cervical fascia,—and, again becoming fleshy, ascends beside and parallel to the outer edge of the sterno-hyoid, to which it is closely united,—to be inserted into the lower border of the hyoid bone at the junction of its body and cornu. The very important relations of this muscle will be more fully given in the detailed surgical anatomy of the region. It may for the present suffice to say, that, in crossing the direction of the sterno-mastoid muscle, it furnishes the subdividing line of the great triangles of the neck; that its posterior belly lies parallel to and just above the subclavian artery and brachial plexus, is covered by the platysma and partly by the trapezius, clavicle and subclavius, and crosses the scaleni and phrenic nerve: that its looped tendon is covered by the sterno-mastoid, and lies on the sheath of the carotid vessels, across which its anterior belly continues obliquely to run.
The two omo-hyoid muscles acting in concert are capable of depressing the os hyoides; but their chief action is of a different nature. Being contained in their whole bent course within a sheath of cervical fascia, they affect this membrane by their contraction, tensely spanning it across the median line in a space which extends from the hyoid bone to its clavicular attachment. This appears to be one of the consensual movements in the act of deglutition, designed to give, during that act, additional efficacy to the protection against atmospheric pressure, which Burns has shown to be an important function of the fascia of the neck.[1]
The digastric muscle is likewise, as its name imports, double-bellied; it passes from the mastoid process of the temporal bone to the symphysis of the jaw, but is looped down in its course to the side of the os hyoides. Its temporal attachment is to the groove, which is named from it, on the inner surface of the mastoid process: a large fleshy belly proceeds from this origin downward and forward, contracts to a round tendon, which usually pierces the stylo-hyoid muscle, traverses an aponeurotic ring lined by synovial membrane, which strongly binds it to the hyoid bone, near its lesser cornu, and is then reflected upward, expanding again to a strong muscular belly, which fixes itself by short aponeurotic fibres into the lower border of the jaw, at an oval depression beside the symphysis. Its tendon, just after passing through the fibrous pulley that maintains its curve, gives off a fascial process toward the median line: this attaches itself strongly along the upper edge of the hyoid bone, and
- ↑ Surg. Anat. of Head and Neck, p. 36. Glasgow, 1824.
