Sterno-Hyoid.
Thyro-Hyoid.
Sterno-Thyroid.
Omo-Hyoid.

Dissection. The muscles in this region may be exposed by removing the deep fascia from the front of the neck. In order to see the entire extent of the Omo-hyoid, it is necessary to divide the Sterno-mastoid at its centre, and turn its ends aside, and to detach the Trapezius from the clavicle and scapula, if this muscle has been previously dissected ; but not otherwise.

The Sterno-Hyoid is a thin, narrow, ribband-like muscle, which arises from the inner extremity of the clavicle, and the upper and posterior part of the first piece of the sternum; and, passing upwards and inwards, is inserted, by short tendinous fibres, into the lower border of the body of the os hyoides. This muscle is separated, below, from its fellow by a considerable interval; they approach one another in the middle of their course, and again diverge as they ascend. It often presents, immediately above its origin, a transverse tendinous intersection, analogous to those in the liectus abdominis.

Variations in Origin. The origin of this muscle presents many variations. Thus, it may be found to arise from the inner extremity of the clavicle, and the posterior sterno-clavicular ligament; or from the sternum and this ligament; from either bone alone, or from all these parts; and occasionally has a fasciculus connected with the cartilage of the first rib.

Relations. By its superficial surface, below, with the sternum, sternal end of the clavicle, and the Sterno-mastoid; and, above, with the Platysma and deep cervical fascia. By its deep surface, with the Sterno-thyroid, Crico-thyroid, and Thyro-hyoid muscles, the thyroid gland, the superior thyroid artery, the crico-thyroid and thyro-hyoid membranes.

The Sterno-Thyroid is situated immediately beneath the preceding muscle, but is shorter and broader than it. It arises from the posterior surface of the first bone of the sternum, beneath the origin of the Sterno-hyoid, and occasionally from the edge of the cartilage of the first rib; and is inserted into the oblique line on the side of the ala of the thyroid cartilage. These muscles are in close contact

Muscles of the Neck. Anterior View. at the lower part of the neck by their inner margins; and are frequently traversed by transverse or oblique tendinous intersections, analogous to those in the Recto* abdominis.

Variations. This muscle, at its insertion, is liable to some variations. A lateral prolongation is sometimes continued as far as the os hyoides; and it is sometimes continuous with the Thyro-hyoideus and Inferior constrictor of the pharynx.

Relations. By its anterior surface, with the Steruo-hyoid, Omo-hyoid, and Sterno-mastoid. By its posterior surface, from below upwards, with the trachea, vena innominata, common carotid (and on the right side the arteria innominata), the thyroid gland and its vessels, and the lower part of the larynx. The middle thyroid vein lies along its inner border, this should be remembered in the operation of tracheotomy.

The Thyro-Hyoid is a small quadrilateral muscle, appearing like a continuation of the Sterno-thyroid. It arises from the oblique line on the side of the thyroid cartilage, and passes vertically upwards to be inserted into the lower border of the body, and greater cornu of the hyoid bone.

Relations. By its external surface, with the Sterno-hyoid and Omohyoid muscles. By its internal surface, with the thyroid cartilage, and thyro-hyoid membrane. Interposed between this muscle and the membrane, is the superior laryngeal nerve and artery.

The Omo-hyoid passes across the side of the neck, from the scapula to the hyoid bone. It consists of two fleshy bellies, united by a central tendon. It arises from the upper border of the scapula, and occasionally from the transverse ligament which crosses the supra-scapular notch; its extent of attachment to the scapula varying from a few lines to an inch. From this origin, the posterior belly forms a flat, narrow fasciculus, which inclines forwards across the lower part of the neck; behind the Sterno-mastoid muscle, where it becomes tendinous, it changes its direction, forming an obtuse angle, and ascends almost, vertically upwards, close to the outer border of the Sterno-hyoid, to be inserted into the lower border of the body of the os hyoides, just external to the insertion of the Sterno-hyoid. The tendon of this muscle, which much varies in its length and form in different subjects, is held in its position between two lamella of the deep cervical fascia, which include it in a sheath, and are prolonged down to be attached to the cartilage of the first rib. It is by this means that the angular form of the muscle is maintained.

This muscle subdivides each of the two large triangles at the side of the neck, formed by the Sterno-mastoid, into two smaller triangles. The two posterior ones being the posterior superior or sub-occipital, and the posterior inferior or subclavian; the two anterior, the anterior superior or superior carotid, and the anterior inferior or inferior carotid triangle.

Relations. By its superficial surface, with the Trapezius, Subclavius, the clavicle, the Sterno-mastoid, deep cervical fascia, Platysma, and integument. By its deep surface, with the Scaleni, brachial plexus, sheath of the common carotid artery, and internal jugular vein, the descendens noni nerve, Sterno-thyroid and Thyro-hyoid muscles.

Nerves. All the muscles of this group, excepting the Thyro-hyoid, which is supplied by the hypo-glossal, receive their nerves from the loop of communication between the descendens and communicans noni.

Actions. These muscles serve to depress the larynx and hyoid bone, after these parts have been drawn up with the pharynx in the act of deglutition. The Omo-hyoid muscles not only depress the hyoid bone, but carry it backwards, and to one or the other side. These muscles also are tensors of the cervical fascia. The Thyro-hyoid may act as an elevator of the thyroid cartilage, when the hyoid bone ascends, drawing upwards the thyroid cartilage behind the os hyoides.

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