The infra-hyoid muscles form a group of four long flat muscles, arranged in two layers, and enveloped by the deep layer of the cervical fascia.

Superficial layer

This consists of two muscles: the sterno-hyoid and omo-hyoid.

Sterno-hyoid

The sterno-hyoid, named from its two attachments - is long and ibbon-shaped.

Origin:

(1) The back of the manubrium (presternum) just internal to the notches for the clavicle and first rib ;

(2) the back of the posterior sterno-clavicular ligament ;

(3) the back of the inner end of the clavicle external to the facet for rhe first costal cartilage.

Insertion: The lower border of the body of the hyoid bone, close to the middle bone.

Structure: It arises fleshy, and forms a band of parallel fibers, which approach le middle line as they ascend, and are inserted by a short tendon.

Nerve-supply: From the first three cervical nerves through the descendens and communicans hypoglossi, which send filaments to its deep surface near its upper end.

Action: To draw down the body of the hyoid bone, e.g. after it has been raised, swallowing ; also to fix it when the muscles which pass upwards from it are depressing the tongue, as in suction.

Relations:

Superficially, the deep cervical fascia, sterno-mastoid, sterno-clavicular joint, anterior jugular vein; deeply, the sterno-thyroid, crico-thyroid, tyroid and cricoid cartilages, the trachea, thyroid isthmus, the anterior jugular and inferior thyroid veins.

Variations: The sterno-hyoid may arise from the clavicle alone, and occasionally even from the middle of that bone. It may be absent or double. A tendinous intersection sometimes crosses it.

Omo-hyoid

The omo-hyoid, named from its attachment to the shoulder blade and the hyoid bone, is ribbon -shaped with a tendinous constriction in the middle which divides it into two fleshy bellies.

Origin:

(1) The upper border of the scapula for about an inch behind the suprascapular notch ;

(2) occasionally, the upper border of the transverse ligament which crosses the notch.

Insertion: The lower border of the body of the hyoid bone just external to the preceding, which it also slightly overlaps.

Structure: Arising fleshy and broad, the muscle contracts slightly as it passes forwards, and a little upwards, across the posterior triangle of the neck above the clavicle. Beneath the sterno-mastoid, and over the great vessels of the neck, it becomes tendinous for a short distance; and then, changing its direction, it again expands to a fleshy band which runs upwards and slightly forwards and inward to its insertion, which is by short tendinous, intermingled with fleshy, fibers. An obtuse angle is formed between the two bellies, the lower portion of the musch and its tendon being held down by a strong process of the deep layer of the cervical fascia, which, forming a loop around them, passes downwards to be attached to the posterior surface of the clavicle and to the first rib.

Nerve-supply: The anterior belly is supplied by a branch from the descender hypoglossi, which enters the back of its deep surface, while the posterior receive a branch from the loop of communication between this nerve and the communicantes hypoglossi which enters the deep surface of the muscle close to its junction with the tendon.

Action:

(1) To draw down the hyoid bone ;

(2) very slightly to help in raising the scapula ;

(3) to make tense the lower part of the cervical fascia. In this way it assists the platysma myoides in diminishing the inward suction of the soft parts, which tends to compress the great vessels and the apices of the lungs during prolonged inspiratory efforts. The part of the muscle between the sterno-mastoid and the carotid sheath, being tendinous, will not vary in thickness, otherwise during contraction it might tend to obstruct the vessels beneath.

Relations: Superficially, the deep cervical fascia, sterno-mastoid, clavicle, subclavius and trapezius, the external jugular vein, and the descending branch ; of the superficial cervical plexus ; deeply, the thyro-hyoid, sterno-thyroid, scalen and the first digitation of the serratus magnus, the sheath of the common carotid artery and internal jugular vein, the upper part of the brachial plexus, often the third part of the subclavian artery with the transversalis colli and suprascapular arteries and the suprascapular nerve.

Variations: These are very frequent. It may be absent or double. The posterior belly may be attached to the clavicle and scapula, or to the clavicle alone. It may receive a slip from the manubrium. The anterior belly may blend with the sterno-hyoid, and it may send a slip to the thyroid cartilage or the mandible.

Second Layer of Infra-hyoid Muscles

This consists of two muscles: the sterno-thyroid and thyro-hyoid.

Sterno-thyroid

The sterno-thyroid, named from its attachment to the sternum and thyroid cartilages, is flat and ribbon- shaped.

Origin:

(1) The lower part and side of the back of the manubrium (pre-sternum), from the middle line to the notch for the first rib cartilage ;

(2) the back the first rib cartilage, and occasionally that of the second ; and

(3) occasionally from the back of the clavicle near the facet for the first costal cartilage.

Insertion: The oblique line on the outer surface of the ala of the thyroid cartilage.

Structure: Arising fleshy, the two muscles form broad bands of parallel fibers, which at first lie close to one another, and afterwards diverge slightly as they acend. Finally, they are inserted by short tendinous fibers into the thyroid cartilage. Occasionally the muscle is crossed by a tendinous intersection.

Nerve-supply: Branches from the loop between the descendens and the comunicantes hypoglossi which enter the outer part of the deep surface of the muscle.

Action:

(1) To draw down the thyroid cartilage, e.g. after swallowing ;

(2) in injunction with the thyro-hyoid it will depress the hyoid bone.

Relations: Superficially, the cervical fascia and first piece of the sternum,the anterior jugular vein, the sterno-mastoid and sterno-hyoid and omo-hyoid muscles ; deeply, the thyroid cartilage, inferior constrictor, the cricoid cartilage, crico-thyroid muscle, thyroid gland, inferior thyroid veins and trachea, the common carotid artery and left innominate vein.

Variations: This muscle may be absent, or double ; the muscles of the two sides may be united. It may send a slip to the carotid sheath.

Thyro-hyoid

The thyro-hyoid, named from its attachment to the thyroid cartilage and hyoid bone, is a quadrilateral sheet.

Origin: The oblique line on the outer surface of the ala of the thyroid cartilage.

Insertion: The lower border of

(1) the outer third of the body of the hyoid bone ; and of

(2) the inner half of the greater cornu.

Structure: This muscle is a continuation upwards of the preceding muscle, .any of its fibers being derived from those of the sterno-thyroid without any attachment to the cartilage. It consists of parallel fleshy fibers which run nearly vertically upwards to their insertion into the hyoid bone.

Nerve-supply: A special branch of the hypoglossal nerve which enters the deep irface of the muscle near its posterior border.

Action:

(1) To draw up the thyroid cartilage, as in swallowing, or in the production of a high note in singing ;

(2) in association with the sterno-thyroid to draw down the hyoid bone.

Relations:

Superficially, the sterno-hyoid, omo-hyoid, and sterno-mastoid ;

Deeply, the thyro-hyoid membrane, the thyro-hyoid bursa and ala of the thyroid cartilage, the superior laryngeal vessels and nerve.

Variations: Slips are occasionally seen passing from the cricoid cartilage to the hyoid bone, or between the upper border of the thyroid cartilage in front and the body of the hyoid one.