Together with branches of the inferior thyroid arteries, the recurrent laryngeal nerves are clearly related to, and may pass through ligaments, one on each side, that bind the thyroid gland to the trachea and to the cricoid cartilage of the larynx. These relationships need to be considered when surgically removing or manipulating the thyroid gland.
The parathyroid glands are two pairs of small, ovoid, yellowish structures on the deep surface of the lateral lobes of the thyroid gland. They are designated as the superior and inferior parathyroid glands (Fig. 8.180). However, their position is quite variable and they may be anywhere from the carotid bifurcation superiorly to the mediastinum inferiorly.
Derived from the third (the inferior parathyroid glands) and fourth (the superior parathyroid glands) pharyngeal pouches, these paired structures migrate to their final adult positions and are named accordingly.
The arteries supplying the parathyroid glands are the inferior thyroid arteries, and venous and lymphatic drainage follows that described for the thyroid gland.
Location of structures in different regions of the anterior triangle of the neck
The regional location of major structures in the anterior triangle of the neck is summarized in Table 8.14. Structures can be identified as being within a specific subdivision, passing into a specific subdivision from outside the area, originating in one subdivision and passing to another subdivision, or passing through several subdivisions while traversing the region.
Posterior triangle of the neck
The posterior triangle of the neck is on the lateral aspect of the neck in direct continuity with the upper limb (Fig. 8.183). It is bordered: anteriorly by the posterior edge of the sternocleidomastoid muscle, posteriorly by the anterior edge of the trapezius muscle, basally by the middle one-third of the clavicle, and apically by the occipital bone just posterior to the mastoid process where the attachments of the trapezius and sternocleidomastoid come together.
The roof of the posterior triangle consists of an investing layer of cervical fascia that surrounds the sternocleidomastoid and trapezius muscles as it passes through the region.
The muscular floor of the posterior triangle is covered by the prevertebral layer of cervical fascia; and from superior to inferior consists of the splenius capitis, levator scapulae, and the posterior, middle, and anterior scalene muscles.
Numerous muscles participate in forming the borders and floor of the posterior triangle of the neck (Table 8.15).
In addition, the omohyoid muscle passes across the inferior part of the posterior triangle before disappearing under the sternocleidomastoid muscle and emerging in the anterior triangle (Fig. 8.184). It is enclosed in the investing layer of cervical fascia and crosses the posterior triangle from lateral to medial as it continues in a superior direction. It originates on the superior border of the scapula, just medial to the scapular notch and eventually inserts into the inferior border of the body of the hyoid bone. It has two bellies connected by a tendon, which is anchored by a fascial sling to the clavicle:
The superior belly is in the anterior triangle.
The inferior belly crosses the posterior triangle, subdividing it into a small, omoclavicular or subclavian triangle inferiorly and a much larger occipital triangle superiorly.
The omohyoid is innervated by branches of the ansa cervicalis (anterior rami from C1 to C3) and it depresses the hyoid bone.
One of the most superficial structures passing through the posterior triangle of the neck is the external jugular vein (Fig. 8.185). This large vein forms near the angle of the mandible, when the posterior branch of the retromandibular and posterior auricular veins join, and descends through the neck in the superficial fascia.
After crossing the sternocleidomastoid muscle, the external jugular vein enters the posterior triangle and continues its vertical descent.