The neck is divided by the sternocleidomastoid muscle into the anterior, or medial, triangle and the posterior, or lateral, triangle. These are illustrated in Figure 9-3.
The sternocleidomastoid is a strong muscle that serves to raise the sternum during respiration. The sternocleidomastoid has two heads: The sternal head arises from the manubrium sterni, and the clavicular head originates on the sternal end of the clavicle. The two heads unite and insert on the lateral aspect of the mastoid process. The sternocleidomastoid is innervated by the spinal accessory, or eleventh cranial, nerve.
Anterior to the sternocleidomastoid muscle is the anterior triangle. The other boundaries of the anterior triangle are the clavicle inferiorly and the midline anteriorly. The anterior triangle contains the thyroid gland, larynx, pharynx, lymph nodes, submandibular salivary gland, and fat.
The thyroid gland envelops the upper trachea and consists of two lobes connected by an isthmus. It is the largest endocrine gland in the body. As seen from the front, the thyroid is butterfly shaped and wraps around the anterior and lateral portions of the larynx and trachea, shown in Figure 9-4.
The thyroid isthmus lies across the trachea just below the cricoid cartilage of the larynx. The lateral lobes extend along both sides of the larynx, reaching the level of the middle of the thyroid cartilage of the larynx. On occasion, the thyroid gland may extend downward and enlarge within the thorax, producing a substernal goiter. The function of the thyroid gland is to produce thyroid hormone in accordance with the needs of the body.
Isthmus Thyroid lateral lobes muscle
Figure 9-3 Boundaries of the triangles of the neck.
Isthmus Thyroid lateral lobes
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