VISCERAL LAYERS OF THE NECK
The visceral region of the neck has three layers. From anterior to posterior, the layers are an endocrine layer (the thyroid and parathyroid glands), a respiratory layer (the trachea and larynx), and an alimentary layer (the pharynx and esophagus).
The thyroid gland is an endocrine gland with two lobes connected by a central isthmus. The thyroid gland is located below the thyroid cartilage overlying the second through the fourth tracheal rings, all enclosed within the pretracheal fascia (Figure 26-1A–C).
Thyroid hormones (T3 or T4). Regulate basal metabolic rate, increase body temperature and blood flow, and regulate growth rate.
Calcitonin. Decreases blood calcium concentration.
Anterior (A) and posterior (B) views of the visceral triangle of the neck. C. Cross-section through the thyroid gland.
Vessels of the thyroid gland are as follows:
Superior thyroid artery. Arises from the external carotid artery and courses with the superior laryngeal nerve (CN X).
Inferior thyroid artery. Arises from the thyrocervical trunk and is near the recurrent laryngeal nerve (CN X).
Middle and superior thyroid veins. Generally, middle and superior thyroid veins course anterior to the common carotid artery and drain into the internal jugular vein.
Inferior thyroid veins. Varied number of vessels that drain into the brachiocephalic veins.
Goiter. A goiter is a pathologic enlargement of the thyroid gland. Consequently, a goiter presents as a swelling in the anterior part of the neck, inferior to the thyroid cartilage. A goiter is usually caused by iodine deficiency. Iodine is necessary for the synthesis of thyroid hormones; when there is a deficiency of iodine, the gland is unable to produce thyroid hormones. When the levels of thyroid hormones decrease, the pituitary gland secretes more thyroid-stimulating hormone, which stimulates the thyroid gland to produce more faulty thyroid hormone, causing the gland to enlarge.▼
The parathyroid glands are four small endocrine glands located on the posterior surface of the thyroid (usually two on each side) (Figure 26-1B).
Parathyroid hormone. The major function of the parathyroid gland is the homeostatic maintenance of calcium and phosphate levels to ensure proper functioning of the muscular and nervous systems. Parathyroid hormone increases bone resorption of calcium, which increases blood calcium and phosphate concentration. Parathyroid hormone is an antagonist to calcitonin secreted by the thyroid.
Embryological origin. Derived from the epithelium of the third and fourth branchial pouches.
At the C6 vertebral level the trachea extends inferiorly from the cricoid cartilage (Figure 26-1A–C).