Thyroid AnatomyThyroid Anatomy 13/13Thyroid Anatomy 12/13 Most of the anterolateral surface of the neck is covered by the thin platysma muscles, remnants of the panniculus carnosus of mammals (watch a dog shake its coat). This muscle is at the level of the superficial fascia and its underside provides an excellent plane of dissection exposing the deep fascia of the neck. Because of the inverted V- shaped gap in this layer anteriorly, dissection of flaps in a collar incision should begin laterally.Thyroid Anatomy 11/13 The sternocleidomastoid muscles flank the straps and cover the internal jugulars and carotid sheaths. They are enclosed in an encircling girdle of deep cervical fascia called the investing layer. This layer continues posteriorly to enclose the trapezius muscles (see neck clinical folio). The external jugular veins descend across the posterior portions of the sternocleidomastoids and anterior jugular veins descend along the straps anteriorly.Thyroid Anatomy 10/13 The thyroid is covered by the sternohyoid and sternothyroid (strap) muscles. These are enclosed in an intermediate layer of deep cervical fascia (see Thryoid Anatomy: 1) with the sternohyoids being apposed in the midline and the underlying sternothyroids being slightly more lateral. When mobilizing the straps it is important to stay in the plane deep to both muscles. The omohyoid muscles ascend diagonally across the upper part of the straps and are automatically mobilized with those muscles. Thyroid Anatomy 9/13 Venous drainage of the thyroid runs with the superior laryngeal arteries above, passes laterally (middle thyroid veins) directly into the internal jugular veins, and drains inferiorly into the brachiocephalic veins. The dominant lymphatic drainage accompanies the vessels within the loose areolar carotid sheaths. The veins form a plexus on the surface of the gland within the areolar thyroid (visceral) fascia. Remaining external to this capsule provides a relatively bloodless dissection. Entry into the capsule is a bloody mess. Thyroid Anatomy 4/13 Two common variants are the presence of a pyramidal lobe extending upward from the isthmus and a thyroid ima artery ascending from below. The pyramidal lobe is a remnant of the track the thyroid followed in embryologic life from the foramen cecum at the base of the tongue, through the site of the future hyoid bone to its final location. Incomplete descent of the thyroid can be mistaken for a tumor. In the face of a high midline neck mass, it is imperative to document thyroid tissue in the normal location by scan before excision. The thyroid ima artery arises from any of the great vessels of the chest and neck. Thyroid Anatomy 2/13 The arterial supply to the thyroid comes principally from the superior and inferior thyroid arteries. The former arise from the external carotid and the latter are usually a branch of the thyrocervical trunk off the subclavian arteries. The inferior thyroid arteries pass behind the common carotids to reach the lower third of the thyroid gland on each side. Note the positions of the vagus and recurrent laryngeal nerves (details in subsequent imagesThyroid Anatomy 3/13 The thyroid gland isthmus saddles one or two of the uppermost tracheal rings and usually blocks the site of an optimal tracheostomy. It is routinely divided in such cases. The lobes of the gland wrap posteriorly to embrace the upper trachea and lower larynx. The posterior face of each lobe lies on the carotid sheath (see also neck anatomy).Thyroid Anatomy 1/13The thyroid gland is enclosed in a thin layer of pretracheal or visceral fascia which also includes the trachea and esophagus. The visceral compartment of the neck is deep to the intermediate fascia surrounding the strap muscles and the investing fascia enclosing the sternocleidomastoid and trapezius muscles (see neck anatomy images). Behind the visceral compartment is the prevertebral fascia overlying the cervical vertebrae (see brachial plexus anatomy). The space between the esophagus and prevertebral fascia extends down into the mediastinum. The recurrent laryngeal nerve is in the tracheoesophageal groove or anterior to it in the majority of individuals (details in subsequent images).Thyroid Anatomy 5/13 The larynx is innervated by branches of the vagus nerve (cranial nerve X). All the muscles of the larynx except the cricothyroid (see larynx) are supplied by the recurrent laryngeal nerves. The terminal portion of this nerve passes beneath the inferior pharyngeal constrictor to enter the larynx behind the articulation of the inferior cornu of the thyroid cartilage with the cricoid cartilage. Recurrent laryngeal nerve injury results in paralysis of the vocal cord in a relatively adducted position. Bilateral recurrent nerve injury during total thyroidectomy may result in acute postoperati