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Triangles of the Neck
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Bounded by the mandible above and the clavicle below, the neck is subdivided by the sternocleidomastoid muscle into an anterior and a posterior triangular region, each of which is further divided into smaller triangles by the omohyoid and digastric muscles (Figure 1–17). The surface markings of these muscles help to visibly define the borders of the triangles of the neck.
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The posterior triangle is bounded by the sternocleidomastoid muscle in front, the trapezius muscle behind, and the clavicle below. It is divided by the omohyoid muscle into an occipital triangle and a supraclavicular triangle.
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The occipital triangle has a muscular floor formed from above, downward by the semispinalis capitis, splenius capitis, levator scapulae, and scalenus medius muscles. After emerging from behind the sternocleidomastoid muscle, the spinal accessory nerve (XI) courses across the muscular floor of the posterior triangle to pass deep to the trapezius muscle. In addition, the cutaneous nerves of the neck, discussed below, course through the deep fascia of the neck that covers the posterior triangle.
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Supraclavicular Triangle
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The supraclavicular triangle lies above the middle of the clavicle. It contains the terminal portion of the subclavian artery, roots, trunks, and divisions of the brachial plexus, branches of the thyrocervical trunk, and cutaneous tributaries of the external jugular vein. The cupola of the pleural cavity extends above the level of the clavicle and is found deep to the contents of the supraclavicular triangle.
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The anterior triangle is bounded by the sternocleidomastoid muscle behind, the midline of the neck in front, and the mandible above. It is subdivided into submental, digastric, carotid, and muscular triangles.
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The submental triangle is bounded by the anterior belly of the digastric muscle, the midline of the neck, and the hyoid bone. The mylohyoid muscle forms its floor.
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The digastric triangle is bounded by the mandible above and the two bellies of the digastric muscle. In addition, the stylohyoid muscle lies with the posterior belly of the digastric muscle. The mylohyoid and hyoglossus muscles form the floor of this triangle. The submandibular salivary gland is a prominent feature of this area, which is also referred to as the submandibular triangle. The hypoglossal nerve (XII) runs along with the stylohyoid muscle and posterior belly of the digastric muscle, between the hyoglossus muscle and the submandibular gland, on its course into the tongue. The facial vessels course across the triangle, with the facial artery passing deep to the submandibular gland while the facial vein passes superficial to it.
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The carotid triangle is bounded by the sternocleidomastoid muscle behind, the posterior belly of the digastric muscle above, and the omohyoid muscle below. Its floor is formed by the constrictor muscles of the pharynx. It contains the structures of the carotid sheath—namely, the common carotid artery as it divides into its external and internal carotid branches, the internal jugular vein and its tributaries, and the vagus nerve (X) with its branches.
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The muscular triangle is bounded by the omohyoid muscle above, the sternocleidomastoid muscle below, and the midline of the neck in front. It contains the infrahyoid muscles in its floor. Deep to these muscles are the thyroid and parathyroid glands, the larynx, which leads to the trachea, and the esophagus. The hyoid bone forms the superior attachment for the infrahyoid muscles, and the prominent thyroid cartilage and cricoid cartilage are also contained in this region.
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Sternocleidomastoid Muscles
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The sternocleidomastoid muscles act together to flex the cervical spine while extending the head at the atlantooccipital joint. Acting independently, each muscle turns the head to face upward and to the contralateral side. By virtue of their attachment to the sternum, the sternocleidomastoids also serve as accessory muscles of respiration.
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The trapezius muscles have fibers running in several directions. The uppermost fibers pass downward from the skull to the lateral end of the clavicle and help to elevate the shoulder. The middle fibers pass laterally from the cervical spine to the acromion process of the scapula and help to retract the shoulder. The lowest fibers pass upward from the thoracic spine to the spine of the scapula and help to laterally rotate the scapula, making the glenoid fossa turn upward. This action assists the serratus anterior muscle in rotating the scapula when the arm is abducted overhead.
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The scalene muscles attach to the cervical spine and pass downward to insert on the first rib. They are contained within the prevertebral layer of deep fascia and help to laterally bend the cervical spine. The roots of the brachial plexus and the subclavian artery pass between the anterior and middle scalene muscles on their course to the axilla. In contrast, the subclavian vein passes anterior to the anterior scalene muscle as it leaves the neck to pass behind the clavicle and reach the axilla. Also, the phrenic nerve lies immediately anterior to the anterior scalene muscle as it runs down the neck into the thorax.
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The infrahyoid muscles, the omohyoid, sternohyoid, sternothyroid, and thyrohyoid, are named for their attachments. Together, they act to depress the hyoid bone and the thyroid cartilage during movements of swallowing and speech.
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The suprahyoid muscles, the mylohyoid, stylohyoid, geniohyoid, and digastric, act together to elevate the hyoid bone during movements of swallowing or speech. In addition, with the infrahyoid muscles holding the hyoid bone in place, the suprahyoid muscles help to depress the mandible and open the mouth.
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The arch of the aorta has three branches: (1) the brachiocephalic artery, (2) the left common carotid artery, and (3) the left subclavian artery. The brachiocephalic artery branches into the right subclavian and right common carotid arteries.
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The subclavian artery gives off the vertebral artery, the internal thoracic artery, the thyrocervical trunk, and the costocervical trunk (see Figure 1–2).
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The vertebral artery courses up through the transverse foramina of the upper six cervical vertebrae. It enters the vertebral canal, passes through the foramen magnum, and goes on to supply blood to the hindbrain, the midbrain, and the occipital lobe of the forebrain.
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Internal Thoracic Artery
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The internal thoracic artery leaves the root of the neck and passes into the thorax, where it supplies blood to the anterior chest wall and eventually to the upper part of the anterior abdominal wall through its superior epigastric branch.
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The thyrocervical trunk gives off the following branches: (1) the inferior thyroid artery, which supplies blood to the thyroid gland; (2) the transverse cervical artery, which passes backward across the neck to supply blood to the trapezius and rhomboid muscles; and (3) the suprascapular artery, which courses laterally across the neck toward the suprascapular notch and participates in the elaborate anastomosis of vessels that surround the scapula. The inferior thyroid artery has a branch, the inferior laryngeal artery, which enters the larynx by passing between the lowest fibers of the inferior pharyngeal constrictor muscle and the upper fibers of the circular muscle of the esophagus. The inferior thyroid artery anastomoses with the superior thyroid artery, a branch of the external carotid artery.
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The costocervical trunk gives off branches that supply blood to the first two intercostal spaces and the postvertebral muscles of the neck.
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Common Carotid Artery
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The common carotid artery courses up into the neck and terminates at the level of the thyroid cartilage by dividing into the internal and external carotid arteries. It has no branches.
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Internal Carotid Artery
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The internal carotid artery also has no branches in the neck. It travels up to the base of the skull, where it enters the carotid canal and passes through the petrous part of the temporal bone and the cavernous sinus before turning sharply upward and backward at the carotid siphon to pierce the dura mater. It supplies blood to the frontal, parietal, and temporal lobes of the forebrain. Its main branch to the head is the ophthalmic artery, which supplies blood to the orbit and the upper part of the nasal cavity.
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External Carotid Artery
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The external carotid artery is the main source of blood supply to the head and neck (see Figure 1–2). In the neck, it has a number of branches.
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Superior Thyroid Artery
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The superior thyroid artery passes downward to supply blood to the upper part of the thyroid gland. It has a branch, the superior laryngeal artery, which pierces the thyrohyoid membrane to pass into the larynx. The superior thyroid artery anastomoses with the inferior thyroid artery, a branch of the thyrocervical trunk of the subclavian artery.
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Ascending Pharyngeal Artery
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The ascending pharyngeal artery supplies blood to the pharynx.
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Posterior Auricular Artery
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The posterior auricular artery passes upward, behind the auricle, and supplies blood to the scalp.
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The occipital artery passes upward and backward to supply blood to the scalp on the back of the head.
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The facial artery passes upward and forward, deep to the submandibular salivary gland. It then crosses the mandible, where its pulsations can be palpated just in front of the masseter muscle, to supply blood to the face.
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The lingual artery passes upward and forward, behind the posterior edge of the hyoglossus muscle, and into the substance of the tongue, to which it supplies blood.
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The external carotid artery then ascends into the substance of the parotid gland, where it gives off two terminal branches.
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Superficial Temporal Artery
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The superficial temporal artery crosses the zygomatic arch just in front of the auricle, where its pulsations can be palpated. It then goes on to supply blood to the scalp.
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The maxillary artery passes medially into the infratemporal fossa and is responsible for the blood supply to the deep structures of the face and the nose.
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The venous drainage of the head and neck is best understood by comparing it with the arterial distribution described above. Many variations exist in the pattern of venous drainage, but each of the arteries has a vein that corresponds to it (see Figure 1–3).
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The veins that correspond to the two terminal branches of the external carotid artery, the superficial temporal and maxillary veins, come together within the substance of the parotid gland to form the retromandibular vein. At the angle of the mandible, the retromandibular vein divides into an anterior and a posterior division.
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External Jugular Vein
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The two veins that correspond to the arteries that pass backward from the external carotid artery, the posterior auricular and occipital veins, join the posterior division of the retromandibular vein and become the external jugular vein. In addition, the suprascapular and transverse cervical veins drain into the external jugular vein.
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Internal Jugular Vein
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The two veins that correspond to the arteries that pass forward from the external carotid artery, the facial and lingual veins, join the anterior division of the retromandibular vein and drain into the internal jugular vein. The internal jugular vein drains blood from the areas to which the internal carotid artery supplies blood. In addition, the superior and middle thyroid veins drain into the internal jugular vein.
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Inferior Thyroid Veins
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The inferior thyroid veins lie in front of the trachea and drain blood from the isthmus of the thyroid gland into the left brachiocephalic vein as it lies behind the manubrium of the sternum.
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The external jugular vein drains into the subclavian vein, which joins the internal jugular vein at the root of the neck to become the brachiocephalic vein. The two brachiocephalic veins come together to form the superior vena cava.
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The superficial lymph nodes of the head and neck are named for their regional location (Figure 1–18). The occipital, retroauricular, and parotid nodes drain lymph from the scalp, auricle, and middle ear. The submandibular nodes receive lymph from the face, sinuses, mouth, and tongue. The retropharyngeal nodes, although not truly superficially located, receive lymph from deeper structures of the head, including the upper parts of the pharynx. All of these regional nodes drain their lymphatic efferents into the deep cervical nodes, which lie along the internal jugular vein. Two of these deep nodes are commonly referred to as the jugulodigastric and the juguloomohyoid nodes. They lie at locations at which the internal jugular vein is crossed by the digastric and omohyoid muscles, respectively. The jugulodigastric node is concerned with the lymphatic drainage of the palatine tonsil; the juguloomohyoid node is concerned primarily with the lymphatic drainage of the tongue. The deep cervical nodes drain their lymph into either the thoracic duct or the right lymphatic duct. The thoracic duct empties into the junction of the left internal jugular vein and the left subclavian vein. The right lymphatic duct drains into a similar location on the right side of the root of the neck.
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The cutaneous innervation of the anterior skin of the neck is by the ventral rami of cervical spinal nerves that form the cervical plexus (C2–4), whereas the posterior skin of the neck is innervated by the dorsal rami of cervical spinal nerves (C2–5) (see Figure 1–4). The cutaneous branches of the cervical plexus emerge from just behind the sternocleidomastoid muscle, at a point about halfway between its attachments to the sternum and the mastoid process. They are named for the areas of skin from which they carry sensation.
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Transverse Cervical Nerve
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The transverse cervical nerve turns forward and courses across the neck, with its branches carrying sensation from the anterior neck.
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Supraclavicular Nerves
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The supraclavicular nerves course down toward the clavicle and carry sensation from the skin of the lower neck, extending from the clavicle in front to the spine of the scapula behind.
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Greater Auricular Nerve
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The greater auricular nerve courses up toward the auricle, with its branches carrying sensation from the skin of the upper neck, the skin overlying the parotid gland, and the auricle itself.
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Lesser Occipital Nerve
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The lesser occipital nerve courses upward to carry sensation from the skin of the scalp that lies just behind the auricle.
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The infrahyoid muscles are innervated by branches of the ansa cervicalis, which is formed by the descending cervical nerve and the descending hypoglossal nerve. The descending cervical nerve (C2 and 3) arises from the cervical plexus. The descending hypoglossal nerve contains fibers from the first cervical spinal nerve, some of which initially joined the hypoglossal nerve (XII) before dropping from that nerve to form the ansa cervicalis (Figure 1–19). Other fibers from the first cervical spinal nerve continue on the hypoglossal nerve and later branch off to supply the thyrohyoid muscle.
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Of the suprahyoid muscles, the mylohyoid muscle and the anterior belly of the digastric muscle are innervated by the nerve to the mylohyoid muscle, which is a branch of the inferior alveolar nerve from the mandibular division of the trigeminal nerve (V3). The stylohyoid muscle and the posterior belly of the digastric muscle are innervated by the facial nerve (VII). The geniohyoid muscle is innervated by C1 fibers carried by the hypoglossal nerve (XII).
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The prevertebral musculature and the scalene muscles receive motor innervation from direct branches of the cervical plexus. The sternocleidomastoid muscles and the trapezius muscles are innervated by the spinal accessory nerve (XI).
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The vagus nerve (X) travels in the carotid sheath with the internal jugular vein and the carotid artery (Figures 1–20 and 1–21). In the neck, it has branches to the larynx, the pharynx, and the heart. The laryngeal and pharyngeal branches of the vagus nerve carry motor fibers that originate in the cranial component of the accessory nerve (XI).
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Superior Laryngeal Nerve
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The superior laryngeal nerve gives off two branches, the external and the internal laryngeal nerves. The external laryngeal nerve provides motor innervation to the cricothyroid muscle. The internal laryngeal nerve pierces the thyrohyoid membrane to enter the larynx. It carries sensation from the part of the larynx that lies above the vocal folds and also carries sensation from the piriform recess of the laryngopharynx.
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Recurrent (Inferior) Laryngeal Nerve
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The recurrent (inferior) laryngeal nerve provides motor innervation to all the muscles of the larynx, with the exception of the cricothyroid muscle, as previously described. In addition, it carries sensation from the part of the larynx that lies below the vocal folds and from the upper part of the trachea. It courses up the neck in the groove between the trachea and the esophagus. As a result of the differing development of the aortic arches on the right and left sides of the body, the right recurrent laryngeal nerve passes in front of the right subclavian artery and turns up and back around this vessel to course toward the larynx. In contrast, the left recurrent laryngeal nerve passes into the thorax and lies in front of the arch of the aorta before turning up and back around the aorta, behind the ligamentum arteriosum, to reach the larynx.
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The pharyngeal branches provide motor innervation to all the muscles of the pharynx, with the exception of the stylopharyngeus muscle, and to all the muscles of the palate, with the exception of the tensor veli palatini muscle.
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The cardiac branches descend into the mediastinum and provide parasympathetic innervation to the heart. Additional branches arise in the chest to provide parasympathetic innervation to the lungs.
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The vagus has sensory branches that serve the meninges and the external ear.
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The phrenic nerve arises from the ventral rami of cervical spinal nerves C3–5 and courses down in the prevertebral fascia, in front of the anterior scalene muscle, into the thorax between the subclavian artery and vein. It provides motor innervation to the diaphragm. In addition, it carries sensation from the mediastinal and diaphragmatic parietal pleura, the pericardium, and the parietal peritoneum under the diaphragm.
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The sympathetic trunk in the neck is an upward continuation of the thoracic part of the trunk and reaches the base of the skull, lying medial to the carotid sheath in the prevertebral fascia. Unlike the thoracic part of the trunk, which has a sympathetic ganglion associated with each spinal nerve, the cervical part of the trunk has only three ganglia. The inferior cervical ganglion lies near the first rib and is frequently fused with the first thoracic ganglion to form the stellate ganglion. The middle cervical ganglion lies at the level of the cricoid cartilage. The superior cervical ganglion lies at the base of the skull, just below the inferior opening of the carotid canal. The cervical sympathetic ganglia get preganglionic input from fibers that originate in the upper thoracic spinal cord and ascend in the sympathetic trunk to reach the neck. Postganglionic outflow from these ganglia passes to the cervical spinal nerves, the cardiac plexus, the thyroid gland, the pharyngeal plexus, and the neurons that form plexuses around the internal and external carotid arteries as those vessels course up to the head.
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The deep fascia of the neck is thickened into several well-defined layers that are of clinical significance (Figure 1–22).
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The investing fascia surrounds the neck, attached below to the sternum and the clavicle, and above to the lower border of the mandible, the zygomatic arch, the mastoid process, and the superior nuchal line of the occipital bone. The fascia splits to enclose the sternocleidomastoid and trapezius muscles and the submandibular and parotid salivary glands.
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The prevertebral fascia surrounds the prevertebral and postvertebral muscles, and is attached to the ligamentum nuchae in the back. It is attached to the base of the skull above and extends down into the mediastinum below. There is a potential space, the retropharyngeal space, between this fascial layer and the pharynx and esophagus, allowing for the free movement of these structures against the vertebral column. However, this arrangement also provides a communicating space that extends from the base of the skull down into the mediastinum, allowing for infections to easily track in either direction.
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The carotid sheath surrounds the carotid arteries, the internal jugular vein, the vagus nerve (X), and the deep cervical lymph nodes.
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The visceral fascia surrounds the thyroid and parathyroid glands and the infrahyoid muscles. It extends from its attachment to the thyroid cartilage above to the pericardium below and is fused with the carotid sheath and the investing fascia.