# Lecture 13: Neck Triangles and Fascia ## Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the boundaries of the anterior and posterior triangles of the neck 2. Identify the subdivisions and contents of each triangle 3. Describe the layers of cervical fascia and their clinical significance 4. Identify the fascial spaces and potential routes of infection spread 5. Describe the anatomy of the thyroid and parathyroid glands 6. Correlate anatomical features with clinical conditions --- ## Overview of the Neck The neck extends from the skull base superiorly (defined by the inferior border of the mandible, the mastoid process, and the superior nuchal line) to the thoracic inlet inferiorly (defined by the manubrium, the clavicles, the first ribs, and the T1 vertebra). This region contains vital structures including the major vessels supplying the brain (carotid arteries, jugular veins), the airway (larynx and trachea), the alimentary passage (pharynx and esophagus), the endocrine thyroid gland, and major neural structures (cervical spinal cord, brachial plexus, cranial nerves). The organization of these structures into anatomically defined triangles and fascial compartments provides a framework for both anatomical study and surgical approach. The sternocleidomastoid muscle is the key surface landmark that divides the neck into the anterior triangle (in front of the muscle) and the posterior triangle (behind the muscle). Each of these triangles is further subdivided by smaller muscles into more specific regions, each with characteristic contents. Panel A: Lateral view of the neck showing the sternocleidomastoid muscle dividing the neck into anterior and posterior triangles. Panel B: External jugular vein crossing the sternocleidomastoid and carotid pulse in the anterior triangle. Panel C: Trapezius muscle forming the posterior boundary of the posterior triangle. Panel D: Surface landmarks labeled including hyoid bone, thyroid cartilage, and clavicle. --- ## Sternocleidomastoid Muscle The sternocleidomastoid is a prominent strap-like muscle that dominates the lateral contour of the neck. It arises by two heads: a sternal head from the anterior surface of the manubrium and a clavicular head from the medial third of the clavicle. These heads unite and ascend obliquely across the neck to insert onto the mastoid process and the lateral portion of the superior nuchal line. Motor innervation is provided by the spinal accessory nerve (CN XI), which enters the deep surface of the muscle. Proprioceptive fibers come from the ventral rami of C2 and C3. When the sternocleidomastoid contracts unilaterally, it tilts the head toward the same side while rotating the face toward the opposite side. Bilateral contraction flexes the neck and extends the head. When the head is fixed, the muscle can assist in forced inspiration by elevating the sternum. Congenital muscular torticollis (wry neck) results from fibrosis of the sternocleidomastoid, possibly related to birth trauma. Affected infants hold the head tilted toward the affected side with the face rotated away. If untreated, facial asymmetry and permanent cervical spine deformity may develop. Panel A: Lateral view of the sternocleidomastoid with two heads arising from manubrium (sternal head) and medial clavicle (clavicular head). Panel B: Anterior view showing the two heads merging to insert on mastoid process and superior nuchal line. Panel C: Spinal accessory nerve entering muscle substance from the deep surface. Panel D: Inset showing head position in unilateral contraction with head tilted ipsilaterally and face rotated contralaterally. --- ## Anterior Triangle The anterior triangle occupies the front of the neck between the two sternocleidomastoid muscles. Its boundaries are the midline of the neck anteriorly (from the chin to the sternal notch), the anterior border of the sternocleidomastoid posteriorly, and the inferior border of the mandible superiorly. This triangle contains the major visceral structures of the neck and the great vessels and is subdivided into four smaller triangles by the digastric and omohyoid muscles. The submandibular (digastric) triangle is bounded superiorly by the mandible and inferiorly by the two bellies of the digastric muscle. Its floor is formed by the mylohyoid and hyoglossus muscles. This triangle contains the submandibular salivary gland, which wraps around the posterior border of mylohyoid. The facial artery grooves the deep surface of the gland and emerges to cross the mandible. The facial vein accompanies the artery. The hypoglossal nerve courses through the triangle between the mylohyoid and hyoglossus, and the mylohyoid nerve (from V3) runs on the inferior surface of the mylohyoid muscle. Submandibular lymph nodes are embedded in and around the gland. The submental triangle is a single midline triangle bounded by the anterior bellies of the two digastric muscles, the hyoid bone inferiorly, and the mylohyoid muscle as its floor. It contains submental lymph nodes and small veins. Lymph from the tip of the tongue, the floor of the mouth, the lower lip, and the mental region drains to these nodes. The carotid (superior carotid) triangle is bounded by the posterior belly of digastric superiorly, the superior belly of omohyoid anteroinferiorly, and the anterior border of sternocleidomastoid posteriorly. This triangle provides access to the carotid vessels and internal jugular vein within the carotid sheath, as well as the external carotid artery and its branches. The vagus nerve lies within the sheath between the vessels. The hypoglossal nerve crosses the internal and external carotid arteries. The superior laryngeal nerve descends to the larynx. The ansa cervicalis lies on or superficial to the carotid sheath. The muscular (inferior carotid) triangle is bounded by the superior belly of omohyoid superiorly, the midline anteroinferiorly, and the anterior border of sternocleidomastoid posterolaterally. This triangle contains the infrahyoid (strap) muscles (sternohyoid, sternothyroid, thyrohyoid, and omohyoid), the thyroid and parathyroid glands, and the larynx and trachea. The lower parts of the carotid vessels and internal jugular vein, along with the esophagus, also lie within this region. Panel A: Submandibular triangle (blue) bounded by digastric bellies and mandible containing the submandibular gland. Panel B: Submental triangle (green) between anterior digastric bellies with submental lymph nodes. Panel C: Carotid triangle (yellow) containing carotid sheath structures with key vessels and nerves labeled. Panel D: Muscular triangle (orange) containing thyroid gland and strap muscles. --- ## Posterior Triangle The posterior triangle is the space behind the sternocleidomastoid muscle. Its boundaries are the posterior border of the sternocleidomastoid anteriorly, the anterior border of the trapezius posteriorly, and the middle third of the clavicle inferiorly. The floor is formed by the prevertebral muscles covered by prevertebral fascia: from above downward, these are the splenius capitis, levator scapulae, and the posterior and middle scalene muscles. The roof is formed by the investing layer of deep cervical fascia. The inferior belly of the omohyoid muscle crosses the posterior triangle, dividing it into a larger upper portion (the occipital triangle) and a smaller lower portion (the supraclavicular or subclavian triangle). The occipital triangle, the larger upper subdivision, contains the spinal accessory nerve (CN XI) as its most important structure. This nerve emerges from the posterior border of the sternocleidomastoid at approximately its midpoint (Erb's point), crosses the triangle superficially on the prevertebral fascia, and disappears beneath the anterior border of the trapezius, which it innervates. The nerve is vulnerable to injury during surgical procedures in this region, including lymph node biopsies. Cutaneous branches of the cervical plexus (lesser occipital, great auricular, transverse cervical, and supraclavicular nerves) also emerge at Erb's point and radiate across the triangle. The occipital artery may be encountered in the upper part of the triangle. The supraclavicular triangle, below the omohyoid, contains critical neurovascular structures passing between the neck and the upper limb. The third part of the subclavian artery crosses the first rib here, accompanied superiorly and posteriorly by the trunks of the brachial plexus emerging from behind the anterior scalene muscle. The external jugular vein descends superficially across the sternocleidomastoid to pierce the investing fascia and drain into the subclavian vein. Supraclavicular lymph nodes lie in this region; left-sided supraclavicular lymphadenopathy (Virchow's node) may indicate intraabdominal malignancy. Panel A: Posterior triangle boundaries with sternocleidomastoid anteriorly, trapezius posteriorly, and clavicle inferiorly, divided by the inferior belly of omohyoid into occipital and supraclavicular triangles. Panel B: Spinal accessory nerve crossing superficially with cervical plexus branches emerging at Erb's point. Panel C: External jugular vein on the surface and in the supraclavicular triangle the brachial plexus trunks and third part of subclavian artery. Panel D: Floor muscles (splenius capitis, levator scapulae, scalenes) indicated beneath the investing fascia. --- ## Cervical Fascia The cervical fascia is divided into superficial and deep layers, with the deep fascia further subdivided into three distinct layers that invest the various structures of the neck and create compartments and potential spaces. The superficial cervical fascia is a thin layer of subcutaneous tissue containing fat, the platysma muscle, superficial veins (including the external and anterior jugular veins), and cutaneous nerves. The platysma is a thin sheet of muscle extending from the upper chest to the lower face that tenses the skin of the neck. The deep cervical fascia consists of three layers: the investing layer, the pretracheal layer, and the prevertebral layer. The investing (superficial) layer of deep cervical fascia encircles the entire neck like a collar. Superiorly, it attaches to the mandible, the mastoid process, and the superior nuchal line. Inferiorly, it attaches to the clavicles, the manubrium, and the spine of the scapula. It splits to enclose the sternocleidomastoid and trapezius muscles, and also forms capsules for the parotid and submandibular glands. The pretracheal layer is limited to the anterior neck and has two components. The muscular layer invests the infrahyoid muscles (strap muscles). The visceral layer surrounds the thyroid gland, trachea, and esophagus, blending with the fibrous pericardium in the superior mediastinum. This continuity allows infection or air from the neck to track into the mediastinum. The prevertebral layer covers the prevertebral muscles and the vertebral column, extending laterally as the axillary sheath to surround the brachial plexus and axillary vessels as they enter the upper limb. The cervical and brachial plexuses and the scalene muscles lie within this fascial compartment. The carotid sheath is a condensation of all three layers of deep cervical fascia that envelops the common and internal carotid arteries, the internal jugular vein, and the vagus nerve as they pass through the neck. The vagus lies in the posterior groove between the artery and vein. Cranial nerves IX, X, XI, and XII all pass through or adjacent to the carotid sheath. Panel A: Cross-section of the neck at the thyroid gland level showing the investing layer (blue) surrounding the entire neck and splitting around sternocleidomastoid and trapezius. Panel B: Pretracheal layer (green) with muscular component around strap muscles and visceral component around thyroid, trachea, and esophagus. Panel C: Prevertebral layer (yellow) covering vertebral column and prevertebral muscles. Panel D: Carotid sheath (red outline) containing carotid artery, internal jugular vein, and vagus nerve with retropharyngeal space indicated between visceral and prevertebral layers. --- ## Fascial Spaces and Infection Spread The fascial planes of the neck create potential spaces that normally contain only loose areolar tissue but may become distended with blood, pus, or air in pathological states. Understanding these spaces is essential because they provide routes for the spread of infection, potentially to the mediastinum. The retropharyngeal space lies between the buccopharyngeal fascia (covering the posterior wall of the pharynx and esophagus) and the prevertebral fascia. It extends from the skull base to approximately the level of T1-T4, where it is closed by fusion of the two fasciae. Infections from the pharynx, adenoids, or cervical vertebrae may collect in this space. Although the space does not extend directly into the posterior mediastinum, infection may track there via the danger space. The danger space (or alar fascial space) lies between the alar fascia anteriorly and the prevertebral fascia posteriorly. Unlike the retropharyngeal space, it extends continuously from the skull base to the diaphragm, providing a direct route for infection spread from the neck to the posterior mediastinum. The alar fascia is a thin layer between the retropharyngeal space and the danger space. The pretracheal space surrounds the trachea and thyroid gland and communicates with the anterior mediastinum. Infections in this space, such as from thyroid surgery or neck trauma, may spread to the anterior mediastinum and pericardium. Ludwig's angina is a rapidly spreading cellulitis of the submandibular space, typically arising from dental infections of the lower molars. The infection spreads through the sublingual and submandibular spaces, causing massive swelling of the floor of the mouth that elevates and posteriorly displaces the tongue, potentially compromising the airway. Panel A: Sagittal section showing retropharyngeal space (blue) between pharynx and prevertebral fascia extending to upper thorax. Panel B: Danger space (red) between alar and prevertebral fascia extending to the diaphragm. Panel C: Pretracheal space (green) around trachea and thyroid communicating with the anterior mediastinum, with arrows indicating potential paths of infection spread. Panel D: Inset showing Ludwig's angina with floor of mouth elevation and potential airway compromise. --- ## Root of the Neck The root of the neck is the transitional zone between the neck proper and the thorax, located above the first ribs, clavicles, and manubrium and posterior to the sternoclavicular joints. This region contains the major vessels entering and leaving the thorax, along with important neural structures. On the right side, the brachiocephalic artery arises from the aortic arch and divides behind the sternoclavicular joint into the right common carotid and right subclavian arteries. On the left side, the common carotid and subclavian arteries arise directly from the aortic arch and ascend through the superior mediastinum into the neck. The subclavian artery arches over the apex of the lung and crosses the first rib to become the axillary artery. The anterior scalene muscle divides the artery into three parts and separates it from the subclavian vein, which lies anterior and inferior to the muscle. The phrenic nerve descends on the anterior surface of the anterior scalene, passing between the subclavian artery and vein to enter the thorax. The vagus nerves descend through the neck in the carotid sheath. The right recurrent laryngeal nerve loops around the right subclavian artery, while the left recurrent laryngeal nerve has a longer course, looping around the aortic arch. Both nerves ascend in the tracheoesophageal groove to reach the larynx. The thoracic duct, carrying lymph from most of the body, arches over the left subclavian artery and drains into the junction of the left internal jugular and left subclavian veins. The right lymphatic duct, draining the right upper quadrant of the body, enters the corresponding junction on the right. The sympathetic trunks lie posterior to the carotid sheaths on the prevertebral fascia. The cervical ganglia give rise to cardiac branches and postganglionic fibers that distribute along the arteries. Panel A: Anterior view of the root of the neck showing brachiocephalic artery dividing into right common carotid and subclavian, and left common carotid and subclavian arising directly from aortic arch. Panel B: Subclavian arteries arching over lung apices with phrenic nerves descending on anterior scalenes. Panel C: Vagus nerves in carotid sheaths with recurrent laryngeal nerves looping around subclavian (right) and aortic arch (left). Panel D: Thoracic duct arching on the left side and draining into the venous angle. --- ## Thyroid Gland The thyroid gland is a highly vascularized endocrine organ located in the anterior neck at the level of the C5-T1 vertebrae. It consists of two lateral lobes connected across the midline by the isthmus, which overlies tracheal rings two through four. A pyramidal lobe, representing a remnant of the thyroglossal duct, is present in approximately fifty percent of individuals and ascends from the isthmus toward the hyoid bone. The gland has two capsules. The true capsule is a thin fibrous layer adherent to the gland parenchyma. The false capsule is formed by the pretracheal (visceral) layer of deep cervical fascia, which creates a surgical plane around the gland. The ligament of Berry is a condensation of the false capsule that firmly attaches the gland to the cricoid cartilage and upper tracheal rings; it contains small blood vessels and is closely related to the recurrent laryngeal nerve. The thyroid's anatomical relations are clinically important. Anteriorly lie the strap muscles (sternohyoid and sternothyroid), the investing fascia, and skin. Posteriorly, the gland is related to the trachea and esophagus medially and the carotid sheath laterally. Most critically, the recurrent laryngeal nerve ascends in the tracheoesophageal groove and passes posterior to or through the ligament of Berry before entering the larynx; it is vulnerable during thyroid surgery. The thyroid receives an exceptionally rich blood supply. The superior thyroid artery, the first branch of the external carotid, descends to the upper pole of the gland. The inferior thyroid artery, arising from the thyrocervical trunk (a branch of the subclavian), approaches the lower pole from laterally. A thyroid ima artery is present in approximately three to ten percent of individuals, arising from the brachiocephalic artery or aortic arch and ascending on the anterior trachea to supply the isthmus; it must be recognized to avoid hemorrhage during tracheostomy. Venous drainage is via the superior thyroid vein (to internal jugular), the middle thyroid vein (directly to internal jugular), and the inferior thyroid veins (which descend along the trachea to drain into the brachiocephalic veins). The inferior veins form a plexus on the anterior trachea that poses a bleeding risk during tracheostomy. Lymphatic drainage follows the arterial supply, with nodes in the prelaryngeal, pretracheal, and paratracheal chains, ultimately reaching the deep cervical nodes. Understanding the lymphatic drainage is essential for staging thyroid carcinoma. Panel A: Anterior view of thyroid gland with two lobes connected by isthmus over tracheal rings 2-4 and pyramidal lobe ascending toward hyoid. Panel B: Arterial supply with superior thyroid artery descending from external carotid and inferior thyroid artery approaching from thyrocervical trunk. Panel C: Venous drainage showing superior and middle thyroid veins to internal jugular and inferior thyroid veins descending to brachiocephalic veins. Panel D: Posterior view showing recurrent laryngeal nerve ascending in the tracheoesophageal groove and passing near the ligament of Berry. --- ## Parathyroid Glands The parathyroid glands are small endocrine organs that secrete parathyroid hormone (PTH) to regulate calcium homeostasis. Most individuals have four glands, though the number may vary from two to six. The glands are typically located on the posterior surface of the thyroid lobes, within the pretracheal fascia (false capsule), but their positions are variable, particularly for the inferior pair. The superior parathyroid glands develop from the fourth pharyngeal pouch and have a relatively constant position at the level of the middle third of the thyroid lobe, typically posterior to the recurrent laryngeal nerve. The inferior parathyroid glands develop from the third pharyngeal pouch along with the thymus and descend during development; consequently, they are found in more variable positions, from the lower thyroid pole to the superior mediastinum within or adjacent to the thymus. Blood supply to the parathyroids comes predominantly from the inferior thyroid artery, though the superior thyroid artery may contribute, particularly to the superior glands. The precarious blood supply makes the parathyroids vulnerable to devascularization during thyroid surgery. During thyroidectomy, the parathyroid glands must be identified and preserved. They are distinguished from thyroid tissue by their tan or brown color (compared to the reddish-brown thyroid) and their characteristic appearance likened to a chocolate chip on a cookie. Inadvertent removal or devascularization causes hypoparathyroidism with resultant hypocalcemia, which may present acutely with tetany or chronically with neuromuscular irritability. Panel A: Posterior view of thyroid gland with superior parathyroids at mid-thyroid level shown in tan color. Panel B: Inferior parathyroids in variable position near inferior pole or lower, shown slightly displaced. Panel C: Blood supply from inferior thyroid artery to both pairs indicated with recurrent laryngeal nerve ascending posterior to the gland. Panel D: Inset showing parathyroid appearance compared to surrounding thyroid tissue. --- ## Lymphatic Drainage of the Neck The cervical lymph nodes are organized into superficial and deep groups, with the deep nodes ultimately draining to the thoracic duct (on the left) or the right lymphatic duct. The superficial cervical lymph nodes form a ring around the junction of the head and neck, including the submental nodes (below the chin), submandibular nodes (beneath the mandible), preauricular nodes (in front of the ear), postauricular nodes (behind the ear), and occipital nodes (at the base of the skull posteriorly). These nodes receive lymph from the superficial structures of the face and scalp and drain to the deep cervical nodes. The deep cervical nodes are arranged along the internal jugular vein and are divided into superior and inferior groups by the omohyoid muscle. The jugulodigastric node (at the level of the posterior belly of digastric) is particularly prominent and receives drainage from the palatine tonsil; it is often called the tonsillar node. The jugulo-omohyoid node (at the omohyoid crossing) receives lymph from the tongue. Other deep node groups include the retropharyngeal nodes (behind the pharynx), the pretracheal and paratracheal nodes (around the trachea and thyroid), and the supraclavicular nodes (above the clavicle). For surgical purposes, the cervical lymph nodes are classified into six levels. Level I includes the submental and submandibular nodes. Level II comprises the upper jugular nodes (from skull base to the level of the hyoid). Level III includes the middle jugular nodes (from hyoid to the omohyoid crossing). Level IV contains the lower jugular nodes (from omohyoid to the clavicle). Level V encompasses the posterior triangle nodes. Level VI includes the anterior compartment nodes (pretracheal and paratracheal). This level system guides the extent of neck dissection for head and neck malignancies. Panel A: Superficial cervical lymph nodes forming a collar around the head-neck junction with submental, submandibular, and occipital groups. Panel B: Deep cervical chain along the internal jugular vein with jugulodigastric and jugulo-omohyoid nodes highlighted. Panel C: Levels I-VI demarcated with different colors and corresponding to specific anatomical landmarks. Panel D: Arrows showing drainage patterns from various head and neck regions to the respective node groups. --- ## Clinical Correlations Thyroid surgery carries specific anatomical risks. Injury to the recurrent laryngeal nerve causes vocal cord paralysis; unilateral injury produces hoarseness, while bilateral injury may cause airway obstruction requiring tracheostomy. Injury to the external branch of the superior laryngeal nerve causes subtle voice weakness and difficulty with high-pitched sounds. Inadvertent removal of or damage to the parathyroid glands causes hypocalcemia, which may present immediately postoperatively or after a delay. Hemorrhage can compress the airway, constituting a surgical emergency requiring evacuation. Lymphadenopathy in the neck requires careful evaluation. Tender, mobile nodes suggest infection. Hard, fixed nodes raise concern for metastatic carcinoma. Rubbery, painless nodes may indicate lymphoma. The location of enlarged nodes helps identify the likely primary site: posterior triangle nodes often drain the scalp and nasopharynx; supraclavicular nodes (particularly Virchow's node on the left) may indicate thoracic or abdominal malignancy. Central venous catheterization in the neck may target the internal jugular vein (approaches include anterior, central, and posterior routes relative to the sternocleidomastoid) or the subclavian vein (approached below the clavicle, aiming toward the sternal notch). Ultrasound guidance reduces complications, which include carotid artery puncture, pneumothorax, hemothorax, and air embolism. Neck infections may spread along fascial planes. Ludwig's angina (submandibular space infection) can obstruct the airway. Retropharyngeal abscess may compress the airway or spread to the mediastinum via the danger space. Bezold's abscess is a complication of mastoiditis in which infection tracks inferiorly along the sternocleidomastoid. Erb's point, located at the midpoint of the posterior border of the sternocleidomastoid, is where the spinal accessory nerve emerges and where cutaneous branches of the cervical plexus radiate. The accessory nerve is vulnerable during procedures in the posterior triangle, and its injury causes shoulder droop (trapezius weakness) and difficulty turning the head against resistance (sternocleidomastoid weakness). The scalene triangle (interscalene triangle), between the anterior and middle scalene muscles, contains the subclavian artery and brachial plexus trunks. Thoracic outlet syndrome results from compression of these neurovascular structures by cervical ribs, fibrous bands, or hypertrophied scalene muscles, producing upper limb pain, paresthesias, and vascular symptoms. Panel A: Thyroid surgery showing relationship of recurrent laryngeal nerve to inferior thyroid artery and parathyroid glands. Panel B: Internal jugular vein cannulation showing landmarks (SCM heads, carotid pulse) and ultrasound-guided needle trajectory. Panel C: Ludwig's angina with sublingual and submandibular space involvement elevating the tongue. Panel D: Scalene triangle demonstrating subclavian artery and brachial plexus passing between anterior and middle scalene muscles with potential compression site in thoracic outlet syndrome. --- ## Summary The sternocleidomastoid muscle divides the neck into the anterior triangle (further subdivided into submandibular, submental, carotid, and muscular triangles) and the posterior triangle (divided into occipital and supraclavicular triangles by the omohyoid). The carotid triangle contains the carotid vessels, internal jugular vein, and vagus nerve within the carotid sheath. The posterior triangle contains the spinal accessory nerve, cervical plexus cutaneous branches, and (in the supraclavicular portion) the brachial plexus trunks and third part of the subclavian artery. The deep cervical fascia comprises three layers: the investing layer surrounds the entire neck, the pretracheal layer encloses the thyroid and viscera, and the prevertebral layer covers the vertebral column and deep muscles. Fascial spaces including the retropharyngeal space and danger space provide potential routes for infection spread to the mediastinum. The thyroid gland has a dual arterial supply from the superior and inferior thyroid arteries, with the recurrent laryngeal nerve lying posterior to the gland. The parathyroid glands lie on the posterior thyroid surface and are at risk during surgery. Cervical lymph nodes are organized into levels I-VI for surgical planning. --- ## Key Terms | Term | Definition | |------|------------| | Carotid triangle | Anterior triangle subdivision containing carotid vessels, internal jugular vein, and vagus nerve; bounded by digastric, omohyoid, and sternocleidomastoid | | Carotid sheath | Condensation of deep cervical fascia surrounding the common and internal carotid arteries, internal jugular vein, and vagus nerve | | Retropharyngeal space | Potential space between the pharynx and prevertebral fascia allowing infection spread toward the mediastinum | | Ligament of Berry | Condensation of pretracheal fascia anchoring the thyroid to the cricoid cartilage; contains small vessels and is close to the recurrent laryngeal nerve | | Erb's point | Point on the posterior border of the sternocleidomastoid where the spinal accessory nerve and cervical plexus cutaneous branches emerge | | Thoracic outlet | Space between the scalene muscles and first rib through which the subclavian vessels and brachial plexus pass; site of potential neurovascular compression | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*