Human Anatomy Lesson 19
Lesson 19 - Neck
Think of the neck as a tube that runs from the head superiorly to the thorax inferiorly, and passing nerves, arteries, and veins from one region to the other. In this lesson, we will cover these structures in addition to the muscles in this region.
Learning Objectives - By the end of this lesson, you should be able to ...
- List and describe fascial layers and compartments in the neck, and understand what structures are contained within these spaces
- List five "triangles" in the neck, including 4 anterior triangles and the posterior triangle
- List and describe the origins, insertions, innervations, and actions of suprahyoid and infrahyoid muscles in the anterior triangle of the neck
- Review nerves, arteries, and veins that run from the head to the thorax through the anterior triangle of the neck
- List and describe the origins, insertions, innervations, and actions of 8 posterior triangle muscles, including the structurally-important scalene muscles
- List and describe nerves, arteries, and veins that run through the posterior triangle of the neck
To date, we haven't concerned ourselves too much with fascia, but it is very important for defining regions in the neck:
- Superficial fascia: in the anterior part of the neck, the platysma muscle runs in this fascial layer between the thorax and mandible.
- Investing layer: completely surrounds the neck, attaching posteriorly to the nuchal ligament and splitting to enclose the trapezius, sternocleidomastoid, and infrahyoid muscles. The investing fascia runs from the external occipital protuberance and superior nuchal line, mastoid process, and zygomatic arch to the scapular spine, acromion, clavicle, and manubrium. The investing fascia is pierced by the external and anterior jugular veins and branches of the cervical plexus.
- Prevertebral layer: surrounds the vertebral column and associated muscles, including the prevertebral muscles, anterior, middle, and posterior scalene muscles, and the deep muscles of the back. The prevertebral fascia inserts onto the nuchal ligament and the base of the skull, along the basioccipital, area around the jugular foramen and carotid canal, mastoid process, superior nuchal line and external occipital protuberance. More inferiorly, the prevertebral fascia attaches to the anterior surfaces of transverse processes and cervical vertebral bodies.Anterolaterally, the prevertebral fascia surrounds the brachial plexus and subclavian artery and is called the axillary sheath.
- Pretracheal layer: surrounds the trachea, esophagus, and thyroid gland, running from the hyoid bone superiorly to the thorax inferiorly. Posteriorly, the pretracheal layer is referred to as the buccopharyngeal fascia, and separates the pharynx and esophagus from the prevertebral layer.
- Carotid sheath: column of fascia that surrounds the common carotid artery, internal carotid artery, internal jugular vein, and vagus nerve, receiving contributions from the investing, prevertebral, and pretracheal fasciae along its length.
Fascial Compartments and Spaces
The fascial coverings form a number of compartments and spaces. It is important to understand the relationships between fascia, compartments, and spaces, since infection can easily spread through the head, neck, and thorax through these spaces.
- The first compartment contains the other three, and is defined by the area covered by the investing layer.
- Vertebral compartment: area covered by the prevertebral fascia, which includes the vertebral column and surrounding deep muscles.
- Visceral compartment: area covered by the pretracheal fascia, and includes the pharynx, trachea, esophagus, and thyroid gland.
- Vascular compartment: area covered by the carotid sheath, and includes the neurovascular structures anterolateral to the vertebral column.
- Pretracheal space: space between the investing fascia and pretracheal fascia, which includes areas in the neck and the anterior part of the superior mediastinum.
- Retropharyngeal space: space between the buccopharyngeal fascia on the posterior surface of the pharynx and esophagus and prevertebral fascia on the anterior surfaces of the cervical vertebrae, which extends from the skull base to the posterior mediastinum.
- "Prevertebral" space: space within the prevertebral layer, which extends from the skull base to the posterior mediastinum all the way down to the diaphragm.
Triangles of the Neck
The posterior triangle is defined by the sternocleidomastoid anteriorly, trapezius posteriorly, and the lateral half of the clavicle inferiorly. The anterior triangle is defined by the inferior border of the mandible superiorly, midline of the neck medially, and anterior border of the sternocleidomastoid laterally. It is commonly split into the different smaller triangles listed below.
- Submandibular triangle: inferior border of the mandible superiorly, anterior and posterior bellies of the digastric inferiorly
- Submental triangle: hyoid bone inferiorly, anterior belly of digastric laterally, midline medially
- Muscular triangle: hyoid bone superiorly, superior belly of omohyoid and anterior border of sternocleidomastoid laterally, midline medially
- Carotid triangle: superior belly of omohyoid anteroinferiorly, stylohyoid and posterior belly of digastric superiorly, anterior border of sternocleidomastoid muscle posteriorly
Muscles in the Anterior Triangle
Eight muscles run in the anterior triangle of the neck, The muscles superior to the hyoid bone are called suprahyoid muscles, and those inferior to the hyoid bone are called infrahyoid muscles.
- Stylohyoid muscle: runs from the base of the styloid process to the body of the hyoid bone. Innervated by CN VII. Functions to elevate hyoid posterosuperiorly.
- Anterior belly of digastric muscle: runs from digastric fossa on mandible to tendon on the hyoid body. Innervated by mylohyoid nerve (from inferior alveolar branch of V3). Functions depress mandible or elevate hyoid bone.
- Posterior belly of digastric muscle: runs from medial side of mastoid process to tendon on hyoid body. Innervated by CN VII. Functions to elevate hyoid posterosuperiorly.
- Mylohyoid muscle: runs from mylohyoid line on inner surface of mandible to hyoid body. Innervated by mylohyoid nerve (CN V3). Functions to elevate hyoid, but mainly a structural muscle at the base of the tongue.
- Geniohyoid muscle: runs from inferior mental spine on inner surface of mandible to anterior surface of hyoid body. Innervated by branch from anterior ramus of C1. Functions to fix mandible, elevate and pull hyoid bone anteriorly, or to depress mandible when the hyoid is fixed.
- Sternohyoid muscle: runs from sternoclavicular joints and manubrium to hyoid body medial to the attachment of the omohyoid muscle. Innervated by anterior rami of C1 to C3 through ansa cervicalis. Functions to depress hyoid bone.
- Omohyoid muscle: runs from superior border of scapula medial to the scapular notch to the lower border of the hyoid body just lateral to attachment of sternohyoid. Innervated by anterior rami of C1 to C3 through the ansa cervicalis. Functions to depress and fix hyoid bone.
- Thyrohyoid muscle: runs from oblique line on lamina of thyroid cartilage to greater horn and adjacent body of hyoid. Innervated by fibers from anterior ramus of C1. Functions to depress hyoid bone or raise larynx when hyoid bone is fixed.
- Sternothyroid muscle: runs from posterior surface of manubrium to oblique line on lamina of thyroid cartilage. Innervated by anterior rami of C1 to C3 through ansa cervicalis. Functions to draw larynx inferiorly.
Arteries and Veins
You know some of the important arteries and veins in the neck, since we covered them already when we covered the thorax:
- Common carotid arteries: remember, the right common carotid artery is a branch of the brachiocephalic trunk, whereas the left common carotid artery branches directly from the aortic arch. In the neck, the common carotid arteries run in the carotid sheath, and branch into external and internal carotid arteries near the superior edge of the thyroid cartilage, in the carotid triangle. The carotid sinus and carotid body are both located in close approximation to the bifurcation. The carotid sinus, innervated by CN IX, contains receptors that monitor changes in blood pressure; whereas the carotid body, innervated by CN IX and CN X, detects changes in oxygen content.
- Internal carotid arteries: gives off no branches in the neck, but ascends toward the carotid canal to enter the head to supply the brain, orbits, and forehead.
- External carotid arteries: this artery gives off two branches we haven't talked about (superior thyroid artery, which runs in an anteroinferior direction to supply the thyroid gland, internal laryngeal structures, and surrounding muscles, and ascending pharyngeal artery, which runs posterosuperiorly to supply pharyngeal muscles and surrounding structures), and several which we will discuss in lessons on the superficial face and head (lingual artery, facial artery, occipital artery, posterior auricular artery, superficial temporal artery, maxillary artery).
- Veins: the sigmoid sinus/inferior petrosal sinus meet just superior to the jugular foramen to for the internal jugular vein, which runs through the neck in the carotid sheath posterior to the vagus nerve. The internal jugular veins meet the brachiocephalic veins posterior to the sternal end of the clavicle. The facial, lingual, pharyngeal, occipital, superior thyroid, and middle thyroid veins all flow into the internal jugular vein.
The facial, glossopharyngeal, vagus, accessory, and hypoglossal nerves all run through the anterior triangle of the neck.
- Facial nerve: besides providing motor innervation to the muscles of facial expression, CN VII innervates the posterior belly of the digastric and the stylohyoid muscle.
- Glossopharyngeal nerve: descends between the internal carotid artery and internal jugular vein deep to the styloid process, then passes forward between the internal and external carotid arteries around the lateral border of the stylopharyngeus muscle and deep to the hyoglossus muscle. CN IX provides motor innervation to the stylopharyngeus muscle and carotid sinus, and sensory innervation to the pharynx.
- Vagus nerve: runs through the neck in the carotid sheath between the internal jugular vein and internal carotid and common carotid arteries. Gives off a motor branch to the pharynx, branch to the carotid body, and superior laryngeal nerve.
- Accessory nerve: runs between the internal jugular vein and internal carotid artery, but passes posteroinferiorly, crossing the internal jugular vein, and crosses the posterior triangle.
- Hypoglossal nerve: starts medial to internal jugular vein and internal carotid artery, but passes inferolaterally to these two structures as it descends. It hooks around the occipital artery deep to the posterior belly of the digastric and stylohyoid, then disappears medial to hyoglossus.
- Ansa cervicalis: this is a loop of nerve fibers from cervical nerves C1 to C3 that innervates the muscles in the anterior triangle, joining CN XII. The superior root innervates the superior belly of the omohyoid muscle and the upper parts of the sternohyoid and sternothyroid muscles. The inferior root innervates the inferior belly of the omohyoid and the lower parts of the sternohyoid and sternothyroid muscles.
Posterior Triangle Muscles
- Sternocleidomastoid: runs from the manubrium and superior part of the medial 1/3 of the clavicle to the lateral 1/2 of the superior nuchal line and lateral surface of mastoid process. Innervated by CN XI and branches from anterior rami of C2 and C3. Functions to tilt and rotate head or, acting together, to draw the head forward.
- Trapezius: review
- Splenius capitis: review
- Levator scapulae: runs from transverse processes of C1 to C4 to the upper part of the medial border of the scapula. Innervated by C3, C4, and the dorsal scapular nerve. Functions to elevate the scapula.
- Three scalene muscles: run from transverse processes of cervical vertebrae to upper surfaces of ribs 1 and 2. Innervated by anterior rami of C3 through C7. Elevate ribs to which they attach.
- Omohyoid: runs from superior border of scapula medial to scapular notch to inferior border of body of hyoid bone. Innervated by ansa cervicalis; anterior rami of C1 to C3. Depresses the hyoid bone.
Posterior Triangle Vessels
Below is a list of arteries and veins to be found in the posterior triangle of the neck. We briefly touched on these vessels, so you don't need to know any details about their course through the posterior triangle.
- External jugular vein: runs superficially along the sternocleidomastoid muscle and enters the posterior triangle of the neck near its inferior border, where it meets with various tributaries and empties into the subclavian vein in the superior mediastinum.
- Subclavian artery and branches: can be split into 3 parts by the scalene muscles - the 1st part runs to the medial border of the anterior scalene muscle, the 2nd part runs between the anterior and middle scalene muscles, and the 3rd part runs over the 1st rib after emerging from between the anterior and middle scalene muscles.
- Transverse cervical and suprascapular arteries: these two small arteries, which are branches of the thyrocervical trunk, arise from the 1st part of the subclavian artery and run through the posterior triangle.
- Subclavian vein: the external jugular vein enters the subclavian vein in the posterior triangle.
Posterior Triangle Nerves
- Accessory nerve: innervates the sternocleidomastoid and trapezius muscles (review).
- Cervical plexus: a number of nerves come off the cervical plexus. For our purposes, the ansa cervicalis is important since it contributes C1 to C3 fibers to innervate the infrahyoid muscles.
Next is the pharynx and larynx, and then we move on to the upper limb.
© 2014 Robert McCarthy