Anterior triangle of the neck

• Triangles of the neck

  •Sternocleidomastoid (SCM) divides the anterior from the posterior triangle

  A large muscle called sternocleidomastoid lies between the triangles and therefore forms the posterior boundary of the anterior triangle and the anterior boundary of the posterior triangle.

  
• Boundaries of the anterior triangle

  •Anterior border = imaginary midline of the neck

  •Posterior border = anterior border of SCM

  •Base = mandible

  
• Divisions of the anterior triangle

  •Submental – submental lymph nodes, beginning of anterior jugular vein

  •Submandibular – submandibular salivary gland, lymph nodes, facial artery  & vein, carotid sheath, hypoglossal nerve etc

  The submental division lies under the chin and other structures in this area also have ‘mental’ as part of their name – the mentalis muscle for example. The submandibular division lies inferior and posterior to the mandible.

  
• Divisions

  •Carotid – carotid sheath, branches of external carotid artery, internal jugular vein, hypoglossal, accessory and vagus nerves etc

  •Muscular – sternohyoid and sternothyroid form the floor beneath which lies the thyroid gland, larynx, trachea and oesophagus

  The muscular division is named after the infrahyoid or strap muscles it contains.

  
• Fascia

  Once the skin is removed from the neck it is apparent that there are many fascial layers in the neck. The superficial cervical fascia lies close to the skin and the deep fascia is divided into sections that surround structures such as the carotid arteries and muscles of the neck.

  
• Platysma

  -From fascia above pectoralis major and clavicle to lateral neck and onto mandible

  -Very variable

  -Facial nerve CN VII

  . It is classed as a muscle of facial expression, even though it acts to tighten the skin of the neck. All of the muscles of facial expression are supplied with motor innervation by cranial nerve seven (CN VII) the facial nerve.

  
• Veins

  •External jugular anterior to SCM (post triangle)

  •Internal jugular posterior to SCM (ant triangle)

  •Anterior jugular midline, may be absent

  Most individuals have external jugular veins that cross the anterior surface of sternocleidomastoid. The posterior division of the retromandibular vein usually joins with the external jugular vein which carries blood down the neck and into the subclavian vein

  
• Veins

  The facial vein joins with the anterior division of the retromandibular vein and this usually drains into the internal jugular vein. This lies deeper in the neck, within the carotid sheath, it will join with the subclavian vein to form the brachiocephalic vein (one each side). The anterior jugular veins can drain into the external or internal veins depending on the individual.

  
• Cervical plexus

  •Anterior rami C1-4

  •Joined into loops that lie anterior to levator scapulae and scalenus medius

  •Covered by pre-vertebral layer of deep cervical fascia

  •Somatic motor and sensory (plus sympathetic)

  
• the sensory branches of the plexus and their general distribution pattern to the skin.

  
• The motor branches supply some of the supra and infra hyoid muscles of the neck and form a loop of nerve tissue – an ‘ansa’ to do this. The ansa cervicalis contains nerve fibres from C1 to C3

  
• Hyoid bone and related structures

  The hyoid bone lies between the mandible and the larynx. Muscles attach to the hyoid bone – some from above the hyoid – ‘suprahyoid’ muscles and some from below the hyoid – ‘infrahyoid’ muscles.

  
• Suprahyoid muscles

  •Supra-hyoid muscles

  •Raise hyoid bone or depress mandible (mylohyoid and digastric)

  •Facial nerve (posterior belly of digastric and stylohyoid)

  •Mandibular division of trigeminal CNV3 (anterior belly of digastric and mylohyoid)

  •Suprahyoid muscles also include geniohyoid – deep to mylohyoid seen later in course innervated by C1 via hypoglossal CN XII

  
• Suprahyoid muscles

  The anterior belly attaches to the mandible and the posterior belly to the mastoid process of the temporal bone. Stylohyoid attaches to the styloid process of the temporal bone and the hyoid.

  Mylohyoid attaches to the mylohyoid line of the mandible and to the hyoid bone. It forms a diaphragm in the floor of the oral cavity. Geniohyoid lies deep to mylohyoid and attaches to the inferior genial tubercle of the mandible and the hyoid bone.

  
• Suprahyoid muscles

  The facial nerve exits the skull via the stylomastoid foramen so will pass close to both the posterior head of digastric and to stylohyoid. The mandibular division of the trigeminal nerve supplies the muscles of mastication. The ‘odd one out’ is really geniohyoid as it is supplied by nerve fibres from C1 root that are carried to geniohyoid via the hypoglossal nerve.

  
• Infrahyoid muscles

  •Depress hyoid bone / thyroid cartilage

  •Names indicate sites of attachment

  •Ansa cervicalis except thyrohyoid (C1 via hypoglossal CN XII)

  The superficial muscles are omohyoid and sternohyoid, the deeper muscles are sternothyroid and thyrohyoid.

  Omohyoid is named due to is shape. It has a superior belly that attaches to the hyoid bone, this becomes tendinous near the clavicle and passes through a small fascial sling that is attached to the clavicle. The inferior belly attaches to the scapula.

  
• the innervation pattern and also the ansa cervicalis lying on the anterior surface of the carotid sheath.

  
• Common carotid artery

  •LCA from arch of aorta

  •RCA from brachiocephalic

  •No branches but terminal 2 = internal and external

  •Internal carotid to brain

  •External carotid to head, face, neck

  •Within carotid sheath with internal jugular vein and vagus nerve

  everyone the internal carotid lies external to the external carotid at the bifurcation but comes internal as it passes into the skull, whereas the external carotid remains on the external surface of the head to supply the face and neck.

  
• . Once the fascia of the sheath is removed the internal jugular vein and the common carotid artery are revealed, and lying between them, the vagus nerve. The vein is easy to damage during dissection as it is very thin walled, but the artery is more robust.

  
• Anterior triangle 

  The external carotid artery forms many branches and three anterior branches lie in the anterior triangle of the neck. The first branch is the superior thyroid artery that passes down the neck to the thyroid gland. The lingual artery is the second branch and this passes into the floor of the oral cavity to supply the tongue. The third branch is the facial artery that crosses the anterior surface of the mandible to supply the face.

  
• Other vessels

  The subclavian artery supplies blood to the upper limb but branches from the first part of the subclavian artery also supply the thoracic wall (internal thoracic branch), the brain (the vertebral branch) and structures in the neck (thyrocervical trunk).

  
• Vagus nerve

  •Cranial nerve  (CN X)

  •Descends in carotid sheath

  •Recurrent laryngeal branches to muscles of larynx = L around arch of aorta, R around subclavian artery

  •Internal and external laryngeal branches

  •Through thoracic inlet into superior mediastinum – parasympathetic to thorax and abdomen

  
• The vagus nerve lies within the carotid sheath of fascia, sitting between the common carotid artery and the internal jugular vein. The sheath also surrounds lymph nodes that sit near the vessels.

  
• Thyroid gland

  •Endocrine – regulates metabolic rate

  •2 lobes, isthmus and capsule

  •From lamina of thyroid cartilage down to 6th tracheal ring

  •Isthmus over tracheal ring 2 or 3

  •Parathyroid glands embedded into posterior surface

  
• Arterial supply

  The superior thyroid artery is the first anterior branch of the external carotid artery. The inferior thyroid artery is a branch of the thyrocervical trunk (from the subclavian).

  
• Venous drainage

  There are three thyroid veins on each side – a superior, middle and inferior vein. The superior and middle veins drain into the internal jugular veins and both inferior veins drain into the left brachiocephalic vein.

  
• Submandibular salivary gland

  •Inferior to mylohyoid, plus part in floor of mouth superior to mylohyoid

  •One duct each side opens between the first and second lower incisor (either side of frenelum)

  •CN VII (facial nerve)

  The submandibular salivary gland lies in the anterior triangle and is crossed by the facial vein. This is only the small outer part of the gland, the rest lies in the floor of the mouth. It passes saliva into the mouth via a duct between the first and second lower incisor teeth. The facial nerve supplies parasympathetic secretomotor supply to the submandibular gland.

  
• Thoracic duct

  The thoracic duct empties lymph into the venous system deep in the anterior triangle. This normally occurs at the junction of the internal jugular and subclavian veins (as they form the brachiocephalic vein).