anterior triangle
Introduction
The anterior triangle of the neck lies between the midline of the neck and the sternocleidomastoid muscle. It is subdivided into smaller triangles.
Surface Landmarks
Mandible: Forms the lower jaw (Fig. 4.1). The lower border is the base of the mandible. Anteriorly, this base forms the chin, and posteriorly it can be traced to the angle of the mandible.
Hyoid Bone: The body of the U-shaped hyoid bone can be felt in the median plane just below and behind the chin, at the junction of the neck with the floor of the mouth. On each side, the body of the hyoid bone is continuous posteriorly with the greater cornua which is overlapped in its posterior part by the sternocleidomastoid muscle.
Thyroid Cartilage: Forms a sharp protuberance in the median plane just below the hyoid bone. This protuberance is called the laryngeal prominence or Adam’s apple. It is more prominent in males.
Cricoid Cartilage: Lies below the thyroid cartilage at the upper end of the trachea.
Trachea: Runs downwards and backwards from the cricoid cartilage. It is identified by its cartilaginous rings and is partially masked by the isthmus of the thyroid gland against the second to fourth tracheal rings. It can be palpated in the suprasternal notch.
Structures in the Anterior Median Region of the Neck
Features: A strip 2 to 3 cm wide extending from the chin to the sternum.
Superficial to Deep
Skin: Freely movable over the deeper structures.
Superficial Fascia:
Upper decussating fibres of the platysma for 1 to 2 cm below the chin.
Anterior jugular veins beginning in the submental region below the chin. It descends in the superficial fascia about 1 cm from the median plane. About 2.5 cm above the sternum, it pierces the investing layer of deep fascia to enter the suprasternal space where it is connected to its fellow of the opposite side by a transverse channel, the jugular venous arch (Fig. 4.2). The vein then turns laterally, runs deep to the sternocleidomastoid just above the clavicle, and ends in the external jugular vein at the posterior border of the sternocleidomastoid.
Small submental lymph nodes.
Terminal filaments of the transverse or anterior cutaneous nerve of the neck.
Deep Fascia:
Above the hyoid bone, the investing layer splits to enclose the submandibular salivary gland.
Between the hyoid bone and the cricoid cartilage, it is a single layer extending between the right and left sternocleidomastoid muscles.
Below the cricoid, the fascia splits to enclose the suprasternal space.
Deep to the Pretracheal Fascia
The thyrohyoid membrane is deep to the thyrohyoid muscle and is pierced by the internal laryngeal nerve and the superior laryngeal vessels (Fig. 4.7).
Thyroid cartilage.
Cricothyroid membrane with the anastomosis of the cricothyroid arteries on its surface.
Arch of the cricoid cartilage.
Cricothyroid muscle supplied by the external laryngeal nerve.
Trachea, partly covered by the isthmus of the thyroid gland from the second to fourth rings.
Carotid sheaths lie on each side of the trachea (Fig. 3.8).
Deep Structures Lying Above the Hyoid Bone
The mylohyoid muscle is overlapped by:-
Anterior belly of digastric above the hyoid bone.
Superficial part of the submandibular salivary gland (Figs 4.3 and 4.4).
Mylohyoid nerve and vessels.
Submental branch of the facial artery.
The anteroinferior part of the hyoglossus muscle with its superficial relations may also be exposed during dissection. Structures lying in this corner are:-
The intermediate tendon of the digastric muscle with its fibrous pulley (Fig. 4.3).
The bifurcated tendon of the stylohyoid muscle embracing the digastric tendon (Fig. 4.10).
The subhyoid bursa lies between the posterior surface of the body of the hyoid bone and the thyrohyoid membrane. It lessens friction between these two structures during the movements of swallowing (Fig. 4.5).
Structures Lying Below the Hyoid Bone
Grouped into three planes:
Superficial Plane: Infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, superior belly of omohyoid).
Middle Plane: Pretracheal fascia and thyroid gland.
Deep Plane: Larynx, trachea and associated structures.
Infrahyoid Muscles
Sternohyoid, sternothyroid, thyrohyoid, superior belly of omohyoid.
These are described in Table 4.1 and Fig. 4.6.
Pretracheal Fascia
Forms the false capsule of the thyroid gland and the suspensory ligaments of Berry which attach the thyroid gland to the cricoid cartilage (see Fig. 8.4).
Clinical Anatomy
Common anterior midline swellings of the neck:-
Enlarged submental lymph nodes and sublingual dermoid in the submental region.
Thyroglossal cyst and inflamed subhyoid bursa just below the hyoid bone (Fig. 4.5).
Goitre, carcinoma of larynx and enlarged lymph nodes in the suprasternal region.
Tracheostomy is an operation in which the trachea is opened and a tube inserted into it.-
Most commonly done in the retrothyroid region after retracting the isthmus of the thyroid gland (Fig. 4.8).
Suprathyroid tracheostomy is liable to stricture, and an infrathyroid one is difficult due to the depth of the trachea and is also dangerous because numerous vessels lie anterior to the trachea here.
Cut throat wounds are most commonly situated just above or just below the hyoid bone. The main vessels of the neck usually escape injury due to voluntary extension of the neck.
Skin incisions to be made parallel to natural creases or Langer’s lines (Fig. 4.9).
Ludwig’s angina is cellulitis of the floor of the mouth.-
The infection spreads above the mylohyoid forcing the tongue upwards.
Mylohyoid is pushed downwards causing swelling within the mouth and below the chin.
Anterior Triangle Boundaries
The anterior median plane of the neck medially
Sternocleidomastoid laterally
Base of the mandible and a line joining the angle of the mandible to the mastoid process, superiorly
Subdivisions
The anterior triangle is subdivided (by the digastric muscle and the superior belly of the omohyoid) into:-
Submental
Digastric
Carotid
Muscular triangles (Fig. 4.10).
Submental Triangle
Boundaries
On each side, anterior belly of the corresponding digastric muscles.
Base: body of the hyoid bone.
Apex: the chin.
Floor: right and left mylohyoid muscles and the median raphe uniting them (Fig. 4.3).
Contents
Submental Lymph Nodes:
Two to four small submental lymph nodes are situated in the superficial fascia between the anterior bellies of the digastric muscles (Fig. 4.3).
They drain:-
Superficial tissues below the chin
Central part of the lower lip
The adjoining gums
Anterior part of the floor of the mouth
The tip of the tongue
Their efferents pass to the submandibular nodes.
Submental veins:
Small submental veins join to form the anterior jugular veins.
Digastric Triangle (Submandibular Region)
Boundaries
Anteroinferiorly: Anterior belly of digastric
Posteroinferiorly: Posterior belly of digastric and the stylohyoid
Superiorly or base: Base of the mandible and a line joining the angle of the mandible to the mastoid process (Fig. 4.10).
Roof
Skin
Superficial fascia, containing:
The platysma
The cervical branch of the facial nerve
The ascending branch of the transverse or anterior cutaneous nerve of the neck.
Deep fascia:
Splits to enclose the submandibular salivary gland (see Fig. 7.6).
Floor
Mylohyoid muscle anteriorly, and by the hyoglossus posteriorly.
A small part of the middle constrictor muscle of the pharynx appears in the floor (Fig. 4.11).
Contents: Anterior Part of the Triangle
Structures superficial to mylohyoid are:
Submandibular Salivary Gland:
Superficial part of the submandibular salivary gland (Fig. 4.3).
Facial vein and lymph nodes:
The facial vein and the submandibular lymph nodes are superficial to it and the facial artery is deep to it.
Submental artery
Mylohyoid nerve and vessels (Fig. 4.4)
Hypoglossal nerve
Posterior Part of the Triangle
Superficial structures:
Lower part of the parotid gland.
The external carotid artery before it enters the parotid gland.
Deep structures, passing between the external and internal carotid arteries:
The styloglossus
The stylopharyngeus
The glossopharyngeal nerve (Fig. 4.13)
The pharyngeal branch of the vagus nerve
The styloid process
A part of the parotid gland.
Deepest structures include:
The internal carotid artery (Fig. 4.13)
The internal jugular vein
The vagus nerve (see Fig. 3.8b).
Carotid Triangle
Boundaries
Anterosuperiorly: Posterior belly of the digastric muscle; and the stylohyoid (Fig. 4.12).
Anteroinferiorly: Superior belly of the omohyoid.
Posteriorly: Anterior border of the sternocleidomastoid muscle.
Roof
Skin
Superficial fascia containing:
The plastysma
The cervical branch of the facial nerve
The transverse cutaneous nerve of the neck.
Investing layer of deep cervical fascia.
Floor
Parts of:-
The middle constrictor of pharynx
The inferior constrictor of the pharynx (Fig. 4.12)
Thyrohyoid membrane.
Contents
Arteries:
The common carotid artery with the carotid sinus and the carotid body at its termination
Internal carotid artery
The external carotid artery with its superior thyroid, lingual, facial, ascending pharyngeal and occipital branches (Fig. 4.12).
Veins:
The internal jugular vein
The common facial vein draining into the internal jugular vein.
A pharyngeal vein which usually ends in the internal jugular vein.
The lingual vein which usually terminates in the internal jugular vein.
Nerves:
The vagus running vertically downwards.
The superior laryngeal branch of the vagus, dividing into the external and internal laryngeal nerves.
The spinal accessory nerve running backwards over the internal jugular vein.
The hypoglossal nerve running forwards over the external and internal carotid arteries. The hypoglossal nerve gives off the upper root of the ansa cervicalis or descendens hypoglossi, and another branch to the thyrohyoid (Fig. 4.15).
Sympathetic chain runs (see Fig. 3.8b) vertically downwards posterior to the carotid sheath.
Carotid sheath with its contents (see Fig. 3.8).
Lymph Nodes:
The deep cervical lymph nodes are situated along the internal jugular vein, and include the jugulodigastric node below the posterior belly of the digastric and the jugulo-omohyoid node above the inferior belly of the omohyoid (see Fig. 8.31).
Muscular Triangle
Boundaries
Anteriorly: Anterior median line of the neck from the hyoid bone to the sternum.
Posterosuperiorly: Superior belly of the omohyoid muscle (Fig. 4.10).
Posteroinferiorly: Lower part of anterior border of the sternocleidomastoid muscle (Fig. 4.14).
Contents
The infrahyoid muscles are the chief contents of the triangle (Fig. 4.6).-
Sternohyoid
Sternothyroid
Thyrohyoid
Omohyoid
They are arranged in two layers: superficial (sternohyoid and omohyoid) and deep (sternothyroid and thyrohyoid) (Fig. 4.6).
All of them are supplied by the ventral rami of the first, second and third cervical spinal nerves.
They move the hyoid bone and the thyroid cartilage.
Sternohyoid, superior belly of omohyoid, and sternothyroid lie superficial to the lateral or superficial convex surface of the thyroid gland (see Fig. 8.4).
The anterior surface of the isthmus of the thyroid gland is covered by right and left sternothyroid and sternohyoid muscles (see Fig. 3.3).
Ansa Cervicalis or Ansa Hypoglossi
A thin nerve loop that lies embedded in the anterior wall of the carotid sheath over the lower part of the larynx.
Formation
Formed by a superior and an inferior root.
The superior root is the continuation of the descending branch of the hypoglossal nerve, derived from the first cervical nerve. It descends over the internal carotid artery and the common carotid artery.
The inferior root or descending cervical nerve is derived from the second and third cervical spinal nerves. As this root descends, it winds around the internal jugular vein, and then continues anteroinferiorly to join the superior root in front of the common carotid artery (Fig. 4.15).
Distribution
Superior root: To the superior belly of the omohyoid.
Ansa cervicalis: To the sternohyoid, the sternothyroid.
Inferior root: To the inferior belly of the omohyoid.
The thyrohyoid and geniohyoid are supplied by separate branches from the first cervical nerve through the hypoglossal nerve (Fig. 4.17).
Common Carotid Artery
The right common carotid artery is a branch of the brachiocephalic artery. It begins in the neck behind the right sternoclavicular joint (Fig. 4.16, also see Fig. 8.17).
The left common carotid artery is branch of the arch of the aorta.
Carotid Sinus
The termination of the common carotid artery, or the beginning of the internal carotid artery. It acts as a baroreceptor or pressure receptor and regulates blood pressure.
Carotid Body
A small, oval reddish brown structure, it acts as a chemoreceptor and responds to changes in the oxygen, carbon dioxide and pH content of the blood.
External Carotid Artery
One of the terminal branches of the common carotid artery. Chief artery of supply to structures in the front of the neck (Fig. 4.16, also see Fig. 8.17).
Course and Relations
Begins in the carotid triangle at the level of the upper border of the thyroid cartilage opposite the disc between the third and fourth cervical vertebrae. It terminates behind the neck of the mandible by dividing into the maxillary and superficial temporal arteries.
In the carotid triangle, the external carotid artery is comparatively superficial, and lies under cover of the anterior border of the sternocleidomastoid.
Branches
Superior thyroid
Lingual
Facial
Occipital
Posterior auricular
Ascending pharyngeal
Maxillary
Superficial temporal (Fig. 4.16).
Superior Thyroid Artery
Arises from the external carotid artery just below the level of the greater cornua of the hyoid bone. It supplies the superior laryngeal artery.
Lingual Artery
Arises from the external carotid artery opposite the tip of the greater cornua of the hyoid bone (Fig. 4.17).
Facial Artery
Arises from the external carotid just above the tip of the greater cornua of the hyoid bone.
Gives off ascending palatine, tonsillar, submental, and glandular branches.
Occipital Artery
Arises from the posterior aspect of the external carotid artery, opposite the origin of the facial artery. In the carotid triangle, the artery gives two sternocleidomastoid branches.
Posterior Auricular Artery
Arises from the posterior aspect of the external carotid just above the posterior belly of the digastric (Fig. 4.16).
Ascending Pharyngeal Artery
This is a small branch that arises from the medial side of the external carotid artery (Fig. 4.16).
Maxillary Artery
The largest terminal branch.
Superficial Temporal Artery
The smaller terminal branch
Potential Tissue Spaces in Head and Neck
Submental space: Lies below inferior border of mandible. Corresponds to submental triangle. It communicates with submandibular spaces of both sides.
Submandibular space: Lies between anterior and posterior bellies of digastric muscle and inferior border of mandible. It communicates with sublingual and submental spaces.
Parotid space: Localized around parotid gland behind ramus of mandible, communicates with retropharyngeal space and even mediastinum.
Parapharyngeal space: Lies in suprahyoid region of neck, lateral to pharynx. It is continuous with retropharyngeal space.
Retropharyngeal space: Lies between anteriorly placed buccopharyngeal fascia and posteriorly placed prevertebral fascia. It communicates with peritonsillar space, submental spaces and may reach mediastinum.
Peritonsillar space: Lies between anterior and posterior fauces of the palatine tonsil. It communicates with sublingual space.
Mnemonics
External carotid artery branches: “Some Anatomists Like Freaking Out Poor Medical Students” (Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Maxillary, Superficial temporal).