L25. Neck Triangles

The inferior margin of the mandible, the mastoid process of the temporal bone, and the superior nuchal line of the occipital bone.

The top of the sternum, the entire length of the clavicle, and the acromion process.

The area between the acromion process and the vertebra prominens.

The cervical vertebrae have a smaller body size compared to other vertebrae, except for the first cervical vertebra (C1) which lacks a body. Moreover, they exhibit bifid spinous processes, except for C1.

Each of the cervical vertebrae has a foramen within its transverse process, which is a hole that allows for the passage of blood vessels and nerves.

The first cervical vertebra (C1), also known as the atlas, forms a unique articulation with the second cervical vertebra (C2) and the occipital bone.

The floor of the oral cavity, the larynx, and the pharynx define its boundaries.

The hyoid bone is not attached to other bones and possesses a high degree of mobility. This mobility allows it to serve as a point of attachment for muscles and soft tissues in both the head and neck.

The vertebral compartment encloses the cervical vertebrae, spinal cord, cervical nerves, and associated muscles. The visceral compartment surrounds structures like the thyroid and parathyroid glands, portions of the larynx, and pharynx. The vascular compartments, found on both sides of the neck, contain blood vessels and the vagus nerve (CN X).

The platysma muscle, located within the superficial fascia of the neck, originates from the superficial fascia of the thorax and inserts into the mandible.

The investing layer of the deep cervical fascia, a thick layer of connective tissue, surrounds the external and anterior jugular veins, all cutaneous branches of the cervical plexus, and splits to enclose the sternocleidomastoid, trapezius, and infra- and supra-hyoid muscles.

The prevertebral layer of deep cervical fascia encloses the vertebral column and the deep muscles of the neck. As it passes between the transverse processes, it divides into two layers, with the anterior layer being termed the Alar fascia.

The pretracheal layer of deep cervical fascia encloses the neck viscera, and the posterior part that encloses the pharynx and esophagus is referred to as the buccopharyngeal fascia.

The carotid sheaths are thick parts of the deep cervical fascia that surround the neurovascular structures bilaterally.

The pre-tracheal space is located between the investing and pretracheal layers of the deep cervical fascia, extending from the neck to the anterior part of the superior mediastinum.

The retropharyngeal space lies between the buccopharyngeal fascia and the prevertebral fascia. It stretches from the base of the skull to the upper posterior mediastinum.

The prevertebral space is formed by two layers of the prevertebral fascia as it passes between the transverse processes. It extends from the base of the skull through the posterior mediastinum all the way down to the diaphragm.

The platysma muscle originates from the mandible and the muscles surrounding the mouth.

The platysma muscle is innervated by the facial nerve (CN VII).

The platysma muscle tenses the skin of the neck and helps move the lower lip and corners of the mouth downwards.

The platysma muscle, external jugular vein, and anterior jugular vein are all found within the superficial fascia of the neck.

The posterior triangle of the neck is bordered anteriorly by the posterior edge of the sternocleidomastoid muscle, posteriorly by the anterior edge of the trapezius muscle, inferiorly by the middle one-third of the clavicle, superficially by the investing layer of the deep cervical fascia, and deeply by muscles of the neck covered by the prevertebral fascia.

The sternocleidomastoid muscle originates from the lateral surface of the mastoid process and the lateral half of the superior nuchal line.

The sternocleidomastoid muscle is innervated by the accessory nerve (CN XI).

The sternocleidomastoid muscle tilts the head to the same side, rotates the head to turn the face to the opposite side. When both sternocleidomastoids contract together, they draw the head forward.

The trapezius muscle originates from the lateral third of the clavicle, the acromion, and the spine of the scapula.

The trapezius muscle is innervated by the accessory nerve (CN XI).

The trapezius muscle rotates the scapula during humerus abduction above the horizontal, elevates, depresses, and abducts the scapula.

The following cutaneous nerves of the cervical plexus are found in the posterior triangle: Lesser occipital (C2), Great auricular (C2-C3), Transverse cervical (C2-C3), and Supraclavicular (C3-C4).

The cranial nerve XI (accessory nerve) is located in the investing layer of the deep cervical fascia, between the sternocleidomastoid and trapezius muscles.

The anterior triangle of the neck is bounded posteriorly by the anterior edge of the sternocleidomastoid muscle, superiorly by the inferior border of the mandible, and medially by the midline of the neck.

The anterior triangle of the neck is divided into four smaller subtriangles: Submandibular, Submental, Muscular, and Carotid.

The submandibular triangle is bounded superiorly by the inferior border of the mandible, antero-posteriorly by the digastric muscle, superficially by the investing layer of the deep cervical fascia, and deeply by the mylohyoid and hypoglossus muscles.

The submental triangle is bordered inferiorly by the hyoid bone, laterally by the anterior digastric muscle, and medially by the midline of the body.

The muscular triangle of the neck is bounded superiorly by the hyoid bone, laterally by the superior belly of the omohyoid muscle and the anterior sternocleidomastoid (SCM) muscle, and medially by the midline of the body.

The carotid triangle of the neck is bordered antero-inferiorly by the superior belly of the omohyoid muscle, superiorly by the stylohyoid muscle and the posterior digastric muscle, and posteriorly by the anterior border of the sternocleidomastoid muscle.

The muscular triangle contains four infrahyoid muscles: the omohyoid, sternohyoid, thyrohyoid, and sternothyroid. The thyrohyoid and sternothyroid muscles are deeper and shorter than the other two.

All infrahyoid muscles, except for the thyrohyoid, are innervated by the anterior rami of C1-C3. The thyrohyoid muscle is innervated by the anterior ramus of C1, which travels with the hypoglossal nerve (CN XII).

The external carotid artery has eight branches: superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary, and superficial temporal.

The internal jugular vein originates inside the skull as a dilation of the sigmoid sinus and then exits the skull through the jugular foramen. It travels posterior and then lateral to the internal carotid artery before joining with the subclavian vein.