The boundaries of the anterior triangle are the anterior border of the sternocleidomastoid muscle, the inferior border of the mandible, and the midline of the neck.
The boundaries of the posterior triangle are the posterior border of the sternocleidomastoid muscle, the anterior border of the trapezius muscle, and the middle one-third of the clavicle.
The fascia of the neck has a number of unique features.
The superficial fascia in the neck contains a thin sheet of muscle (the platysma), which begins in the superficial fascia of the thorax, runs upward to attach to the mandible and blend with the muscles on the face, is innervated by the cervical branch of the facial nerve [VII], and is only found in this location.
Deep to the superficial fascia, the deep cervical fascia is organized into several distinct layers (Fig. 8.160). These include: an investing layer, which surrounds all structures in the neck; the prevertebral layer, which surrounds the vertebral column and the deep muscles associated with the back; the pretracheal layer, which encloses the viscera of the neck; and the carotid sheaths, which receive a contribution from the other three fascial layers and surround the two major neurovascular bundles on either side of the neck.
The investing layer completely surrounds the neck and encloses the trapezius and sternocleidomastoid muscles (Fig. 8.162).
Attaching posteriorly to the ligamentum nuchae and the spinous process of the CVII vertebra, this fascial layer splits as it passes forward to enclose the trapezius muscle, reunites into a single layer as it forms the roof of the posterior triangle, splits again to surround the sternocleidomastoid muscle, and reunites again to join its twin from the other side.
Anteriorly, the investing fascia merges with fascia surrounding the infrahyoid muscles.
The investing fascia is attached: superiorly to the external occipital protuberance and the superior nuchal line, laterally to the mastoid process and zygomatic arch, and inferiorly to the spine of the scapula, the acromion, the clavicle, and the manubrium of the sternum.
The external and anterior jugular veins, and the lesser occipital, great auricular, transverse cervical, and supraclavicular nerves, all branches of the cervical plexus, pierce the investing fascia.
The prevertebral layer is a cylindrical layer of fascia that surrounds the vertebral column and the muscles associated with it (Fig. 8.162). Muscles in this group include the prevertebral muscles, the anterior, middle, and posterior scalene muscles, and the deep muscles of the back.
The prevertebral fascia is attached posteriorly along the length of the ligamentum nuchae, and superiorly forms a continuous circular line attaching to the base of the skull. This circle begins: anteriorly as the fascia attaches to the basilar part of the occipital bone, the area of the jugular foramen, and the carotid canal; continues laterally, attaching to the mastoid process; and continues posteriorly along the superior nuchal line ending at the external occipital protuberance, where it associates with its partner from the opposite side.
Anteriorly, the prevertebral fascia is attached to the anterior surfaces of the transverse processes and bodies of vertebrae CI to CVII.
The prevertebral fascia passing between the attachment points on the transverse processes is unique. In this location, it splits into two layers, creating a longitudinal fascial space containing loose connective tissue that extends from the base of the skull through the thorax (Figs. 8.162 and 8.163).
There is one additional specialization of the prevertebral fascia in the lower region of the neck. The prevertebral fascia in an anterolateral position extends from the anterior and middle scalene muscles to surround the brachial plexus and subclavian artery as these structures pass into the axilla. This fascial extension is the axillary sheath.