The sternocleidomastoids and trapezius comprise the outermost layer of the deep cervical muscles. The stermocleidomastoids lie superficial to the carotid sheath and are crossed diagonally by the external jugular vein. Central venous access via the internal jugular vein can be obtained through the hiatus between sternal and clavicular heads of the sternocleidomastoid or at the posterior border of the mid-portion of the muscle.
The deep cervical fascia splits to encompass the sternocleidomastoids and trapezius muscles, forming a girdle around the neck. The cervical plexus nerves and superficial veins penetrate the deep fascia.
The subcutaneous platysma muscle layer is the only remnant in the human of the panniculus carnosus of other mammals which allows them to shake their fur. The plane beneath this layer is easily separated from the underlying deep fascia and facilitates anatomic dissection for thyroid surgery and neck dissection.