The third space is within the prevertebral layer covering the anterior surface of the transverse processes and bodies of the cervical vertebrae. This layer splits into two laminae to create a fascial space that begins at the base of the skull and extends through the posterior mediastinum to the diaphragm.
The external jugular and anterior jugular veins are the primary venous channels for superficial venous drainage of the neck (Fig. 8.164).