Histology of the human carotid sheath revisited.

Using semiserial sections, we histologically observed the carotid sheath and adjacent structures in 8 sides of 5 cadavers. For description, we classified the carotid sheath into 2 parts or laminae: 1) a laminar "adventitia" enclosing each of the cervical great vessels; and 2) a "common sheath" outside the adventitia. Arterial and venous adventitial structures sometimes fused and provided a definite septum between the artery and vein. Contrasting with previous descriptions, the common sheath did not fuse with superficial or prethracheal lamina of the cervical fasciae, but often fused with visceral fascia to provide a thick plate. The common sheath as well as the prevertebral lamina of the cervical fasciae sometimes became interrupted or unclear, but the adventitia was consistently complete circular. The alar fascia was usually considered as one layer of the multilaminar structure behind the cervical viscera, but it was difficult to identify as a single proper lamina. The carotid sheath was thus not a dissection artifact, but a definite histological structure. However, interindividual and/or site-dependent variations were evident in thicknesses of the adventitia and common sheath. Consequently, the author proposed a model of the fascial arrangement around cervical great vessels that unexpectedly differs from most descriptions in textbooks.