The head and neck region is functionally difficult to divide into distinct 'head' and 'neck' regions. For example, the nasal cavity is intimately related with the pharynx, which extends from the base of skull (definitely in the head) to the cricoid cartilage (definitely in the neck). It is therefore easier to consider these head and neck structures as separate, functional units that all reside in the 'head and neck' region.
The central nervous system is a distinct entity and is considered in a different topic. The skin of the face is also covered separately.
Lip and Oral Cavity
The tonsillar fossa (C09) is included with the oropharynx. The pyriform sinus (C12) is included with the hypopharynx.