The oral or buccal cavity and its related structures. The oral cavity forms in the embryo from an in-pocketing of the skin, the stomodeum; it is thus lined by ectoderm and is not, properly speaking, part of the digestive tract. Functionally, however, the mouth forms the first portion of both the digestive and respiratory systems. Various special structures are found in, or associated with, the mouths of most vertebrates.