Origin, communication and course of the vagus nerve auricular branch with special references to the mastoid canaliculus for the nerve: A study using human fetus histological sections.

Background: The vagus nerve auricular branch (Xab) is a well-known sensory nerve and it communicates with the facial and glossopharyngeal nerves. Xab passes through the so-called mastoid canaliculus of the temporal bone (TB) to reach the facial nerve. We aimed to examine the nerve origin, communication and course of Xab as well as bone components along the nerve.

Methods: We observed serial histological sections of 13 embryos (6-8 weeks of gestation) and 31 midterm fetuses (9-17 weeks) as well as semiserial sections of 11 late-term fetuses (25-32 weeks).

Results: The initial Xab originated from both the glossopharyngeal and vagus nerves and these two roots were joined at six weeks. Soon later, Xab ran laterally along the anteromedial aspect of the internal jugular vein to reach the facial nerve. Communication with the sympathetic trunk appeared by nine weeks. By 12 weeks, variations became evident in the nerve course, communication and terminal. A microganglion sometimes accompanied Xab. The early Xab ran through a relatively large space between the future occipital condyle and Reichert's cartilage. Depending on a delayed growth of the otic capsule precondylar process, the nerve became to pass through a narrow space between the otic capsule (or Rechert's cartilage) and occipital. At late-term, Xab passed through a canaliculus between the occipital and TB or within the TB.

Conclusions: Xab had two roots, ran along the anterior wall of the bony jugular fossa and, penetrated the TB or passed between the occipital and TB to reach the facial nerve descending portion.