The unmyelinated fibre spectrum of the avian cervical vagus nerve.

The numbers and diameters of the unmyelinated nerve fibres in the cervical vagus nerve of the domestic fowl were studied electron microscopically. The mean total number was estimated at 7028 in the right nerve and 6774 in the left, the ratio or myelinated to unmyelinated fibres being about 1.3:1. From the numbers of unmyelinated fibres surviving intracranial vagotomy on the right side it was estimated that approximately 5028 (about 72%) were afferent in the right cervical vagus nerve. The diameters of the unmyelinated fibres in the cervical nerve ranged from 0.15 to 2.0 micrometers, about 90% being between 0.45 and 1.04 micrometer. The total number of myelinated and unmyelinated fibres in the cervical vagus nerve of this species of bird appears to be about 16000 on the right side and 15000 on the left.