The nerve periphery in the human trachea was studied histologically by means of SUZUKI's silver method with the following results:
1. The ciliated epithelium was supplied with well developed intraepithelial free endings of sensory nerves. Their branches were to be classified into ascending and horizontal branches. These branches often showed neurofibrillar enlargements either in their course or at their ends. Some of the ascending branches reached the free surface to end just beneath the terminal bars. The horizontal branches were widely spreaded through the basal cell layer of the epithelium, and at their periphery often intersected with those of the neighbouring ending. Neurofibrillar enlargements of the horizontal branches were often found closely lying to basal cell nulei.
2. In the tracheal epithelium, especially of the membraneous portion, there were often found so-called ‘patches of the squamous epithelium’. These patches were also provided with free sensory endings, which were somewhat less developed than in the case of the ciliated epithelium.
3. The mode of the vegetative innervation of the tracheal epithelium was a hypolemmal one, that is, the vegetative nerve periphery gave rise to the intraepithelial nerve net accompanied by a number of stellate cells. Most of these cells were to be considered as interstitial cells of the vegetative nerve periphery, since their cell-bodies were in syncytial connection with protoplasmic nerve strands and the neurofibrils formed intricate networks in the cytoplasm.
4. Trachcal glands received intraepithelial neurofibrils from the surrounding terminal reticulum. These neurofibrils generally ran and ended intercellularly. But between the glandular cells there were often conical cells which were penetrated by the neurofibrils from process to process giving an appearance of intermediate elemnts between glandular cells and the vegetative nerve pathway.
5. Various types of the sensory nerve endings were observed in the tracheal muscle layer and muscularis mucosae. They were mostly to be classified into: a) the glomerular or convoluted endings near or surrounding the small blood vessele and b) the arborized large endings. The latter were generally the same with so-called ‘pressoreceptors’of SETO and others. But they were also characterized by a more or less intimate relaionship with the adjoining small blood vessels in various ways, e. g., encircling the vessels with their arborizations, or giving off side branches with or without the endbody to the vessels. These facts suggests the existence of another possible function of the endings, in addition to a usually accepted pressoreceptor, such as interoceptor of local blood vessels.
6. In the smooth muscle layer the vegetative nerve periphery gave rise to the terminal reticulm consisting of the anastomosing protoplasmic nerve strands and the interstitial cells which were in syncytial continuity with the former. Both of them touched the surface of smooth muscle cells with their protoplasmic processes.