On the Innervation of the Human External Auditory Canal

Abstract

In the outer half, the upper wall of the bony part of the external auditory canal shows a histological picture similar to that of the cartilaginous part, but in other sides and parts, the picture of the wall is quite different. In the former part too, however, the further we go inwards, the epidermis becomes the thinner, the hair follicles in the corium worse developed and the ceruminous glands in the subcutis the fewer, finally disappearing altogether.The development of the str. papillare of the corium of the bony part is very poor, so that in some parts it appears as if the epidermis directly touches the str. reticulare. Stratum reticulare is composed of collagenous fibre bundles arranged circularly, but is only very thin except in the outer half of the upper wall of the part, as is also the subcutis containing a small number of fat cells, and its outer surface joins the periosteum of the temporal bone.The epidermis becomes the thinner, as we go further interiorwards, its str. germinativum being covered by a thin horn plate often with cell nuclei, as found in the tympanic membrane.The inner half of the bony part is abundantly provided with blood vessels. Beside the venous plexus without smooth muscle fibres extending out of the subcutis into the corium, we find some small veins surrounded by longitudinal smooth muscle bundles. These veins are connected with the capillaries formed subepidermally, and in the outer half, the veins gradually gain in size, but the muscle fibres around them decrease more and more. There are besides some smooth muscle fibres running about circularly in the corium independent of any blood vessels.The innervation, especially, the sensory innervation, of the cartilaginous part is not much different from that of any common haired skin. That is to say, most of the sensory nerve fibres end in plexus-like terminations poorly developed in the SETO's so-called hair nerve shields or rings in the necks of the hair follicles, while a minor part of them run into the str. papillare and form unbranched and simple branched terminations therein. The quantity of such fibres here is larger than in the scalp, the eye-lid, the belly and the back skin, and is nearly equal to that in the outer genitals and the perineum, but though this cartilaginous part is susceptible to a similar ticklish feeling, it contains no genital nerve bodies or PACINIan bodies.The outer half of the upper wall of the bony part of the auditory canal is rather similar to that of the cartilagionus part, poorly formed plexus-like terminations being found in the poorly developed hair follicle necks and unbranched and simple branched terminations beneath the epidermis. The quantity of the latter here is larger than in the cartilagionus part.In the wall of the bony part except in the upper side of the outer half, the sensory fibres are markedly abundant, mostly ending in unbranched and simple branched terminations subepidermally. On approaching the tympanic membrane, the number of sensoy terminations grow still larger, some of their terminal fibres showing the appearance as if running into the epidermis itself, as seen in the skin part of the tympanic membrane. Besides, some branched terminations usually formed by thick fibres are not rarely found adjacent to the periosteum of the temporal bone in this part.