Innervation of human back skin.

Abstract

The poor sensory innervation of the human back skin between the 5th and the 8th thoracal spinal processes may be surmised from the peculiar histological picture of this part, which shows very sporadic growth of hairs, small terminal territory of the sensory hair nerve fibres in the hair follicle neck and a weak development of the subepidermal papillae, because, as Seto and his collaborators have established, the majority of sensory fibres in the haired skin terurinates in connection with the hairs and the minority that comes into the papillary layer is developed in proportion to the development of the papillae. In fact, the sensory fibres running into the papillar layer were very small in number, and their terminations were very simple, being represented by sharp unbranched, and in rarer cases, by bifurcated sharp endings. No intraepithelial fibres were ever found. The hairs in this region are downy, but they reach far into the deeper layer of the corium, together with the much elongated sebaceous glands. The terminal areas of the sensory hair nerve fibres, the Seto's hair neuroshields or -bubes formed between the sebaceous glands and the outer root sheathes are generally very poorly formed, and consequently the development of the sensory nerve fibres and their terminations is also extremely weak. The sensory terminations are even simpler than those in the scalp (Seto)1), but more complex than those with the downs on the eyelids (Seto2)). The terminal formation is entirely of simple plexus-like, serrate and indefinite types. The plexus-like terminations are formed by sensory fibres and their branches arranged in plexus form, and are incomparatively simpler in formation than those with cilia (Seto2)) and the downs of the pars cutanea of lip (Seto et al.3)) and even simpler than those in the scalp (Seto1)). The serrate terminations are composed of sensory fibres arranged in serration, but since these belong to the more complex type of hair nerve terminations, only a very few of them are found in the back skin. The indefinite type of termination is represented by a few rather stout sensory fibres running an irregular course, together with their branches, and shows no definite terminal formation. Being of simpler termination type, such endings are frequently found in the back skin. As described above, the back skin is very poor in sensory fibres, but interestingly enough, there were found glomerular terminations in the small artery wall in the subcutaneous tissue and the papillary layer of the back skin, a type of terminations never found in the haired skin except in the external genitals and the papilla mammae and areola mammae, though, may be, in extremely rare cases. This glomerular termination found in the small artery, but is more complex. It is represented by a capsulated oval body composed of sensory fibres running into a ground substance containing numerous cell nuclei together with a few branches and showing glomerular arrangement, and is presumably receptor apparatus connected with adjustment of blood pressure. The other corpuscular body in the papillar layer is composed of non-capsulated glomerular ending formed in the same ground substance, and is perhaps receptor apparatus for delicate stimuli. The vegetative innervation of the back skin is much the same with that of the scalp (Seto)1).