THE SKIN AND ITS APPENDAGES. 



The nerves supplying the skin vary greatly in different regions, the 

 palmar surface of the fingers and the corresponding parts of the toes 

 receiving the richest supply. The larger sterns lie within the sub- 

 cutaneous tissue, from which, in addition to twigs distributed directly 

 to the sweat-glands and the involuntary muscle, numerous 

 branches accompany the blood-vessels into the corium to end in 

 various ways. Upon reaching the superficial portions of the corium, 

 after having given off many lateral branches, the ascending twigs 

 break up into bundles, which form a rich subpapillary plexus, con- 

 taining both medullated and pale fibres, and extending beneath the 

 epidermis and the bases of the papillae. The non-medullated 

 fibres are probably destined for the involuntary muscle of the cutis, 

 the glands, and the blood-vessels ; the medullated fibres, on the 

 other hand, are connected with several forms of special nerve- 

 endings. From the superficial plexus within the corium small twigs 

 ascend to the epidermis, some fibres ending immediately beneath 

 the epithelium, while others pass for different distances between 

 the epithelial elements to terminate either as free endings or in 

 connection with the tactile cells. The branches from the subpap- 

 illary plexus which ascend into the papillae are connected with the 

 large tactile corpuscles of Meissner which occupy the non-vas- 

 cular papillae. Within the subcutaneous layer, in many regions, 

 numerous corpuscles of Vater are present. The hair-follicles 

 receive a considerable part of the nerves of the corium, the medul- 

 lated fibres forming loose net-works around the follicles, which they 

 accompany as far as the sebaceous glands, where they divide into the 

 naked fibrillas which are traceable with certainty as far as the glassy 

 membrane and probably end within the external root-sheath. 



ITS APPENDAGES. 



FIG. 311. 



THE DEVELOPMENT OF THE SKIN AND 



The development of the skin in- 

 cludes the participation of the ecto- 

 derm and the mesoderm, which con- 

 tribute respectively the epidermis and 

 the corium. The history of the epi- 

 dermis is closely identified with that of 

 the ectoderm. In the earliest stage the 

 latter consists of a single layer of 

 low cuboidal cells ; later an addi- 

 tional superficial stratum, the epi- 

 trichium, becomes differentiated, the 

 two layers of the ectoderm probably 

 already indicating the corneous and 

 Malpighian strata of the future epidermis, although the precise 



Section of developing skin from 

 human foetus of three and a half 

 months : a, layer of cuboidal cells rep- 

 resenting stratum Malpighii ; b, polyhe- 

 dral elements forming superficial layers ; 

 c, outermost flattened plates, probably 

 the remains of the epitrichial layer ; d, 

 mesodermic tissue forming corium. 



