STRUCTURE OF THE SKIN. 129 
nucleated columnar cells planted by denticulated extremities on the basement 
membrane of the corium, 
2. The stratum mucosum, consisting of six or eight layers of polygonal, nucleated 
“prickle” or “finger” cells, the processes of which join those of adjacent cells. 
Between the cells of this layer are minute channels, in which leucocytes or pigment 
eranules may be seen. The cells of the stratum mucosum are characterised by the 
presence of numerous epidermic fibrils, which are coloured violet by hematoxylin 
and red by carmine. These fibrils are unaffected by boiling, but swell up under 
the action of acids and alkalies and form the filaments of union between adjacent 
cells. On account of their presence, L. Ranvier (Compt. rend., Paris, Jan. 1899, 
tome ¢xxvill.) has named this layer the stratum filamentosum. The dark colour 
of the negro’s skin is caused by the presence of numerous pigment granules in the 
deeper layers of this stratum; the pigment—of which melanin forms an important 
constituent—is absent from the more superficial layers of the epidermis. 
3. The stratum granulosum, consisting of two or three layers of horizontally 
arranged, flattened cells, scattered around the nuclei of which are elliptical 
or spherical granules of e/ecdin, a substance staining deeply with carmine, and 
probably representing an intermediate stage between the protoplasm of the deeper 
cells and the keratin of the superficial layers. 
4. The stratum lucidum, an apparently homogeneous layer, but in reality made 
up of several strata of flattened or irregular squames, some of which may contain 
eleidin granules. 
5. The stratum corneum, which comprises several layers of flattened, keratinised 
squames, from which the nuclei have disappeared. The superficial layers of this 
stratum are constantly removed by friction and as constantly replaced by the 
deeper cells, which undergo keratinisation as they approach the surface. 
L. Ranvier (op. e7t.) has pointed out that the stratum lucidum is really double, and has named 
the deeper of its two layers the stratum intermedium; this he describes as consisting of two or 
three layers of clear cells with atrophied nuclei, while in the cell-walls the epidermic fibrils “are 
rolled up like the threads of a cocoon.” He has also apphed the term stratum disjunctum to the 
partly-detached cells on the free surface of the stratum corneum. Ranvier (Arch. Anat. Micr. 
1898, tome ii. fase. iv.) has likewise shown that the cells of the stratum corneum contain granules 
of a fatty material having the consistency and plasticity of beeswax. 
Regeneration of the epidermis is generally regarded as taking place by cell proliferation in 
the stratum germinativum, the young cells evadually passing through the polyhedral and 
granular stages, and ultimately bee oming the horny squames of the stratum corneum. Professor 
Thomson of Oxford considers th: it, although this view meets all the requirements in white races, 
a difficulty is met with if it is applied to coloured races, where most pigment is found in the 
deeper cells of the stratum mucosum, while the superfic ‘ial layers are free from colour. If the 
deeper cells advance to the surface, it is only reasonable to suppose that they would carry their 
pigment with them. This theor y, therefore, necessitates a satisfactory explanation of the 
disappearance of the pigment from the superficial | layers. He suggests that possibly the growth 
of the epidermis is analagous to the growth of the cork eambium ‘of plants, the stratum maucosum 
corresponding to the green cells and the stratum corneum to the corky layer of the cambium. If 
such be the case—and he insists that there is much evidence in support of it—the deeper cells would 
advance inwards towards the corium and the superficial cells would grow outwards towards the 
surface. Under this view the active layers would be the stratum eramulosuim and stratum lucidum, 
and by it many of the difficulties would be explained, including the mysterious disappearance of 
the pigment from the superficial layers ; it would also afford a reasonable explanation of how it 
happens that, in old age, a negro’s hair becomes white, while his skin retains its blackness. 
Vessels and Nerves of the Skin.—The arteries form a plexus in the sub- 
cutaneous tissue, from which branches extend into the corium, where they supply 
the hair follicles and glands and form a second plexus under the papille, to which 
small loops are given., The veins and lymphatics commence in the papille, and, 
after forming a subpapillary plexus, open into their respective subcutaneous 
vessels. 
The nerves of the skin vary in number in different parts of the body, being 
extremely numerous where the sense of touch is acute, ¢.g. on the palmar aspect 
of the terminal phalanges, while in the skin of the back, where the sensibility is 
less, they are fewer in number. They form a plexus in the corium, and either 
terminate amongst the cells of the epidermis or in special end organs named 
tactile corpuscles, Those ending in the epidermis form a rich subepithelial plexus, 
7a 
