138 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



lime continuous with the epithelial linings of these. The soft layer which 

 lies in immediate contact with the true skin, was formerly supposed to be a 

 substance of distinct nature, and was described under the name of rete muco- 

 sian ; it has been proved by microscopic examination, however, to consist of 

 the same elements with the ordinary epidermis, in an early stage of their 

 development; and, so far from being the exclusive seat of the colour of the 

 skin, as was formerly supposed, it only participates with the fully-formed 

 epidermis in the possession of pigment-cells ( 163). The thickness of the 

 Epidermis, and consequently the number of layers of which it is composed, 

 vary greatly in different parts; being usually found to be greatest, where there 

 is most pressure or friction, as on the palms of the hands of the labouring 

 man, and on the soles of the feet, particularly at the heel, and the ball of the 

 great toe. It would seem as if the irritation of the true skin produced an 

 augmented determination of blood to the part, and consequently an increased 

 development of epidermic cells. The Epidermis covers the whole exterior of 

 the body, not excepting the Cornea and the Conjunctival membrane; on the 

 latter, however, it has more the character of an Epithelium. This continuity 

 is well seen in the cast skin or slough of the Snake ; in which the covering 

 of the front of the eye is found to be as perfectly exuviated as that of any part 

 of the body. 



162. The Epidermis appears solely destined for the protection of the true 

 Skin, from the mechanical injury and the pain occasioned by the slightest 

 abrasion, and from the irritating influence of exposure to air and of changes 

 of temperature. We perceive the value of this protection, when the Epider- 

 mis has been accidentally removed. It is very speedily replaced, however; 

 the increased determination of blood to the Skin, which is the consequence of 

 the irritation, being favourable to the rapid production of Epidermic cells from 

 its surface. The peculiar character of the tissue appears to depend upon the 

 property possessed by its cells, of secreting horny matter into their cavity ; 

 and this process seems to take place at a period subsequent to the first forma- 

 tion of the cells. For if a thin vertical section of the Epidermis be treated 

 with Acetic acid, or with a strong solution of Potass, it is found that the inner 

 newly-formed layers are dissolved by the re-agent, whilst the outer or scaly 

 ones are unaffected. Recent analysis has shown, that the dense Epidermis 

 from the sole of the foot, and the compact Horny matter of which Nails, 

 Hoofs, Horns, Hair, and Wool, are composed, have the same composition; 

 the formula of all of them being 48 Carbon, 39 Hydrogen, 7 Nitrogen, and 

 17 Oxygen. It is probable that, here as elsewhere, if we could isolate the 

 wall of the cell from its contents, we should find the former to consist of a 

 proteine-compound. 



163. Mingled with the Epidermic cells, we find others which secrete Co- 

 louring-matter instead of Horn ; these are termed Pigment-cells. They are 

 not readily distinguishable in the Epidermis of the fair races of mankind, 

 except in certain parts, such as the areola around the nipple, and in freckles, 

 nacvi, &c. But they are very obvious, on account of their dark hue, in the 

 newer layers of the Epidermis of the Negro and other coloured races; and, 

 like true Epidermic cells, they dry up and become flattened scales in passing 

 towards the surface, thus constantly remaining dispersed through its substance, 

 and giving it a dark tint when it is separated and held up to the light. In all 

 races of men, however, we find the most remarkable development of Pigment- 

 cells on the inner surface of the Choroid coat of the eye : where they form 

 several layers, known as the Pigmcntum nignnn. When examined sepa- 

 rately, these are found to have a polygonal form, and to have a distinct nucleus 

 in their interior. The black colour is given by the accumulation, within the 

 cell, of a number of flat, rounded or oval granules, measuring about 1-20, 000th 



