SECT. 46, 47.] EPIDERMIS. 91 



Pathological colourings of the epidermis (freckles, mother's marks, etc.), 

 according to Simon, Kraiise, Barensprunff, ami what I myself have seen, 

 present entirely the same characters as the more intensely coloured parts in 

 the white races, and the skin of the negro. Different from these are the 

 pigments in the covin m and the papilla, which are observed in cicatrices after 

 chronic inflammation of the skin, — and often, as in Ichthyosis and many ncevi, 

 iu conjunction with a coloured epidermis, — in which the pigment is developed 

 directly from the blood-corpuscles and their colouring matter. Cases of par- 

 tially or tot .illy white negroes and black Europeans, not in consequence of a 

 change of the climate, but from a congenital or subsequently occurring 

 abnormal condition of the skin, have been recorded (see Hildebrandt- 

 Weber, ii., page 526, Flourens, Compt. rend, xvii.) ; still, in respect to future 

 cases of this kind, at least in the dark coloration of Europeans, it must 

 be kept in mind, that it can also arise from deposited colouring matter of 

 the bile. 



§ 46. The thickness of the entire epidermis varies between - n i ~" and 

 1 1'", which depends upon the various depths of the horny layer, 

 and measures in most places between -^'" and T y". 



The absolute thickness of the mucous layer (at the base of the 

 papilla?) varies between o-oo'j'" and o - i6'" ; in many places, as in the 

 face, the genitals, aud on the breasts, it is thicker than the horny 

 layer, and measures 004'", or where it is thinner, from o - oi'" to o - o2'". 

 The horny layer measures only 0'005'" in many places, in others 

 attains the thickness of \" or more; where it exceeds the mucous 

 layer, it amounts to o'l'" to 0-4'" ; where it is less than it, o'oi'". 



§ 47. Physical and Chemical Qualities. — The cells of the epider- 

 mis do not contain, either in their membranes or in their interstices, 

 demonstrable pores (apart from the sudoriparous canals and hair- 

 follicles, which, in a certain measure, have their outermost parts 

 excavated in the epidermis), and form a very firm, scarcely per- 

 meable mass. Many experiments, particularly those of Krause, 

 show that the horny layer of the epidermis does not allow liquids 

 to permeate it (except such as act chemically on the structure, as 

 mineral acids and caustic alkalies), either by pores, imbibition, or 

 endosmose and exosmose, but takes up with facility gaseous and 

 volatile substances (alcohol, ether, acetic acid, ammonia, ethereal 

 solutions of chloride of iron, alcoholic solutions of acetate of lead, 

 etc.), or gives them off (cutaneous transpiration). And this con- 

 clusion is not weakened by the undeniable passage of water and 

 other liquids, unguents, and even solid bodies (sulphur, vermilion) 

 through the uninjured epidermis; for, in these cases absorption is 

 favoured by mechanical intrusion of the substances in and through 



