OF THE SKIN. 147 



jured epidermis, since in these cases a mechanical intrusion of the sub- 

 stances, in and through the sudoriparous ducts and hair-sacs, or their 

 penetration into the sweat-ducts, and mingling with the sweat, explains 

 their absorption. The mucous layer, at any rate, is easily penetrated 

 by liquids, as is sufficiently shown by pathological anatomy (exudations 

 which penetrate the mucous and raise up the horny layer into a vesicle 

 the ready occurrence of absorption after the separation of the horny 

 and the superficial portion of the mucous layer, by the action of vesi- 

 cants). 



In their chemical relations, it is indeed well known how the cells and 

 plates of the epidermis behave with regard to certain reagents, but 

 there exists, at present, no perfectly satisfactory total analysis of the 

 epidermis, with regard to its two layers which differ so widely ; and the 

 organic combinations, also, which occur in it, are not sufficiently known. 



The so-called horn, which forms the membranes of the horny plates, 

 is insoluble in water, easily soluble in concentrated alkalies and con- 

 centrated sulphuric acid, whence the skin, if wetted with these sub- 

 stances, feels slippery and greasy ; there remains, however, a small residue 

 insoluble in alkalies ; concentrated acetic acid, also, dissolves it, first 

 rendering it gelatinous, by which it is distinguished from the protein 

 compound of the hair. It contains less sulphur than the hair and nails, 

 which is perhaps the reason why salts of lead, mercury, and bismuth, 

 color the hair but not the epidermis. Besides these, Mulder finds in 

 the horny layer a gelatinous matter, which is obtained by long boiling 

 in water, and which would appear to be of a collagenous nature. The 

 epidermis does not putrefy — it melts in the fire without bending or 

 swelling up, and burns with a clear flame. 



The behavior of the epidermis towards reagents is particularly of 

 importance for the microscopist, on whose behoof I add the following 

 account. 



After long maceration in water, the epidermis becomes detached in 

 portions, and under moderate pressure is resolved into a white powder, 

 consisting of the isolated horny plates, and the uppermost cell of the 

 rete Malpighii. Boiled in water, pieces of the horny layer break up 

 into their elements much more readily. Boiled in concentrated acetic 

 acid from 15 to 25 minutes, all the horny plates become perfectly isola- 

 ted, forming a cloudy, whitish deposit in the test tube ; they are ex- 

 ceedingly pale, so that they are often hardly visible under a full illu- 

 mination ; and are completely swollen up and changed into globular or 

 elongated, distended, but always more or less flattened vesicles of 0-02- 

 0-032 of a line in breadth, and 0-006-0-01 of a line in thickness, the nuclei 

 when they are present being also pale and hardly to be perceived. The 



