142 SPECIAL HISTOLOGY. 



trated sulphuric acid, in five minutes, causes the horny layer to 

 swell up so much, that its elements, although still remaining 

 flattened and irregular, appear quite distinctly to be vesicles; 

 after half au hour they are somewhat more distended, and easily 

 separable from one another. By boiling with this acid the 

 plates swell up even in a minute, without exhibiting nuclei, 

 and in two minutes they disappear without leaving any trace. 

 Boiling in dilute sulphuric acid renders the horny layer hard 

 and transparent, and dissolves it wholly in 4 — 5 hours. The 

 cells of the stratum Malpighii are little altered by cold sulphuric 

 acid: on boiling, their contours and nuclei at first become more 

 distinct, but in about two minutes the whole is dissolved. 

 Nitric acid colours the epidermis yellow, softens and changes 

 it into xantho-proteic acid. The cells of the horny layers swell 

 up somewhat, after a time, in the cold, and become granular ; 

 the stratum Malpighii is rendered granular and indistinct, and 

 sharply defined from the horny layer. Upon boiling, the 

 whole epidermis is entirely dissolved in half a minute. Hydro- 

 chloric acid does not tinge the epidermis, and in the cold 

 renders the cells of the horny layer somewhat more distinct 

 than nitric acid. After boiling for a minute the horny layer 

 becomes a beautiful cellular tissue, exactly as after the addition 

 of dilute solution of potass. In carbonate of potass the epi- 

 dermis is hardly changed at all. After seventeen weeks it is 

 hardened and easily cut with a knife. Nitrate of silver 

 colours it violet or brownish-black, by the formation of oxide 

 of silver, of chloride of silver, and of black sulphuret of 

 silver, in consequence of the chloride of sodium and sulphur 

 which it contains. Investigated microscopically with the help 

 of acetic acid, the tissue of the epidermis is seen to remain 

 quite unchanged, and minute dark granules are visible between 

 its elements. Nitrate of mercury gives the epidermis a reddish- 

 brown hue, sulphurets of the alkalies render it brown and black: 

 many vegetable colours unite with it. In alcohol and ether it 

 is insoluble, with the exception of the small quantity of fat 

 which it contains. 



From all this, it results, with regard to the elementary parts 

 of the epidermis, that they are cells, which, however, as the 

 alkalies show, do not everywhere present the same characters. 

 In the stratum mucosum they are actual vesicles and easily 



