150 SPECIAL HISTOLOGY. 



the horny layer somewhat more distinct than nitric acid. After boil- 

 ing 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 epidermis is hardly changed at all. After seventeen 

 weeks it is hardened and easily cut with a knife. Nitrate of silver 

 colors 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 epider- 

 mis 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 colors unite with it. In alcohol and ether it 

 is insoluble, with the exception of a 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 soluble in the horny layer, scarcely 

 so ; and here, in fact, a distinction must be drawn between the resisting 

 cell-membrane and the cell-contents, which swell up and disappear more 

 readily; these, in the natural condition, form an apparently homogene- 

 ous simple plate, but the difference between them may be readily ex- 

 hibited by reagents. In what parts the small quantity of collagenous 

 substance, which has been noticed, has its seat, is not clear ; perhaps 

 it forms a portion of the contents, especially of the cells of the mucous 

 layer, or belongs, it may be, to an intermediate substance between the 

 cells, which, however, is not microscopically demonstrable. If the fat 

 of the epidermis is not merely accidental, arising from the cutaneous 

 secretions, it is most probably contained within the Malpighian cells. 



Cruns, Todd and Bowman, Valentin and Bruch, recommend the use 

 of alkalies for the investigation of the epidermic tissues, but their full 

 importance was first shown by Donders (Mulder's " Phys. Chemie," p. 

 257, et seq., and " Hollandische Beitrage," I. u. 11). They are now 

 generally recognized as quite indispensable reagents for the investiga- 

 tion of the horny tissues ; but, as Paulsen (" Obs. Microchem.," &c. 

 Dorpat, 1848) and Reichert (Mull. "Arch," 1847, Jahresbericht.) ad- 

 vise, it is well always to use only definite solutions. I may add, that a 

 great saving of time is effected by the heating of the tissues to be inves- 

 tigated in test tubes, with these and other reagents, as I have already 

 done in examining those tissues of animals which contain cellulose 

 ("Annales d. Sc.Nat.," 1846). 



46. G-rowth and Regeneration. The epidermis possesses no power 

 of continually active growth depending upon intrinsic causes and founded 



