OF THE SKIN. 143 



soluble — in the horny layer, scarcely so; and here, in fact, 

 a distinction must be drawn between the resisting cell-mem- 

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

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

 mogeneous simple plate, but the difference between them may be 

 readily exhibited 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. 



Bruns, Todd and Bowman, Valentin and Bruch, recom- 

 mend 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. II). They are now generally recognised as 

 quite indispensable reagents for the investigation of the homy 

 tissues; but, as Paulsen ( f Obs. Microchem./ &c. Dorpat, 18-18) 

 and Reichert (Mull. 'Arch/ 1847, Jahresbericht.) advise, 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 investigated, 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. 



Growth and Regeneration. — The epidermis possesses no 

 power of continually active growth depending upon intrinsic 

 causes and founded upon the vital relations of its cells, or 

 those which it has with the coriuni; it is essentially a stable 

 tissue, which does not change in its elementary parts, but, 

 somewhat like a cartilage, 1 has all its vital energies directed 

 to its unchanged self-maintenance as a whole (thickness of the 

 whole epidermis, proportion of the rete Malpighii to the horny 

 layer), and in its separate parts. However, since a throwing- 

 off of the external layers, if not necessarily, yet accidentally, 

 takes place almost continually over the whole body to a greater 



1 [See, however, the note upon the desquamation of cartilage, infra. — Eds.] 



