414 



INNERVATION. 



[CHAP. xiv. 



Fig. 84. 



through a series of changes, till they are cast off from the surface.* 

 These changes are not confined to their figure : the laminae they 

 first form are moist, and comparatively soft, and rest like a cushion 

 011 the highly sensitive surface to which they are adapted, and 

 whose vessels supply the materials for their development. The 

 more external ones are hard, horny, and much drier. Schwann 

 has also pointed out that their chemical properties become modi- 

 fied; that at first they are 

 soluble, but afterwards inso- 

 luble, in acetic acid : and this 

 circumstance of a chemical 

 -<L change occurring in the stages 

 of their development seems 

 to us so important that we 

 shall illustrate it by two views, 

 fig. 84, A. and B. In the 

 former the action of weak so- 

 lution of potass is shown: the 

 layer of cells immediately rest- 

 ing on the basement membrane 

 together with the more super- 

 ficial scales is but slightly 

 or not at all dissolved; while 

 several intermediate layers are 

 swollen and rendered very 



A. Section of the skin of the heel, treated with weak , , , i 4.1, 



solution of potass: a. Basement membrane of pa- transparent, IiaVing lOSt tneir 



pilla. 6. Layer of nucleated cells resting on the base- , . 7 , f ,-, 



ment membrane, c. Several succeeding layers, par- nuclei. I he abruptness OI the 



change is remarkable, andcon- 

 solution tinues after the whole specimen 

 is saturated. In the latter 

 figure a stronger solution has 



not represented i been employed . the deep layer 



is dissolved, but the superficial scales are still unaffected, while 

 the intermediate part is reduced to a semi-fluid mass, in which 

 scarcely any vestige of structure remains. It is very possible that 

 other agents might disclose further varieties of chemical constitu- 

 tion in smaller subdivisions 6f the cuticular lamellae. 



* The cuticle of reptiles and amphibia is periodically cast off in a more or 

 less entire state, a new one being previously formed beneath it. In amphibia 

 the epidermis is tesselated ; the scales adhering to one another by their edges, 

 and being usually pentagonal. A similar ecdysis, or shedding, occurs in the 

 larva state of insects, and in the arachnidans. 



