2 IS GENERAL ANATOMY. 



fixed alkalies resolve it completely into a saponaceous sub- 

 stance. Nitric acid turns it yellow almost at once, thickens, 

 softens, and renders it opaque in about fifteen minutes, and in 

 twenty-four hours reduces it to a yellow pulp. If ammonia is 

 applied on the epidermis, rendered yellow by nitric acid, it 

 changes to a deep orange colour. Now Hatchett has proved 

 that similar effects took place on coagulated albumen. The 

 epidermis appears to consist of a layer of albuminous mucus, 

 coagulated and dried. 



309. The epidermis is neither irritable nor sensible; of all 

 parts of the body it possesses the most active force of forma- 

 tion; it results from the concretion of a fluid exuded on the 

 surface of the skin, constantly renewed, never absorbed, but 

 destroyed externally as fast as it is produced on the internal 

 face. 



310, Numerous hypotheses have been broached upon the 

 formation of the epidermis; the most ancient, is that which 

 teaches us to regard it as the drying of a fluid furnished by the 

 surface of the dermis. Others, with Leuwenhoeck, saw no- 

 thing in it but an expansion of the vessels of the skin. Others 

 again, as Ruysch, made it to consist of the expansion and dry- 

 ing of the papillae. Heister attributed its formation to these 

 two causes; Morgagni to the callification or induration of the 

 surface of the skin, from the pressure of the waters of the am- 

 nios at first, and subsequently, from that of the atmosphere; 

 and Garangeot to the induration of the rete mucosum. All 

 those opinions, particularly the first and the last, contain some 

 truth. It results, in fact, from an exudation or excretion of 

 the dermis. It is the indurated surface of the corpus muco- 

 sum; so that from the dermis to the free surface of the epider- 

 mis, there is a successive deterioration of organization, and of 

 vitality, which makes a kind of varnish of the epidermis, and 

 participating in organization and life, only by its origin, a cir- 

 cumstance which renders it very fit to support the action of 

 external bodies, and to protect the vessels, nerves, and other 

 parts of the skin. 



311. The skin, formed by the dermis, the vessels and the 

 nerves which are distributed through its thickness, and par- 



