24 THE HUMAN BODY. 



place by imbibition or endosmosis, it is plain that the skin 

 is permeable, at least under certain conditions. But it is 

 not equally so throughout its whole extent; the thicker the 

 epidermis the slower and more difficult is the absorption ; in 

 short, the skin, like all the other tissues, absorbs certain sub- 

 stances to the exclusion of others. 



We shall have occasion to discuss these phenomena in 

 treating of absorption. 



After enveloping the body, the skin folds back upon the 

 openings which give access to its cavities, and modifying its 

 nature, becomes, under the name of the mucous membrane, 

 an internal skin, analogous, as we shall see further on, to 

 the external skin in its structure, its functions, and in the 

 intimate connection which is established by their reciprocal 

 influence and unity. 



