6 Introduction. 



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differentiation in relation to the animal kingdom, which is 

 grounded upon the assumption of all animal tissues having 

 in their systematic distribution three membranes, a serous,, 

 a mucous, and a contractile membrane, in contra-distinction 

 to vegetables, which have only two membranes an outer 

 and an inner, or a serous and mucous membrane, if such 

 nomenclature is permissible. 



The term membrane is used much in the sense of rock in 

 geology. It does not necessarily mean a continuous struc- 

 ture, but it refers rather to function running along with 

 certain kinds of structure, and as such shows itself in a 

 variety of forms and differentiations. Thus muscles are 

 called contractile membrane, and so is dartos ; the latter 

 being a lower form of contractile membrane. Still further,, 

 the elastic tissue of arteries, where elasticity is in associa- 

 tion with some low degree of contractility, places this struc- 

 ture in the category of contractile membranes. 



In whatever tissue active vital functions, either of a 

 chemico-vital, or cell-destructive power, are going on, there 

 mucous membrane is recognized, purely and solely from its 

 active vital functions, altogether irrespective of the form 

 of differentiation it may assume ; i.e. if in its totality it 

 includes active vital processes that are not contractile 

 processes, there the functions of mucous membrane exhibit 

 a certain special active property ; which declares what is its 

 proper place in the grouping of the membranes in any special 

 tripartite membrane. 



