THE DIFFERENTIATION OF TISSUES, 33 



kidneys, skin, and elsewhere, whose main business it is to get 

 rid of the waste products of the various parts of the Body. 



Respiratory tissues. These are concerned in the gaseous 

 interchanges between the Body and the surrounding air. 

 They are constituted by the cells lining the lungs and by the 

 colored corpuscles of the blood. 



As regards the nutritive tissues it requires especially to be 

 borne in mind that although such a classification as is here 

 given is useful, as helping to show the method pursued in the 

 domestic economy of the Body, it is only imperfect and 

 largely artificial. Every cell of the Body is in itself assimi- 

 lative, respiratory, and excretory, and the tissues in this class 

 are only those concerned in the first and .ast interchanges 

 of material between it and the external world. They provide 

 or get rid of substances for the whole Body, leaving the feed- 

 ing and breathing and excretion of its individual tissues to be 

 ultimately looked after by themselves, just as even the mandarin 

 described by Robinson Crusoe who found his dignity promoted 

 by having servants to put the food into his mouth, had finally 

 to swallow and digest it for himself. Moreover, there is no 

 logical distinction between a secretory and an excretory cell: 

 each of them is characterized by the separation of certain sub- 

 stances which are poured out on a free surface on the exterior 

 or interior of tl^ * ^ . Many secretory cells too have no 

 concern with the digestion of food, as for example those 

 which form the tears and sweat. 



4. STORAGE TISSUES. The Body does not live from hand 

 to mouth: it has always in health a supply of food-materials 

 accumulated in it beyond its immediate needs. This lies in 

 part in the individual cells themselves, just as in a prosperous 

 community nearly every one will have some little pocket- 

 money. But apart from this reserve there are certain cells, 

 a sort of capitalists., which store up considerable quantities of 

 material and constitute what we will call the storage tissues. 

 These are especially represented by the liver-cells and fat- 

 cells, which contain in health a reserve fund for the rest of 

 the Body. Since both of these, together with secretorv and 

 excretory cells, are the seats of great chemical changes they 

 are all often called metabolic tissues. 



5. IRRITABLE TISSUES. The maintenance, or at any rate 

 the best prosperity, of a nation is not fully secured wheu a 

 division of labor has taken place in food-supply and food-dis- 



