TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 33 



in mind that although such a classification as is here given is use- 

 ful, as helping to show the method pursued in the domestic econ- 

 omy of the Body, it is only imperfect and largely artificial. Every 

 cell of the Body is in itself assimilative, respiratory, and excre- 

 tory, and the tissues in this class are only those concerned in the 

 first and last interchanges of material between it and the external 

 world. They provide or get rid of substances for the whole Body, 

 leaving the feeding 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 substances 

 which arc poured out on a free surface on the exterior or interior 

 of the Body. 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 ac- 

 cumulated in it beyond its immediate needs. This lies in part in 

 the individual cells themselves, but apart from this reserve there 

 are certain cells, 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 con- 

 tain in health a reserve fund for the rest of the Body. 



5. IRRITABLE TISSUES. These include those tissues which are 

 especially susceptible to changes in their surroundings and are 

 therefore useful in giving to the Body information of what is going 

 on around it. Any change in the environment which serves to 

 arouse response in an irritable tissue constitutes a stimulus. 



6. CONDUCTIVE TISSUES. While most, if not all, of the cells of 

 the Body retain the property of conductivity in some degree, the 

 nervous tissues exhibit it in very high degree. They serve there- 

 fore to bring into communication the various parts of the Body. 

 As an incident in the conveying of messages from one part of the 

 Body to another certain nervous structures have the power of 

 modifying the messages which pass through them. 



7. MOTOR TISSUES. These have the contractility of the original 



