GENERAL PROPERTIES OF TISSUES 49 



All these cells possess the common attributes of proto- 

 l)lasiu. The specialization of one of these properties: 

 contraction, excretion, or conduction, makes them different, 

 and gives its special tissue name to the collection of which 

 these cells are a part. 



AVhat happens when we combine bodies of men of dif- 

 ferent specialties? Take a factory, for example. If it is 

 a cotton mill, some are carders, some spinners, some 

 weavers, and the like. Each body performs its own 

 special work, and all together produce the common result, 

 the production of cotton cloth. So in our bodies. The 

 various organs when dissected are found to be composed 

 of different tissues; that is, collections of specialized cells, 

 which work together for some comumon result, such as 

 seeing, digestion, smelling, and feeling. We may carry 

 the comparison further. Just as different factories supply 

 the needs of a community, so in the body we find organs 

 combined into systems, each organ contributing its action 

 to a conamon pm-pose. Thus the stomach, intestines, liver, 

 etc., are organs which by their combined action prepare 

 the food for the body, and hence we group them under 

 the name of the digestive system. Finally, all these sys- 

 tems are placed under the control of a governing board 

 in the form of the nervous system, and so their work is 

 coordinated, and all work for the maintenance of the 

 individual which they make. 



The individual man consists of a collection of sys- 

 tems, organs, tissues, cells, and the total actions of the 

 individual are but the sum of the actions of his indi- 

 vidual cells. This division of the work of the body among 

 different specialized cells is called physiological division 

 of labor. The significance of this term lies in the fact that 



