CHAPTER II 



TISSUES AND ORGANS 



10. The human body, beginning as a single cell, is 

 gradually built up by a process of division and subdivision 

 of that cell, so that the complete, adult man is but a 

 mass of cells with some cementive and connective matter. 

 It is found, however, that differences early appear in the 

 characteristics of different cells, and these differences 

 increase as development proceeds. A group of similar 

 cells having a similar function is called a tissue. 



11. Differentiation of Tissues. In the lowest animals, 

 composed of but a single cell, all the different parts of 

 the body are essentially alike (leaving the nucleus out of 

 consideration) and have the same functions. One part 

 may move as well as another. All parts share in the 

 process of nutrition, and one part responds as well as 

 another to stimulus. But the higher animals are found 

 to be made up of unlike parts, which minister in differ- 

 ent ways to the life of the whole being. As the cells 

 multiply, certain groups of cells become changed in 

 such a manner as to adapt them to the performance 

 of some special function, while other parts are adapted 

 to other functions. A number of cells lying together 

 become modified so as to make up a tissue adapted to 

 a certain purpose. Other cells become modified in a 

 different way to form a tissue adapted to a different 



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