74 



HUMAN TISSUES 



Skin cells colonial proto^oorv iwcmovi/' 



In most organisms, cells are not separate and distinct 

 units, but are joined to one another by delicate strands of 

 cytoplasm. 



fore, it may be said that all of the tissue cells work together to 

 accomplish the life processes of the organism. This is brought 



about by differentia- 

 tion of structure for 

 division of labor. 



The cells of multi- 

 cellular animals and 

 plants are not isolated 

 individuals, but are 

 probably held together 

 by some kind of cyto- 

 plasmic continuity. 

 Biologists have found in certain cases, and have reason to believe 

 that in most cases, slender cytoplasmic bridges connect the various 

 cells. This gives direct continuity to the cytoplasm throughout 

 the entire organism, and probably enables the 

 organism to act as a unit even though it is 

 composed of millions of different cells. 



Many cells of the body deposit intercellular 

 materials, which give additional structural 

 strength. The amount of the deposit varies. 

 Bone cells deposit a large amount of such 

 material ; epithelium produces a very small 

 amount. Protoplasm is the foundation sub- 

 stance of cells. Groups of similar cells with 

 their intercellular materials make up tissues. 

 When tissues are grouped to make a larger 

 structure to perform a certain function, that 

 structure is called an organ. Thus, the en- 

 tire plant or animal organism is a combi- 

 nation cf organs built of tissues, which in turn are built of cells. 

 The tissues of the human body may be classified into epithelial, 

 supporting or connective, muscular, blood, and nervous. 



<» ^O U <D 2^ i- 0. 



1 



Cells 



The cell is the unit of 

 structure and function in 

 every organism. 



