CHAPTER IV 

 CELLS 



Protoplasm, or living substance, occurs only in the form 

 of cells. These " units of life" are in a certain sense like 

 the bricks from which men make houses. Bricks are of 

 various colors and shapes, they may be built into houses 

 or they may exist as separate chunks of burnt clay, yet 

 anyone recognizes a brick when he sees it. Brick houses 

 may have doors, various arrangements of windows, porches, 

 and other embellishments without changing the essential 

 character of the bricks which furnish the bulk of their 

 mass. 



FIG. 22. Cells scraped from the inside of a man's cheek. 



Living substance always occurs in the form of cells. 

 Many of these " units of life" are independent (Protozoa); 

 they are able to reproduce themselves, carry on metabolism, 

 and maintain all the other activities characteristic of living 

 things, as described in Chapter III. Other cells live as 

 small units in the midst of great living machines and are 

 incapable of independent existence. A recent estimate 

 places the number of the nerve cells in the cortex of 

 the human brain at 9,280,000,000, and yet these would 

 together occupy only about one cubic inch of space. The 



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