THE CELL AND PROTOPLASM. 127 



but that there is such a mechanism can not be 

 doubted, and the structure of the cell is certainly 

 complex enough to give plenty of room for it. 

 The irritability of the cell is easily understood ; 

 for, since it is made of very unstable chemical 

 compounds, any slight disturbance or stimulation 

 on one part will tend to upset its chemical stabil- 

 ity and produce reaction ; and this is what is meant 

 by irritability. 



Or, again, we may look upon the cell as a little 

 chemical laboratory, where chemical changes are 

 constantly occurring. These changes we do not 

 indeed understand, but they are undoubtedly 

 chemical changes. The result is that some com- 

 pounds are pulled to pieces and part of the frag- 

 ments liberated or excreted, while other parts are 

 retained and built into other more complex com- 

 pounds. The compounds thus manufactured are 

 retained in the cell body, and it grows in bulk. 

 This continues until the cell becomes too big, and 

 then it divides. 



If a machine is broken it ceases to carry on 

 its proper duties, and if the parts are badly broken 

 it is ruined. So with the cell. If it is broken by 

 any means, mechanical, thermal, or otherwise, it 

 ceases to run we say it dies. It has within 

 itself great power of repairing injury, and there- 

 fore it does not cease to act until the injury is 

 so great as to be beyond repair. Thus it only 

 stops its motion when the machinery has become 

 so badly injured as to be beyond hope of repair, 

 and hence the cell, after once ceasing its action, 

 can never resume it again. 



There are, of course, other functions of living 

 things besides the few simple ones which we have 

 considered. But these are the fundamental ones; 



