CHAPTER 11. 



THE ACTIONS OF FORCES ON ORGANIC MATTER. 



§ 10. To some extent, the parts of every body are changed 

 in their arrangement by any incident mechanical force. 

 But in organic bodies, and especially in animal bodies, the 

 changes of arrangement produced by mechanical forced are 

 usually conspicuous. It is a distinctive mark of colloids 

 that they readily yield to pressures and tensions, and that 

 they recover, more or less completely, their original shapes, 

 when the pressures or tensions cease. Evidently without 

 this pliability and elasticity, most organic actions would be 

 impossible. ISTot only temporary but also permanent 



alterations of form are facilitated by this colloid character 

 of organic matter. Continued pressure on living tissue, by 

 modifying the processes going on in it (perhaps retarding 

 the absorption of new material to replace the old that has 

 decomposed and diffused aAvay), gradually diminishes and 

 finally destroys its power of resuming the outline it had at 

 first. Thus, generally speaking, the substances composing 

 organisms are modifiable by arrested momentum or by con- 

 tinuous strain, in far greater degrees than are inorganic sub- 

 stances. 



§ 11. Sensitiveness to certain forces which are quasi- 

 mechanical, if not mechanical in the usual sense, is seen in 

 two closely-related peculiarities displayed by organic matter 

 as well as other matter which assumes the same state of 

 molecular aggregation. 



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