CHAPTEE H. 



THE ACTIONS OF FORCES ON ORGANIC MATTES. 



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

 in their arrangement by any incident mechanical force. 

 But in organic bodies, the changes of arrangement produced 

 by mechanical forces are usually conspicuous. It is a dis- 

 tinctive mark of colloids, that they yield with great readiness 

 to pressures and tensions ; and that they yet recover, more 

 or less completely, their original shapes, when the pres- 

 sures or tensions cease. It is clear that without this 

 pliability and elasticity, most organic actions would be im- 

 possible. Not only temporary but permanent alter- 

 ations 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 away,) gradually diminishes and 

 finally destroys its power of resuming the outline it had at 

 first. Thus the matter of which organisms are built up, is 

 modifiable by arrested momentum or by continuous strain, 

 in a far greater degree than is ordinary matter. 



§ 11. Sensitiveness to certain forces that are quasi- 

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

 two closely- related peculiarities displayed by organic matter 



