KINETOGENESIS. 289 



Now, in all bones the external walls are composed of 

 dense material, while the centers are spongy and com- 

 paratively soft. The first bone of the foot (astragalus) 

 is narrower, from side to side, than the tibia which 

 rests upon it. Hence the edges of the dense side-walls 

 of the astragalus fall within the edges of the dense 

 side-walls of the tibia, and they have pressed into the 

 more yielding material that forms the end of the bone, 

 and causing bone absorption, pushed it upward, thus 

 allowing the side-walls of the tibia to embrace the 

 side-walls of the astragalus. Now, this is exactly what 

 would happen if two pieces of plastic dead material, 

 similarly placed, should be subjected to a continual 

 pounding in the direction of their length. And in 

 view of the facts already cited we cannot ascribe any 

 other immediate origin to it in the living material. 



The same active cause that produced the two 

 grooves of the lower end of the leg produced the groove 

 of the middle of the upper end of the astragalus. Here 

 we have the yielding lower end of the tibia resting on 

 the equally spongy material of the middle of the as- 

 tragalus. There is here no question of the hard ma- 

 terial cutting into soft, but simply the result of con- 

 tinuous concussion. The consequence of concussion 

 would be to cause the yielding faces of the bones to 

 bend downward in the direction of gravity, or to re- 

 main in their primitive position while the edges of the 

 astragalus were pushed into the tibia. If they were 

 flat at first they would begin to hollow downward, and. 

 a tongue above and groove below would be the result. 

 And that is exactly what has happened. This inclu- 

 sion of the astragalus in the tibia does not occur in the 

 reptiles, but appears first in the Mammalia, which de- 

 scended from them. See Figs. 68-69. ^ na ve shown 



