K1NE TO GENESIS. 377 



growth of the latter is due to a pulling strain on them, 

 as well as to the greater mobility of the element which 

 becomes convex, as supposed by Fick. 



It is claimed in the preceding pages, that impacts 

 on the extremities of a bone or tooth, gradually in- 

 crease its length. It may be hastily supposed that in 

 this assumption I derive elongation of the shaft of a 

 bone from the same stimulus which produces excava- 

 tion and therefore abbreviation of its extremities. In 

 the gross this charge is correct j but the position I 

 have assumed is defensible, because in detail it is easy 

 to perceive that effect of the use of a limb on an ar- 

 ticular surface of a bone is quite distinct from that 

 which it has on the shaft. At the articular faces we 

 have discontinuity; and therefore frtctwn ; in the shaft 

 we have only the concussions produced by impact, to- 

 gether with some torsion strain. That the former 

 movement stimulates the development and activity of 

 the osteoclasts has been shown by Koelliker ; that the 

 latter may stimulate the activity of osteoblasts is ren- 

 dered highly probable from the facts of pathological 

 anatomy. These show that a very slight modification 

 of stimulus is sufficient to change the building cells 

 into the absorbent cells and back again. For the same 

 reason belief in the elongation of bones under stretch- 

 ing strain may not be inconsistent with belief in an 

 elongation under impacts. 



Gary makes specific objections against the kineto- 

 genesis of the articulations of the mammalian skele- 

 ton. 1 After a study of the carpus of the Eocene peris- 

 sodactyle genus Palaeosyops he concludes that the 

 trapezoid bone is too small to express properly the di- 

 rect result of purely mechanical causes. He says that 



^American Journal of Morphology, 1892, p. 305. 



