KINETOGENESIS. 285 



tion may produce metabolic changes in osseous tissue. 

 For this reason it is possible to account for the length- 

 ening of the limb-bones in heavy animals, as an effect 

 of impact, while the astragalus of bats may have been 

 elongated by a stretching strain. 



c. The Physiology of Bone Moulding. 



Dr. Koelliker has summarized the results of the 

 observations made by himself and his predecessors on 

 the processes of the growth and absorption of bone, 

 which determine the forms of the elements of the skel- 

 eton. 1 



Bone is deposited through the agency of uninuclear 

 cells, or osteoblasts, which may under peculiar condi- 

 tions become enlarged and multicellular, when they 

 are termed osteoclasts. These osteoclasts produce an 

 absorption or destruction of the bone or dentine with 

 which they are in contact, the bone or dentine being 

 passive under the operation. How this is done is not 

 known. Pieces of ivory which have been used to re- 

 place bone removed by surgical methods, have been 

 found to be both corroded by osteoclasts, and overlaid 

 by layers of living bone by osteoblasts. 



In explanation of the causes which induce the for- 

 mation and action of the osteoclasts, Koelliker remarks 

 that : " the totality of changes of the jaws during the 

 development of teeth appears to show that it is pres- 

 sure by the soft parts which causes the absorption of 

 bone. One can admit in the case of the jaw that the 

 dental sacs in process of growth produce by their en- 

 largement a state of irritation in the layer of osteo- 

 blasts which originally border the alveolar edge, and 



1 " The Normal and Typical Absorption of Bones and Teeth," Verhandl. 

 der Pliys. Med. Ges. run }l'iirzbnrg, II., III., 1872. 



