﻿212 Proceedings of the Royal Physical Society. > 



cartilage cells being more easily absorbed than the inter- 

 vening columns of calcified matrix. 



While this process of removal of the unsuccessful cartilage 

 is in progress, the rapidly multiplying cells of the successful 

 race begin to feel the pinch of shortening food supply, and ' 

 specialisation is forced on. The specialisation adopted is 

 designed to enable the cells to avoid increasing the amount 

 of their protoplasm, and to enable them healthily to exist 

 upon a supply of nutriment formerly inadequate. Histo- 

 logically they now appear as small cells which secrete a 

 mineral envelope. Thus is formed the primitive central 

 spongy bone. In the course of time, however, the cells 

 begin to suffer, even their specialisation is insufficient ta 

 ensure due economy, and they and their bony envelopes 

 are removed by the osteoclasts. 



It is unnecessary to follow out all the changes leading 

 to a higher and higher specialisation towards economy,, 

 which are undergone before the adult bone is formed, but 

 the type of the adult osteoblast is important. Its specialisa- 

 tion is all towards reduction of mass and increase of absorb- 

 ing surface. The fact that it gets rid of a large part of 

 its absorbed nutriment rather than adds to its bulk, is 

 evidence that it can deal with nutriment in excess of its 

 momentary needs ; its flattened shape, with its numerous 

 branching processes, is typical of a cell prepared against 

 a threatened food shortage, for the flat surfaces and branch- 

 ing processes yield far more surface per unit of mass than 

 could be given by any shape approaching the spherical. 



The conclusions, consequent upon this interpretation of the 

 phenomena of ossification are — (first) the cartilage cells of the 

 primitive shafts of long bones are little able to endure high 

 nutrition, and are, in the higher animals, soon destroyed by 

 their own excessive growth ; (second) all osteoblasts are 

 specialised to ensure economy of nutrition, but they are not 

 adversely affected by high nutrition, as they have acquired 

 the power of excreting a great part of their absorbed nutri- 

 ment. From these conclusions it follows that in conditions 

 of high nutrition, so long as the epiphyseal plates persist the 



