306 JAMES CRAWFORD WATT. 



as creeping replacement occurs whereby this material is gradually 

 absorbed and replaced by new bone. It has been shown by Gallie 

 and Robertson that it is the osteoblasts which invade the graft, 

 gradually absorb the old matrix and bone salts immediately sur- 

 rounding them, and then replace this by newly formed substance. 



If the bone salts were laid down by precipitation of tribasic 

 calcium carbonophosphate due to a change of the carbon dioxide 

 content, it seems to me it would be impossible for this precipitate 

 to be removed again as easily as Wells states it is, in response to 

 the body's need for calcium. Would pregnancy or a pancreatic 

 fistula so change the carbon dioxide content of the bone, as to make 

 possible a reversal of the conditions that brought about the pre- 

 cipitation, and so either provide for increased solubility of the two 

 salts or else make possible the recombination of the carbonate and 

 phosphate of calcium into the double salt, with its consequent solu- 

 tion and carrying off in the blood? 



It seems to me that the carbon dioxide content in the bone will 

 not vary enough to permit of such a reversal, as to raise it would 

 indicate a high activity in a tissue whose metabolism is low. This 

 low-grade activity is given as a reason for obtaining the precipitate. 



The taking of the calcium salts out of the bone probably depends 

 on some balance that has to be maintained between the blood, the 

 osteoblast, and the bony matrix. If the blood is rich in calcium, 

 the cell can take it and pass it on into the matrix. If the blood is 

 poor in calcium, the cell to maintain its relation in balance to the 

 blood takes from the matrix to add to the blood. This is an 

 instance comparable to the relations between the blood and its 

 dextrose, and the liver cell and its stored glycogen, to maintain a 

 certain carbohydrate balance. 



This view also follows logically upon that of Wells, who regards 

 the bony matrix as a great reserve store of alkaline bases for the 

 body, where the calcium salts are in a state of flux, being constantly 

 added to or drawn upon according to the needs of the body. 

 Changes in the bones are much greater in extent and more rapid 

 in their occurrence than is generally believed. It seems to me that 

 this idea of the reversibility of the direction in which calcium salts 



