274 HENRY H. DONALDSON 



must be higher than those for the entire skeleton, and table 16 

 shows that this is true for both the cranium and the vertebrae, 

 which form the greater part of the axial skeleton. On the 

 other hand, while the percentage of water in the proximal bones 

 of both the fore and hind limbs (humerus and femur, respectively) 

 is close to that in the axial skeleton, the percentages in the middle 

 and distal bones are much lower, being least in the distal group 

 (charts 15 and 16). 



A consideration of these various differences renders it prob- 

 able that they are largely due to mechanical causes : the crevices 

 in the cranium, the cavities in the long bones, and possibly the 

 greater porosity of the vertebrae would tend to give these parts a 

 higher percentage of water than was found in the more solid 

 (distal) bones of the limbs — the radius and ulna and the bones 

 of the fore and hind feet, which lack a central cavity. 



These water values, therefore, are anatomically useful, but no 

 general biological significance should be attached to the differ- 

 ences between them. Without question, the percentage of water 

 will vary in a marked way according to the nutritional condition 

 of the animal when this condition departs from the normal, as 

 when, for some reason, calcification is delayed or incomplete. 



On the loss of water 



Between birth and maturity the densest bones, e.g., those of 

 the feet, show a loss of sixty to sixty-five points in their percent- 

 age of water (tables 17 and 18). Just how this loss occurs we 

 do not know. It is possible, however, to get a general notion 

 of the process by a simple computation. If we assume that at 

 birth the bones in question are entirely uncalcified, but at 

 maturity have 60 per cent of their dry weight in the form of 

 salts (assumed in the following argument to retain no water) 

 and, at the same time, that the organic matter present main- 

 tains its initial water content of 83.2 (or 80.1) per cent, then 

 the mixture of 60 per cent salts and 40 per cent organic matter 

 would show at maturity 25 (or 24) per cent of water. We find, 

 however, about 18.5 per cent in the bones of both the fore and 



