GROWTH OF THE SKELETON 303 



is responsive to nutritive conditions, and in reality is much 

 more plastic than one might at first be inclined to suppose. We 

 do not allude to the changes induced by recognized disease (Wolff, 

 '92) or starvation (Voit, '05) or the great loss in the weight of 

 the bones in extreme old age in man, as, for example, in the case 

 of the centenarian described by Waldeyer ('10), a loss which is 

 merely an extreme instance of the senile atrophy that is generally 

 recognized, nor to those atrophies which appear in the skull of 

 the horse (Ussow, '01), of old dogs (Schmey, '15), and in the 

 brain case of old mustelidae as described by Thomas ('86), 

 nor the modifications in the skulls of 'park reared' lions as 

 reported by Hollister ('17), but to those differences in the weights 

 or lengths of the bones which depend on diet in the narrower 

 sense and which may be present during the prime of life. 



It has been shown by Burnett ('08, '11) that the weight, 

 strength, and composition of the bones of the pig can be modified 

 in a very striking way by foods of different composition, and 

 Weiske ('95) brought about changes in the chemical composition 

 of the bones in rabbits by modification of the diet. Lactation in 

 cows changes the salt content of the bones (Forbes, '18) and ini 

 rats which are underfed at an early age calcification can be at. 

 least greatly retarded. Moreover, observations by Weiske ('95)) 

 on rabbits, by Waters ('08, '09) on cattle, and by Aron ('11) 

 on dogs have shown that during prolonged starvation of growing 

 (but not very young) animals the long bones of the limbs still 

 continue to increase in length, and the entire skeleton to increase 

 in weight, even when the animal as a whole is losing weight. 



It is worth noting in this connection that the skeleton and the 

 central nervous system, despite the very different ways in which 

 they grow, are alike in their ability to increase in weight under 

 conditions which may bring about a loss in total body weight. 



In addition to these differences there is of course some varia- 

 bility, which must be expected, even among animals living 

 under the same conditions. This sort of variability is indicated 

 on our charts by the relation of the several entries to the 

 smoothed graphs. 



