296 HENRY H. DONALDSON 



On the relative lengths of the limb bones 



As is shown in chart 22 and table 27, the combined lengths of 

 the femur and tibia first increase more rapidly than those of the 

 humerus and radius up to a body weight of 40 grams, after which 

 the rate of increase is similar, and the length ratios remain nearly 

 constant. The same can be said when the ulna is substituted for 

 radius in the comparison, only in this latter instance the constant 

 relations do not appear until a body weight of 65 grams. 



"When the increase in the length of the femur is compared with 

 that of the tibia, or that of the humerus with the ulna or radius, 

 it appears that in all instances the more distal bones grow the 

 more rapidly. After a body weight of 15 to 35 grams, however, 

 the growth of the distal bone becomes the less rapid and the 

 ratio of lengths falls, nearly regularly, to the end of the record 

 (chart 22, table 27). 



Finally, when the lengths of the leg bones in relation to the 

 length of the body are determined (table 29) , it is seen that after 

 a body length of 125 mm. (equivalent to a body weight of 50 

 grams) these ratios show a high degree of constancy. . 



Taking all the data together, it can be stated that in many 

 cases the mature relations of weight and length among the parts 

 of the skeleton are attained at puberty or earlier. 



DISCUSSION 



In making comparisons with the results for the rat as just 

 recorded we shall consider them very briefly in relation, first, 

 to the available records for mammals below man, and, second, in 

 relation to the records for man himself. So far as we know, 

 there is no study on any lower mammal with which our results 

 can be directly compared. The investigation nearest to our own 

 is that by Jackson and Lowrey ('12) on the ligamentous skeleton 

 of the albino rat. This study was based on seven age groups 

 from to 365 days. As shown in their table 4 and their chart, 

 (page 463, loc. cit.) the maximum relative weight of the skeleton 

 appears a few days after birth — in their ten-day group. Our data 

 show the same relation at six and nineteen days (table 1). There 



