GROWTH OF THE SKELETON 295 



birth and maturity, the dense bones, without cavities, showing 

 the smallest percentage at maturity. It seems probable that the 

 percentage of water in osseous tissue is fairly constant at ma- 

 turity, but in the several bones taken as units it varies according 

 to the presence of cavities, porosity, and other mechanical con- 

 ditions. A certain amount of variation is to be expected also 

 according to the degree of calcification and the possible age 

 changes in the water content of the collagen. Between the 

 room-dried and oven-dried condition there is in general and at 

 all ages a loss of 8.3 per cent of water (charts 13 to 16, tables 

 15 to 19). 



Length of limb bones 



Although it is somewhat incongruous to determine lengths in 

 relation to body weights, yet tests indicate that the fresh lengths 

 of the leg bones are closely related to the body weight of the 

 rat, and in cases where the rat is heavy for its body length, they 

 follow the body weight closely (chart 17, table 20). 



After puberty the change in bone length on drying is less 

 than 1 per cent (table 21). 



The limb-bone lengths on body length (charts 18 and 19, 

 table 22), when cast in the form of percentages (table 23), yield 

 a series of proportional values which after puberty (body length, 

 160 mm.) are fairly constant, notably for the humerus (table 24). 

 By the use of the data in table 24 it is therefore possible to 

 recover the body length of the rat from the lengths of its limb 

 bones. 



Form of the limb bones 



When, on the basis of the weight of a running millimeter 

 (charts 20 and 21), the shape of the bones at maturity is com- 

 pared with that at birth it is found that at maturity the limb 

 bones are relatively more slender than at birth (tables 25 and 26) . 

 They are close to the mature proportions of length to diameter 

 at puberty. The comparison of these determinations with com- 

 pass measurements and a comparison by means of photographs 

 support this statement (fig. 1). 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 26, NO. 2 



