6 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



in percentages in the synoptic tables and text. Bj^ an examination of these 

 indices it will be seen that the medullary canals of the various femora do not 

 bear a constant relation to the bones, but present very important variations. 

 In some bones the canals are relatively large and in others they are relatively 

 small. The larger the canal in a given species the thinner the wall of bone, 

 and vice versa. There are also some individual variations within each species, 

 and quite noticeable differences in tliis respect between the young, adult, and 

 senile bones. 



The averages given below show that the medullary indices of the different 

 classes of animals and even in man vary considerably : 



Per cent 



Amphibians 36.6 



Reptiles (including turtles) 26.1 



Reptiles (excluding turtles) 33.0 



Birds 159.0 



Bats 48.6 



Other mammals 63.3 



Black race 41.9 



fj,„- Yellow-brown race 43.8 



' "I Ancient Egyptian 39.5 



Modern white race 35.8 



Human race as a whole 38.6 



Looking over the above averages it will be noticed: 



First. — That the lowest index is found in the reptiles where it is 26.1%. 

 The average index of the class of reptiles is lowered by the turtles, in most of 

 which the index is zero. In the turtle femora the medullary canals are occupied 

 by heavy cancellous bone with very small meshes filled with marrow. Observed 

 with the naked eye these bones appear to be solid. If the turtles are excluded 

 the average reptile index is 33% instead of 26.1%. As far as the medullary 

 canals are concerned, both in regard to their contents and indices, those of the 

 turtles do not resemble those of other reptilian genera in any respect. Com- 

 paring the indices of the amphibians with those of the reptiles, it will be seen 

 that the index falls quite sharply or that the medullary canal diminishes and 

 the mass of bone increases markedly from amphibian to reptile. 



The highest average index is found in birds, where it is 159%. Fi-om this 

 it will be seen that the medullary canal increases relatively in size and the bone 

 decreases in mass enormously from reptile to bird. In proportion to their 

 weight, the birds have less bone than amphibians or reptiles. The index is 

 higher in those birds which have empty medullary canals (226.4%) thah in 

 those which have full canals (149.7%). That is, the femora with empty canals 

 have thinner walls in relation io tlie size of the bone than those with full canals. 



Second. — The mean index falls, to a marked degree, from birds to mammals, 

 or from 159% to 63.3%. The index in bats is 48.6%., and in mammals without 

 the bats 63.3%. That is, the canal is smaller and the walls of the bone are 



