28 SMITHSONIAX CONTEIBUTIONS TO KNOWLEDGE VOL. 35 



4. The medullary surfaces may be smootli, roughened by depressions, or 

 corrugated. 



5. Medullary contents are composed of marrow and its blood vessels, 

 of marrow and cancellous bone, of trabecular alone, or the contents may be 

 entirely absent. About half of tlie bird femora have full medullary canals,- 

 while the remaining half have no contents. 



6. The medullary index (relative thickness of bone compared with the 

 medullary canal) is lowest in rejjtiles and highest in birds. It falls from am- 

 phibian to reptile, rises from reptile to bird, falls rapidly from bird to mammal, 

 and is about the same in man as in the lower mammals. The reptiles have the 

 highest percentage of bone, the amphibians next, the mammals next, the modern 

 white race of man next, and the bird the lowest percentage. Of the human 

 race the modern white has the most bone, the Egyptian next, the negro next, 

 and the American Indian the least. 



7. Tlie femoi'a of the different animals and in man, even those of different 

 individuals, vary in density, and the single femur varies in the density of the 

 different parts of its wall. 



8. Lacunas and canaliculi })resent various stages of differentiation, the 

 character of the differentiation being harmonious with, and indicative of, tlie 

 degree of l)one development. 



9. Lamellte, laminae, and Haversian systems appear in l)one in the order 

 given, and become the basis of the types and type combinations of bone struc- 

 ture which enter into tlie formation of the different femora. 



10. Basic bone substance is differentiated into lamellse when the diffuse 

 arrangement of lacunje becomes concentric. 



11. Cancellous bone is present in all classes of animals, and is more fre- 

 quently observed in large than in small bones. 



12. Three types of structure form the basis of all femora. They may 

 occur singly or in combination. The first and second predominate in am- 

 phibians, reptiles, and birds, the third in mammals and man. 



13. The first type, composed of lamellae, appears as a uniform structure, 

 or in a twofold or threefold division, and characterizes the amphibians, lizards, 

 and bats. 



14. The' second type (lamina) appears first in the amphibian, and in an 

 early or late form of differentiation in birds and lower mammals. 



15. The third type (Haversian system) is first outlined in tlie amphibians. 

 It is tbe result, primarily, of a series of differentiations beginning witli the 

 ainplii!)ians and ending in man. 



36. Combinations of types are of frequent occurrence. 



17. In fetal and young femora tlic differentiation of first into second 

 and second into tliird types of bone structni'e was observal)le. 



