28 



SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOLi. 35 



4. The medullary surfaces may be smooth, 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 the 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 reptiles 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. The femora 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. Lacuna? and canalicnli present various stages of differentiation, the 

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

 degree of bone development. 



1). Lamella 1 , lamina 1 , and Haversian systems appear in bone in the order 

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

 ture which enter into the formation of the different femora. 



10. Basic bone substance is differentiated into lamella 1 when the diffuse 

 arrangement of lacuna 1 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 the amphibians. 

 It is the result, primarily, of a series of differentiations beginning with the 

 amphibians and ending in man. 



16. Combinations of types are of frequent occurrence. 



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

 and second into third types of bone structure was observable. 



