8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



exclusive presence of the first type of structure has never been found 

 in the adult human femur. The most primitive form is a combina- 

 tion of the first and third types. The proportions of the structural 

 units varies greatly. In some femora, the first type was found to 

 amount to more than half of the section, while in others it is reduced 

 to a small fraction. In other human femora there will be a second and 

 third, instead of first and third type combination ; and still others, the 

 bone shows all the three types. When the first type is present, it is 

 generally found in the form of a horseshoe band extending under the 

 external surface of the bone — the heel of the shoe embracing the 

 posterior ridge. Segments of laminae with a first type of bone, fre- 

 quently found in bone sections, are the remains of a disappearing 

 horseshoe band of first type units. In a complete third type bone, the 

 first and second type units have been entirely displaced by well 

 developed Haversian systems. There may be cancellous bone around 

 the medullary canal. 



Three human races have been examined, namely, the black, the 

 white (including the ancient Egyptian), and the yellow-brown. An 

 early and late differentiation has been found in each. The negro, 

 modern white, Egyptian of the Xllth Dynasty, the Pueblo and 

 Peruvian Indians all show these variations. The variations are the 

 same in kind, but somewhat different in degree. The negro bone, 

 barring individual exceptions, does not perhaps equal that of the 

 other races in the degree of the differentiation. 



The posterior ridge is generally composed of Haversian systems. 

 This occurs in each race. 



Senile changes, absent or very rare in animals, are unexpectedly 

 frequent in human femora, particularly those of civilized races. 



SENILE CHANGES 



After a human bone has reached its developmental limitation and 

 perhaps the climax of its function, it soon begins to undergo certain 

 changes which are properly those of degeneration of senility. Such 

 changes are evidently far more common than is supposed. Age in 

 years is not to be the determining factor. A femur may be more or 

 less senile at 30 or 40 years of age. There is no structural differentia- 

 tion beyond the Haversian system type; when that has been com- 

 pleted it may remain as such for years, but sooner or later a process 

 of aging sets in. The senile changes begin around the canals of the 

 Haversian systems. They involve first the systems around the medul 





