NO. 3 



COMPARATIVE HISTOLOGY OF FEMUR EOOTE 



27 



and although the femora are useless, their bone units are apparently sound ; 

 while in human femora amputated several years before examination the struc- 

 ture is well differentiated, but shows premature senile changes. 



6. INDIVIDUAL VARIATIONS 



Individual variations are by no means as common in the lower femoral 

 vertebrates as they are in the higher forms. They are found to increase in the 

 frequency of occurrence from birds to man. As far as the higher vertebrates 

 are concerned, hardly any two individuals are exactly alike. They conform to 

 a general type of structure which is fundamental, and exhibit special varia- 

 tions which are peculiar to the individual. This was found to be the case even 

 with cats of the same litter. The individuals varied in structure. 



7. HEALTH AND DISEASE 



Variations due to health and disease remain very largely for studies in 

 the future. However, the right femur of an adult white male who had con- 

 genital epilepsy shows an extremely thin wall (1 mm. to 2 mm. in thickness), 

 and an index of 277%. See plate 35, figure 453, for drawing and text fur 

 description. 



8. HEREDITY 



The influence of heredity on bone variation requires a greater amount of 

 selected material of known genetic relationship than the writer has been able to 

 gather. Most of the femora utilized are those of individuals with no obtainable 

 family history. However, the femur of the mule resembles structurally the 

 jackass more than the horse, and the femora of a litter of kittens showed quite 

 marked differences. Excluding other causative factors which do not sufficiently 

 account for the variations observed, heredity offers a most attractive field. The 

 further study of bones of the descendants of known ancestors, and of selected 

 crossings, is especially desirable. 



Conclusions 



If we survey the whole field of bone histology, as it was observed during 

 the present investigation, the following salient points stand out with sufficient 

 clearness : 



1. The predominant shape of cross-sections of the femora of the animals 

 below man is elliptical. 



2. Generally speaking, first and second type femora are circular or elliptical 

 and third types are triangular or related shapes. 



3. Medullary canals are situated centrally, eccentrically, or obliquely, and 

 may be circular, elliptical, or irregular in shape. 



