NO. 3 COMPARATIVE HISTOLOGY OF FEMUR FOOTE 27 



and altlioiigh the femora are useless, their boue units are apparentlj- sound; 

 while in human femora amputated several years before examination tlie 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 

 frequenc}' 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 \ery 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 nmi. to 2 mm. in thickness), 

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

 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 l)ones of the descendants of known ancestors, and of selected 

 crossings, is especially desirable. 



Conclusions 



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

 tlie present investigation, the following salient points stand out witli sufficient 



clearness: 



1. The predominant sliajx' of cross-sec^tioiis of the f(Mnova of I he 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. 



