2 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



and of the arrangements of types as they would appear in reconstructed femora. 

 The exact number of histologic bone units of any section does not, and obviously 

 could not, appear in the drawings. The writer's endeavor has been to represent, 

 in a comprehensive way, the development, proportions, and arrangements of 

 these units, rather than their exact number. 



It was also found impracticable to make the drawings of the various bone 

 sections to a definite, uniform scale, the femora ranging from 0.5 mm. to 13 cm. 

 in diameter. Microphotographs were essayed, but were found unsatisfactory. 



The various diameters of the examined femora, their medullary canals, and 

 medullary indices are given in the synoptic tables and also in the text at the 

 beginning of each detailed description. The sections and measurements were 

 made invariably at the middle of the shaft of the bone. 



The femur was selected for these studies rather than any other bone of tlie 

 body, because it is of good size, because it is in fairly constant use, and on 

 account of its being the sole bone of an important segment of the body. It is, 

 in other words, a good representative bone and perhaps the best adapted to the 

 investigations of this nature. 



The illustrations have, generally, been grouped according to the structural 

 relations of the bones. 



The type or combination of types of structure which any bone was observed 

 to present will be found noted in the tables, so that by a glance at these there 

 may be readily obtained a comprehensive view of the grouping of tlie femora of 

 the various animals. 



The femora examined and included in this report are those of amphibians, 

 reptiles, birds, mammals, and man. Fetal, young, adolescent, adult, and senile 

 femora of the same species were examined whenever possible, and, when circum- 

 stances allowed, observations were also made on other bones of the body. All 

 sections were carefully ground to proper thinness and mounted in liard balsam. 



IL SUMMARY OF RESULTS 



As the mass of detailed observations which follow will be consulted in 

 special instances only, it may be convenient to the reader to liave a general 

 summary of the principal results of the study presented at this place instead of 

 at tlie end of the work. 



The microscopic structure of a long bone described by the early liistologists 

 happened to be that of a third or Haversian system type, and it has been 

 assumed, perhaps without a more defiinte reason, that all long bones have the 

 saine structure. From an examination of a large number of femoia it is evi- 

 dent that they liave not. 



If there is any one distinctive characteristic of bone structure shown by the 

 present investigation, it is that of extensive variation, variation ilue to heredity. 



