NO. 8 HISTOLOGY OF THE FEMUR FOOTE 3 



As to the significations of these types, the subject has been studied 

 from the following standpoints : 



(i) The grade of the animal in biological classification. 



(2) Geographical location. 



(3) Sex - 



(4) Age. 



(5) Function. 



(6) Individuality. 



(7) Health; and 



(8) Heredity. 



1. The first type of bone appears as the basic structure in the 

 amphibians, reptiles, birds, and mammals. It exists, in a pure or but 

 little complicated form, throughout life, in the amphibians, in the liz- 

 ards, in some of the birds, and in the bats, excepting the Pteropus. It 

 exists or predominates in the fetal life of higher animals, including 

 man. It may well be regarded as the simplest and oldest or funda- 

 mental form of bone structure. Its first variation is shown by a change 

 of the round or oval to long and narrow lacunae, by a more con- 

 centric arrangement of the lacunae, and by increase in vascularity, 

 which is accompanied by a change from the first to the second or third 

 type of bone structure. 



The second or intermediary type of bone structure develops as a 

 rule from the first type and represents often, though not invariably, 

 a stage in the differentiation of the bone from the first to the third 

 type. Traces of it are seen first in the amphibians and reptiles, while 

 more pronounced instances of it occur in a few birds, some of the 

 mammals, and at some stages of development, especially in some 

 races, in man. It is best represented in certain mammals, such as the 

 various deer. 



The third. type, foreshadowed in a few amphibians, appears in part 

 in some of the reptiles and a few birds ; it is more strongly repre- 

 sented in certain mammals, and is characteristic of man. 



2. The effect of geographical position upon bone variation is not 

 yet reducible to exact deductions. However, it is a fact that the 

 femora of the African and Asiatic elephants differ from each other 

 very materially. 



3. As to sex, the femora examined showed no evidence that this is 

 an important factor in the minute structural variation of the bone. 



4. Age influences the type of the bone very greatly ; at least so in 

 the higher mammals and particularly in man. All femora of higher 

 mammals, and especially man, change in structure with advancing 



