NO. b HISTOLOGY OF THE FEMUR — FOOTE 5 



systems are composed of Haversian canals communicating' with adja- 

 cent lacunae by a few canaliculi. No concentric arrangement of the 

 lacunas and no Haversian system lamellae are in evidence. 



REPTILIANS 



The reptilian femora show much the same conditions as those of 

 the amphibians, but the differentiation of bone structure has in some 

 forms advanced to a greater extent. Some species present the simple 

 first type of bone. This is especially the case in the lizards. But in 

 the turtles, curiously, a fairly well developed third type of bone 

 structure has made its appearance. Again cancellous bone, which is 

 not a structural feature of the lizards, is generally present in .the 

 turtles. These are remarkable differences and separate widely the 

 two genera. 



BIRDS 



Birds present in general the appearance of an incompleted develop- 

 ment of the structural state. The first, second, as well as the third 

 type of bone structure are found, and also various combinations, but all 

 give the impression of incompleteness. The bone units are rather 

 dim and unsatisfactory. The first type is present in some birds and is 

 generally of a very simple form. The second type appears in a larger 

 number of species and is, perhaps, the most representative type in 

 birds. In some of these femora a few Haversian systems appear, 

 especially in the posterior ridge, and in some birds of large size the 

 second type structure is reinforced by some Haversian systems in 

 the anterior wall. Finally, in a few species the central ring of lamellae 

 has become displaced by Haversian systems and the bone must be 

 classified as that of the third type. As a rule the systems are rather 

 dim and do not stand out clear cut. Their lacunae are oval and their 

 canaliculi bushy. In a few birds the medullary canal is occupied by 

 cancellous bone. In about half the bird femora the medullary canals 

 are full of marrow, while in the remainder they are empty. Com- 

 paring the birds with the reptiles, there is a distinct increase in the 

 proportion of differentiation, although this has not reached full 

 development. 



MAMMALS 



In these animals the bone structure is in general much more differ- 

 entiated than in birds. As a rule the types and their combinations 

 have lost the illy defined characters so frequently present in the fore- 

 going classes. Furthermore, type combinations are more common. 



