no. 3 comparatr'e histology of femuk foote 9 



Cancellous Bone and Teabbcul.e 



Cancellous bone is a special form of first type bone characterized by an 

 arrangement of lamelte enclosing more or less irregularly shaped meshes filled 

 with marrow. The character of the framework varies in the different femora. 

 In some, as in the turtle, it forms a thick, heavy framework with very small 

 meshes (pi. 4, fig. 67). In others, as in some birds, mammals, and in man, it is 

 found as a delicate interwoven lacework with large meshes. In fetal human 

 femora and in the bone of repair there is a channeled bone substance in whicli 

 irregularly shaped meshes are present. 



Cancellous bone, filling the medullary canal, has a wide distribution in the 

 femora of animals. It is found in all classes from amphibian to man, but does 

 not occur in the majority of the species. It was present in only one amphibian, 

 absent in the lizards, present in the turtles, present in a few birds — yellow- 

 hammer, pelican, and domestic pigeon — in many mammals, and in nearly all 

 human femora. It was not found in the order of bats. 



Trabecvlce. — Bone trabeculae are composed of a few lamellae with long nar- 

 row lacunfB and branching canaliculi, with or without Haversian systems. 



In those bones which have marrowless medullary canals, trabeculse form an 

 interlacing network, as in the peahen and turkey-buzzard. The trabeculse ex- 

 tend transversely from wall to wall, or more or less up and down toward the 

 epiphyses of the bone. Near the extremities they generally foi-m a labyrinth, as 

 in man. The femora having trabeculae are generally thin-walled. 



The medullary canals which do not contain marrow have networks of tra- 

 beculae, while those with marrow have cancellous bone or not, according to the 

 animal. 



Trabeculae are very infrequent in amphibians, reptiles, mammals, or man, 

 and reach their greatest degree of frequency in birds. 



Variety of Minute Stbucture of the Wall of the Femuk 



We may now approach the minute structure of the wall of the femur. 

 There is no one type of structure which characterizes all the femora of any 

 single species of animal. Some individuals in each species will sliow single, 

 pure-type bones, while others, and tliese are generally in the majority, present 

 combinations of types. A great variety of combinations occurs. All bones con- 

 sist of the same fundamental structural units, but these are combined and 

 arranged in many ways and in different proportions. Three pure types and 

 several combinations of types, in some stage of differentiation, are more clearly 

 distinguishable and will be defined below. 



