8 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



In size the canal presents several peculiarities. Some femora have rela- 

 tively very small, and others very large canals. In some cases it is reduced to 

 extremely small dimensions, as in the amblystoma, turtles, yellow-hammer, and 

 some embryonic mammals. It reaches its greatest relative size in birds and its 

 smallest in amphibians and reptiles. 



Medullary Contents 



The contents vary. Some medullary canals are full of marrow, as those 

 of amphibians, of reptiles, and of bats ; some contain cancellous bone and mar- 

 row, as those of man; some have trabeculae, as those of the peahen, eagle, and 

 turkey-buzzard. In birds an important variation is found. About one-half of 

 their femora have canals full of marrow, while the remaining half are either 

 empty or contain trabeculfe only. The full or empty condition of the canal 

 seems to bear no relation to the flight of the bird. The peahen is a poor flier 

 and has an empty canal, and the eagle is a good flier and has an empty canal. 

 The wild goose is a good flier and has a full canal, and the domestic turkey ' is 

 a poor flier and has a full canal. The peahen and domestic turkey may be 

 thought to exhibit the flying habits of domestication; but these birds show two 

 opposite conditions of medullary contents — the peahen has an empty canal and 

 the turkey a full canal. Similar results are also observed in the wild and 

 domestic turkeys. Both have full canals although they differ greatly in their 

 abilities to fly. Again, in some birds, as the yellow-hammer, pigeon, and white 

 pelican, the medullary canals are occupied by a heavy cancellous bone with 

 small meshes and present the appearances of nearly solid bones, and yet these 

 birds are good fliers. Generally speaking, the medullary canals of amphibians, 

 reptiles, mammals, and man are full, while those of the birds examined are 

 about equally divided. In most cases the canals were filled with yellow marrow. 

 A few, however, were full of red marrow. 



Medullary Surface 



The medullary canals present a variety of surface. In some instances, as 

 in the peacock and eagle, there is seen extending inward from the wall of the 

 femur an intricate network of trabeculfe, which increases in complexity toward 

 the epiphyses. The surfaces in the larger mammals and in man are generally 

 irregular, from the presence of ridge-like projections. 



In many instances the medullary surface is smooth, as in the whole class of 

 amphibians, in lizards, and in bats ; while in other cases it is rough, uneven, or 

 irregularly corrugated. This is especially true of the larger mammalian femora. 



' The humerus has an empty canal crossed by trabeculse. 



