NO. 3 COMPAEATIVE HISTOLOGY OF FEMUE FOOTE 11 



lamellsB themselves follow several lines of differentiation. Three distinct forms 

 may be distingnished. They are as follows : 



XTNIFORMLY LAMELL.\TED BONE 



Bones of this character, after their development has reached its limits, are 

 composed entirely of concentric lamellae. The structure is uniform in all parts 

 of the section. The lacume are oval or long- and narrow (pi. A, fig. E; pi. 2, 

 fig. 7). 



TWOFOLD DIVISION 



A further stage of differentiation is found in many femora in which a two- 

 fold division has occurred; that is, the section is composed of wide external 

 and very narrow internal lamellar rings in contact with each other (pi. 2, fig. 9). 



THKEEFOLD DIVISION 



In some femora the lamelhu are separated into three concentric rings. The 

 external ring is narrow, the central wide, and the internal again narrow (pi. 2, 

 fig. 27). 



Lamellae appear as the most prominent structures in amphibians, reptiles, 

 bats, and in early fetal liuman femora. 



Laminae 



The lamina, as used in this memoir, is a larger and more complex bone 

 layer than the lamella. It is composed of a variable number of concentric 

 sheaths of lamellas surrounding the bone or its medullary canal, l)ut the char- 

 acteristic feature is that it is separated from adjacent structures by systems 

 or plexuses of vascular canals extending in a direction parallel with the medul- 

 lary and external surfaces of the bones. It occurs in birds, mammals, late fetal, 

 and early childhood, and is often, though not invariably, a stage in the dif- 

 ferentiation of bone from the first to the third. It reaches its highest degree 

 of development in mammals (pi. A, fig. J; pi. B, figs. 2, 5). 



Haversian Systems 



The Haversian systems are more or less cylindrical shaped complexes of 

 varying diameters composed of concentric lamellae enclosing a central or Ha- 

 versian canal. They extend in a direction parallel with the long axis of the bone, 

 and in sections cut at right angles are circular in outline. In many cases, how- 

 ever, and perhaps in the majority, their directions are considerably modified and 

 instead of extending in directions parallel with the long axis they run tortuous 

 courses (pi. A, fig. F; pi. B, figs, o, fi). 



