10 



SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



The Units of Bone Structure 



The units of bone structure are divisible into the basic and the differenti- 

 ated. The basic units are the bone substance proper and in a measure also the 

 individual bone cell with its dendrites or the lacuna with its canaliculi. The 

 differentiated units are the lamella, the lamina, and the Haversian system. 



But little needs to be said in this place about the basic units. The bone 

 substance behaves passively and is wholly subject to the activity of the cells. 

 While the cells with their processes are the all-important, living, constructive, 

 and destructive parts of the bone, they change and act in ways that are, as yet, 

 largely obscure. Certain modifications in their characteristics have, however, 

 been observed in the course of the present work and will be mentioned in the 

 proper place. 



Bone Cells — Lacunje 



Bone cells or osteocytes have formed, and in fresh bone occupy the small 

 spaces called lacuna?. The latter vary in shape from round and oval to long 

 and narrow. Many femora have both. The round lacunae characterize espe- 

 cially the young bone, while the long prevail in bone which is fully formed. The 

 denser the bone, the more probability there is that it will present the long, nar- 

 row lacuna?, while in the rarer, but not senile bone substance, the round and oval 

 forms are more abundant (pi. A, figs. A, B, C). 



Dendrites and Canaliculi 



The dendrites are processes which extend outwardly from the bone cells, 

 the canaliculi being minute canals in the bone substance for the accommodation 

 of the dendrites. In a cross-section of the femur, the exposed canaliculi are 

 seen to vary considerably. In some cases they are short, branching, and bushy 

 in appearance, while in others they are long and but slightly branched. The 

 former belong to the round or oval, the latter to the long and narrow lacuna? 

 (pi. A, figs. A, B, C). 



Differentiated Bone Units — Lamellae 



Although bone first appears as a basic or undifferentiated substance in 

 very young embryos and possibly in the more primitive forms of the lower 

 classes of animals, in the course of time it manifests a tendency towards dif- 

 ferentiation. Perhaps the earliest sign of such tendency is indicated by the 

 concentricity of the lacuna? and next by the formation of what are known to the 

 histologist as lamella? (pi. A, fig. F). 



A lamella can, at present, only be defined as a simple, separate layer of 

 bone. The exact mode of its formation is not as yet known, but it must, of course, 

 be the product of the progressive activity of the bone-forming elements. The 



