12 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



Haversian systems, foreshadowed iii a few ami^hibians, somewhat advanced 

 in some reptiles and birds, much more completely differentiated in the lower 

 mammals, become fully developed in man. 



Types of Bone Structure 



The most interesting and unexpected fact which has been brought out by 

 the investigations herein reported is the existence, in the femur as well as in 

 other bones of the body, of three distinct, easily separable tj-pes of minute bone 

 structure, corresponding, respectively, to the lamellar, laminar, and Haversian 

 system stages of differentiation. These may occur alone, but are very fre- 

 quently found in various though in general readily analyzable combinations. 



The types may be called the primary, intermediate, and advanced; or 

 simply the lamellar, laminar, and Haversian system types. More conveniently 

 tliau either, perhaps, they may be designated as the first, second, and third 

 types respectively. They are shown in plate A, figures 1, 2, 3; plate B, figures 

 1, 2, 3 ; microscopically and grossly in plate B, figures 4, 5, 6. 



These types of bone structure are, in the main, nothing but various stages 

 of osseous development. They do not represent radically distinct varieties of 

 bone, but rather consecutive stages of differentiation of one and the same funda- 

 mental variety which underlies bone structure in all the terrestrial vertebrates. 

 The bone structure advances from simj^le to more complex in conformity with 

 definite laws which affect all the organisms possessing a skeleton, and the pro- 

 cess is never reversed. The main types of bone structure that were determined 

 may be defined briefly, as follows : 



THE first type 



This is composed of basic, or but moderately differentiated, bone substance, 

 enclosing more or less numerous lacunae, from which radiate scant to numerous 

 minute canaliculi. The lacunae are generally round or oval in shape, and their 

 canaliculi are mostly short and bushy. The lacunie may or may not occupy a 

 definitely concentric position (pi. A, fig. D). In the more advanced stage the 

 lacunar assume a concentric arrangement, change in shajie to a longer and 

 narrower form, their canaliculi become longer and straighter, and the basic 

 bone substance becomes separable into parallel layers, which are the lamellae. 

 Here development in many species stops (pi. A, figs. E, F). The first t\^je of 

 bone, therefore, is one varying from wholly basic, unorganized bone substance 

 with no perceivable systematic arrangement of the cells, to that showing dis- 

 tinct lamellation with varying lacunae and canaliculi. The most advanced and 

 characteristic bone unit is the lamella (pi. A, fig. F; pi. B, figs. 1, 4). 



The arrangement of the lamellae is mostly concentric about a larger or 

 smaller canal (pi. A, fig. E), but in some localities in a bone — as in the areas 



