NO. lO THE CIRCULATORY SYSTEM IN BONE FOOTE IQ 



THE STRUCTURAL UNITS OF BONE— FIRST, SECOND, AND 



THIRD, OR LAMELLAE, LAMINAE, AND HAVERSIAN 



SYSTEMS— ARE NOT ESSENTIALLY MECHANICAL 



If all bones were Haversian system bones, or, if there was any one 

 bone which always has Haversian systems as the predominating 

 structural units, no matter in what individual it was found, we might 

 be convinced that their constant presence was sufficient evidence of 

 a mechanical function. But when we know that there are three bone 

 units of structure and that no one of them is constantly present in 

 bones of different animals as the only unit, and not even in bones of 

 individuals of the same birth,^ the idea of a purely mechanical func- 

 tion of bone is severely shaken if not abandoned. The three units, 

 the lamella, lamina, and Haversian system, are found in bones of 

 various animals in such great proportional and locational confusion 

 that their mechanical purposes disappear and we are obliged to 

 look further for an explanation of their strange and unexpected 

 occurrences. 



Again, if we examine these units from a mechanical viewpoint 

 and then observe their locations in the bones of different animals 

 we will find that they are not always found where the same mechan- 

 ical conditions would require them and are very often found where 

 they could serve no mechanical function of any importance. Lamellae 

 are layers of bone substance, laminae are strata composed of lamellae, 

 and Haversian systems are hollow cylinders composed of a variable 

 number of lamellae enclosing a central canal. Of the three units, 

 therefore, the Haversian systems offer the most mechanical service 

 by construction and are the best adapted to support weight, with- 

 stand muscular stress, and serve the general requirements of a 

 skeleton. We would, then, naturally expect to find them constantly 

 present in such long bones as the femur, tibia and fibula of quad- 

 rupeds and bipeds and generally absent from the flat bones of the 

 head and face. But in these respects we are disappointed. They 

 are found, as a pure type, in the long bojies of only a few mammals 

 and are absent as predominating units in the long bones of a large 

 number of mammals including the three races of man, black, yellow- 

 brown and white.^ Haversian systems in their later degrees of dif- 

 ferentiation are not found at all in amphibians, reptiles, birds, bats, 

 monotremes, marsupials and many of the edentates, although the 

 same mechanical functions are demanded by their vocational habits. 

 In the femur, tibia, fibula, humerus, radius, ulna, clavicle, metacarpal 

 and metatarsal bones of man, they are present as predominating 



" Idem. 

 ° Idem. 



