DEVELOPMENT OF BONE 



45 



filling the spaces between them, and consequently attaining their greatest 

 development where the Haversian systems are few. 



The ultimate structure of the lamellae appears to be fibrous. A thin 

 film peeled off the surface of a bone, from which the earthy matter has been 

 removed by acid, is composed of a finely reticular structure, formed apparently 

 of very slender fibers decussating obliquely, but coalescing at the points of 

 intersection, as if here the fibers were fused rather than woven together. 



FIG. 56. Lamellae Torn Off from a Decalcified Human Parietal Bone at some Depth from the 

 Surface, a, a, Lamellae, showing reticular fibers; b, b, darker part, where several lamellae are 

 superposed; c, perforating fibers. Apertures through which perforating fibers had passed, are 

 seen especially in the lower part, a, a, ot the figure. (Allen Thomson.) 



The reticular lamellae are perforated by the perforating fibers of Sharpey, 

 which bolt the neighboring lamellae together, and may be drawn out when 

 the latter are torn asunder, figure 56. These perforating fibers originate from 

 ingrowing processes of the periosteum, and in the adult still retain their 

 connection with it. 



Development of Bone. From the point of view of their develop- 

 ment, all bones may be subdivided into two classes: 



Those which are ossified directly in membrane or fibrous tissue, e.g., the 

 bones forming the vault of the skull, parietal, frontal, and a certain portion 

 of the occipital bones; 



Those whose form, previous to ossification, is laid in down hyaline carti- 

 lage, e.g., humerus, femur, etc. 



The process of development, pure and simple, may be best studied in 

 bones which are not preceded by cartilage, i.e., membrane-formed. Without 

 a knowledge of ossification in membrane it is difficult to understand the much 

 more complex series of changes through which such a structure as the carti- 



