CH. V.] 



DEVELOPMENT OF BOXE. 



matter has been removed by acid, and examined with a high 

 power of the microscope, it will be found composed of very 

 slender fibres decussating obliquely, but coalescing at the points 

 of intersection, as if here the fibres were fused rather than woven 

 together (fig. 86). These are called the intercrossing fibres of 

 Sharpey ; they correspond to the white fibres of connective tissue 

 and form the source of the gelatin obtained by boiling bone. 



In many cases, as in the parietal bone, the lamellae are per- 

 forated by tapering fibres called the perforating fibres of Sharpey, 

 resembling in character 

 the ordinary white or 

 rarely the elastic fibrous 

 tissue, which bolt the 

 neighbouring lamellae 

 together, and may be 

 drawn out when the 

 latter are torn asunder 

 (fig. 87). These per- 

 forating fibres 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 : 



(a.) Those which are 

 ossified directly or from 

 the first in a fibrous 

 membrane afterwards 



called the periosteum e.g., the bones forming the vault of the 

 skull, parietal, frontal, and a certain portion of the occipital 

 bones. 



(6.) Those whose form, previous to ossification, is laid down in 

 hyaline cartilage e.g., humerus, femur. 



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

 studied in bones which are not preceded by cartilage i.e., mem- 

 brane-formed (e.g., parietal) ; and without a knowledge of this 

 process (ossification in membrane), it is impossible to understand 

 the much more complex series of changes through which such 

 a structure as the cartilaginous femur of the foetus passes in its 



Fig. 87. Lamellae torn off from a decalcified human 

 parietal bone at some depth from the surface. 

 a, a, lamellae, showing reticular fibres ; b, darker 

 part, where several lamellae are superposed ; 

 c, perforating fibres. Apertures through which 

 perforating fibres had passed, are seen especially 

 in the lower part, a, a, of the figure. (Allen 

 Thomson.) 



