CH. V.] OSSIFICATION 59 



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. 



(b.) 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 ; and without a know- 

 ledge of this process (ossification in membrane), it is impossible, to 

 understand the more complex series of changes through which such 

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

 transformation into the bony femur of the adult (ossification in 

 cartilage). 



Ossification in Membrane. The membrane, afterwards forming 

 the periosteum, from which such a bone as the parietal is developed, 

 consists of two layers an external fibrous, and an internal cellular or 

 osteo-genetic. 



The external layer is made up of ordinary fibrous tissue. The 

 internal layer consists of a network of fine fibrils with a large number 

 of nucleated cells (pstedblasts), some of which are oval, others drawn 

 out into long branched processes: it is more richly supplied with 

 capillaries than the outer layer. It is this portion of the periosteum 

 which is immediately concerned in the formation of bone. 



In such a bone as the parietal, ossification is preceded by an in- 

 crease in the vascularity of this membrane, and then spicules, starting 

 from a centre of ossification near the centre of the future bone, shoot 

 out in all directions towards the periphery. These primary bone 

 spicules consist of fibres which are termed . osteo-genetic fibres ; they 

 are composed of a soft, transparent substance called osteogen, around 

 and between which calcareous granules are deposited. The fibres in 

 their precalcified state are likened to bundles of white fibrous tissue, 

 to which they are similar in chemical composition, but from which 

 they differ in being stiffer and less wavy. The deposited granules 

 after a time become so numerous as to imprison the fibres, and bony 

 spiculse result. By the junction of the osteo-genetic fibres and their 

 resulting bony spicules a meshwork of bone is formed. The osteo- 

 genetic fibres, which become indistinct as calcification proceeds, persist 

 in the lamellae of adult bone as the intercrossing fibres of Sharpey. 

 The osteoblasts, being in part retained within the bony layers thus 

 produced, form b.one corpuscles. On the bony trabeculse first formed, 

 layers of osteoblastic cells from the osteo-genetic layer of the perios- 

 teum repeat the process just described ; and as this occurs in several 

 thicknesses, and also at the edges of the spicules previously formed, 



