64 THE CONNECTIVE TISSUES. [CH.V. 



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 (osteoblasts), 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 

 increase 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 spiculae 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 bone-corpuscles. On the bony trabeculse 

 first formed, layers of osteoblastic cells from the osteo-genetic 

 layer of the periosteum repeat the process just described ; and as 

 this occurs in several thicknesses, and also at the edges of the 

 spicules previously formed, the bone increases, both in thickness, 

 length and breadth. The process is not completed by the time 

 the child is born ; hence the fontanelles or still soft places on the 

 heads of infants. Fig. 88 represents a small piece of the 

 growing edge of a parietal bone. 



The bulk of the primitive spongy bone is in time converted 

 into compact bony tissue, with Haversian systems. Those portions 

 in the interior not converted into bone become filled with the red 

 marrow of the cancellous tissue. 



Ossification in Cartilage. Under this heading, taking the 



