THE OSSEOUS SYSTEM. 



371 



Fig. 136. 



ceeds exactly in the same way, that it does in the periosteal 

 layers ; the rays of bone as they extend further into the soft 

 blastema becoming softer and paler, and containing less earthy 

 matter, whilst their cells become more and more like the soft 

 formative cells, till, at last, the spicules lose all distinct limitary 

 outline, and are lost in the blastema. At first, the growth of 

 these bones proceeds in a superficial plane only, the rays, as 

 they extend and become connected by transverse branches 

 continuing to add to the size of the original, reticulated lamella, 

 which, however, shortly begins to in- 

 crease in thickness by the deposition 

 of layers upon both sides of it ; the 

 different portions, also, in proportion 

 to their age becoming more and 

 more compact. The formation of the 

 thickening layers is to be referred to 

 the periosteum, which is found on 

 the surfaces of the secondary bones, 

 very soon after their formation has 

 begun, being developed either from 

 their original blastema, or from the 

 contiguous tissues (perichondrium of 

 the primordial cranium, muscular and 

 tendinous coverings), and proceeds 

 exactly in the same way as in the 

 periosteal layers of the primary bones ; 

 that is to say, on the inner side of the 

 periosteum, a soft blastema is de- 

 posited, which gradually ossifies, from 

 the bone outwards, without its ever 

 being cartilaginous (fig. 136). In this 

 way are now formed, chiefly on the 

 outer, but also on the inner sur- 

 face of the primary osseous lamella, 

 and proceeding outwardly from it, 

 successive, new laminse, in consequence of which, the rudi- 

 mentary bone continually increases in thickness. All these 





• w 



Fig. 136. From the inner surface of the parietal bone of a new-born child, x 300 

 diam.: «, bone with lacunae, still pale-coloured and soft; b, border of the same ; 

 c, ossifying blastema with its fibres .and cells. B, three of these cells, x 350 diam. 



