204 



OSSEOUS SYSTEM. 



[sect. 106. 



Fig 89. 





becoming softer, paler, and poorer in salts, and their cells re- 

 sembling more and more the soft formative-cells, till at last, 

 without any line of demarcation they are lost in the soft blastema. 

 In these bones, there exists at first, only a horizontal growth ; the 

 rays, as they further extend and become connected by cross 

 branches, continually adding to the size of the original network, 

 but soon a thickening of the primitive lamella takes place by the 

 deposition of layers both internally and externally, whilst at the 

 same time the bone becomes more compact, and the more so the 

 older it is. The thickening is due to the periosteum, which is 

 found upon the surface of the secondary bones a short time after 

 their appearance, and is developed either from their original 

 blastema or from the neighbouring parts (perichondrium of the 

 primordial skull, muscular and tendinous coverings) . The osseous 

 deposition proceeds exactly as in the sub-periosteal layers of the 

 cartilaginously pre-formed bones, that is, a soft blastema is de- 

 posited upon the inner side of the periosteum, and gradually 

 ossifies from the bone outwards, without ever having been car- 

 tilaginous (fig. 89). In this manner 

 there is now formed, particularly upon 

 the outer, hut also upon the inner side 

 of the primary osseous table, and pro- 

 ceeding from it, a succession of new 

 lamellse, and the young bone becomes 

 thicker and. thicker. All these new 

 lamellse are, like the earliest, at first 

 reticulated ; and then ' rounded or 

 elongated, variously sized interspaces 

 communicate with those of the already 

 existing and the subsequent lamellse, 

 so that the osseous nuclei of the 

 secondary bones, like the sub-periosteal 

 layers, are from the first, penetrated 

 by a network of canals, which, as in. 

 these, soon present the appearance of 

 Haversian canals. At first, filled only 

 by soft blastema, the remains of the 

 formative material of the different 

 layers, these spaces soon become nar- 

 rowed by the advancing ossification 

 in them, which partly crosses them in 

 the form of bridges, as in the osseous 

 rays of the borders, partly appears as 



From the inner surface of a pa- 

 rietal bone of a new-born infant, 

 magnified 300 times, a. Bone with 

 lacunae, still pale and soft. b. Bor- 

 der of the same. c. Ossifying blas- 

 tema with its fibres and cells. 



