332 SPECIAL HISTOLOGY. 



tinuing to add to the size of the original, reticulated lamella, which, 

 however, shortly begins to increase 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 deposited, 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 surface of the primary osseous lamella, arid proceeding 

 outwardly from it, successive, new laminae, in consequence of which 

 the rudimentary bone continually increases in thickness. All these new 

 lamellae, like the primary one, are at first perforated by reticular open- 

 ings, and the various sized, roundish or elongated interstices communi- 

 cate with those of the previously and subsequently formed layers, so 

 that the secondary osseous nuclei, like the periosteal layers, are from 

 the first, penetrated by a network of canals, which, as in those layers, 

 in part at least, soon present the appearance of Haversian canals. At 

 first, filled only with a soft blastema, the remains of the plastic material 

 of the various lamellae, these spaces, in consequence of the advance of 

 ossification in their interior, which sometimes takes the form of bridges 

 stretching across them, sometimes of a deposit on their walls, become 

 more and more contracted. Ultimately, some are entirely closed, whilst 

 others are converted into true vascular canals, the vessels being de- 

 veloped from their contents, which are composed during the time of 

 medulla-cells, and communicating with those of the periosteum. When 

 the bone has arrived at this stage, its subsequent changes are readily 

 followed. It continues to increase in breadth and thickness by the con- 

 stant addition of new blastema on its edges and surfaces, until it has 

 attained its typical form and size, and at the same time, by the solution 

 of its compact substance, additional spongy tissue (or even large cavities), 

 is formed in its interior, so that eventually, like bone developed from 

 cartilage and periosteal layers, it presents, externally, compact sub- 

 stance with Haversian canals ; and internally, medullary spaces (cancelli), 

 although with distinct secondary deposits. 



The secondary cranial bones ossify, in part, earlier than the primary, 

 and mostly with only a single nucleus. The soft blastema out of 

 which they are formed, and which, so long as the bones continue to 

 grow, is to be found on their surfaces and edges, does not, like carti- 



