276 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



pied the cavities, are no longer seen ; and their place is filled with a blastema, 

 composed of cells containing a granular matter, and closely resembling those seen 

 in the intra-membranous ossification, with a few fibres scattered amongst them. 

 It is by a change in this blastema, that the walls of the cavities are gradually 



Fig. 70. 



Transverse sections of growing bone, showing the lateral coales- 

 cence of the primary bony areolae and the thickening of the sides 

 of the enlarged cavities by new osseous deposit. The section A is 

 made almost immediately below the surface of ossification ; B is some- 

 what lower, and shows the cavities still more enlarged, and their 

 sides more thickened than in A. The new osseous lining is transpa- 

 rent, and appears light in the figures ; the dark ground within the 

 areolae is owing to opaque debris, which collected there in grinding 

 the sections. It must be farther noticed, that the letter A, within 

 the larger figure, marks a place where a bony partition had been 

 accidentally broken away, so that the large space was naturally 

 divided into two. 



consolidated ; new deposits of ossific matter being formed in their interior, which 

 occasion the gradual contraction of the cavities, and give an increasing density 

 to the bone. The cancellated structure, which remains for a time in the interior 

 of the long bones, and which continues to occupy their extremities, represents 

 the early condition of the ossifying substance, with very little change ; whilst 

 the cavities which have formed more regular communications with each other, 

 and which have been gradually contracted by the subsequent deposit of concentric 

 lamellae, one within another, form the original Haversian canals. Thus we see 

 that they all form one system in their origin ; as they may be considered to do, 

 notwithstanding the difference of their form, in the complete bone. 



266. The original osseous lamellae, formed by the consolidation of the carti- 

 laginous substance, are entirely composed of granular matter; and exhibit none 

 of the lacunae and canaliculi, which are commonly regarded as characteristic of 

 Bone. These excavations present themselves, however, in all the subsequent 



