8 A MANUAL OF ANATOMY 



excluded from any of the preceding classes. The vertebrae belong 

 to this class. 



Ossiflcation. — All bones are originally membranous. vSome of 

 them— for example, the tegmental bones of the cranium and most 

 of the bones of the face — ossify in membrane, but the majority 

 pass through a cartilaginous stage before becoming ossified. There 

 are, therefore, two modes of ossification, namely intramembranous 

 and intracartilaginous, and bones are consequently spoken of as 

 membrane- and cartilage-hones. 



Centres of Ossification. — These are primary and secondary. The 

 primary centre, which as a rule appears early in intra-uterine life, 



Fig. 4. — Ossification in Membrane. 



IS that from which the ossification of the principal part of the bone 

 proceeds. This, in the case of a long bone, is the shaft or diaphysis. 

 'ihe secondary centres, which for the most part do not appear until 

 after birth, are those from which the ossification of outgrowths of a 

 bone proceeds, these forming what are known as the epiphyses. 



Ossiflcation in Membrane. — In this mode of ossification the bone is preceded 

 by fibrous tissue. The fibres of this tissue are known as osteogenetic fibres, 

 and they are arranged in small bundles. The tissue is very vascular, and 

 contains many nucleated cells, called osteoblasts. At the centre of ossification 

 the osteogenetic fibres, which have a covering of osteoblasts, become calcified, 

 and bony spicnla are thus formed, which radiate towards the circuniference 

 of the bone. These radiating spicula are connected at frequent intervals, and 



