CH. V.] 



OSSIFICATION 



61 



bone is sheathed in a membrane 

 exactly resembles the periosteum 

 layers, in the deeper one of which 

 spheroidal and branched cells 

 predominate and blood-vessels 

 abound, while the outer layer 

 consists mainly of fibres. 



Between the cartilaginous pre-> 

 figurement of which the foetal 

 long bone consists and the adult 

 bone there are several inter- 

 mediate stages. 



The process may, however, be 

 most conveniently described as 

 occurring in three principal 

 stages. 



The first stage consists of two 

 sets of changes, one in the carti- 

 lage, the other under the peri- 

 chondrium. These take place 

 side by side. In the cartilage 

 the cells in the middle * become 

 enlarged and separated from one 

 another. The cartilage-cells on 

 each side get arranged in rows in 

 the direction of the extremities 

 of the cartilaginous rod. If at 

 this stage one cuts the little em- 

 bryonic bone with a knife, the 

 knife encounters resistance, and 

 there is a sensation of grittiness. 

 This is due to the fact that cal- 

 careous particles are deposited in 

 the matrix; and in consequence 

 of this the matrix stains differ- 

 ently with histological reagents 

 from the unaltered matrix. 

 Simultaneously with this, the 

 periosteal tissue is forming layer 

 after layer of true bone ; this is 

 formed exactly in the same way 



* This is the case in nearly all the 

 long bones, but in the terminal pha- 

 langes the change occurs first, not in 

 the middle but at their distal extremities. 



termed the perichondrium, which 

 described above ; it consists of two 



l.m 



FIG. 89. Section of two ftetal phalanges ; the carti- 

 lage-cells in the centre of B are enlarged and 

 separated from one another by calcined matrix. 

 im, Layer of bone deposited under the perios- 

 teum ; o, layer of osteoblasts by which this 

 layer was formed. The rows of cartilage-cells 

 are seen on each side of the centre of calcifica- 

 tion. In A, the terminal phalanx, the changes 

 begin at the tip. (After Dixey.) 



