40 FORMATION OF BONE. 



2d. They are frequently met with in the form of roundish or 

 irregular masses in the substance of the different organs, as the 

 arteries, lungs and ovaries. 



3d. Under the form of smooth flattened concretions, formed 

 originally according to Meckel on the outer side of the synovial 

 membrane of the joints, and which develop themselves towards 

 the centre of the cavity of the joints, till their attachment to 

 the membrane is stretched out, so that it becomes a mere pedicle, 

 which not unfrequently breaks *off. In this way is formed the 

 loose cartilages often met with in the knee joint. 

 All these accidentally developed cartilages have a tendency 

 to be converted into bone, and which are then called accidental 

 ossifications. 



Of the Formation of Bone. 



The generality of bones, and particularly those which are 

 long, are originally formed in cartilage ; some, as those of the 

 skull, are formed between membranes, and the teeth in distinct 

 bags. 



When ossification is about to begin in a particular part of 

 a cartilage, most frequently in the centre, the arteries, which 

 were formerly transparent, become dilated, and receive the red 

 blood from which the osseous matter is secreted. This matter 

 retains, for some time, the form of the vessels which gave it 

 origin, till more arteries being by degrees dilated, and more 

 osseous matter deposited, the bone at length attains its com- 

 plete form. 



During the progress of ossification, the surrounding cartilage 

 by degrees disappears ; not by being changed into bone, but 

 by an absorption of its parts, the new-formed bone occupying 

 its place. 



The ossification of broad bones, as those of the head, begins 

 by one or more points, from which the osseous fibres issue in 

 rays, as seen in Fig. 6. 



The ossification of long bones, as in those of the extremities, 

 begins by central rings, from which the fibres extend towards 

 the ends of the bones. 



