THE TISSUES 



37 



of cartilage between them. Into these canals the processes 

 of the periosteum extend, and fill them with its fibrous 

 tissue. In this fibrous tissue, the deposition of lime salts 

 takes place upon the trabeculae, enclosing cells, and thus 

 forming a crust of bone, while the cartilage also becomes 

 calcified. If this calcification of the cartilage and deposition 

 of bone were to go on unchecked, the block of cartilage 

 would soon be reduced to a solid mass of calcified tissue. 

 But this does not occur. For, as rapidly as the trabeculse 

 become calcified, they are 

 absorbed, while the active 

 changes extend farther and 

 farther from the centre, which 

 is thus reduced to a space 

 filled by fibrous tissue which 

 afterwards becomes the bone 

 marrow. 



The process of absorption 

 does not stop at the original 

 block of cartilage, which is 

 all removed. After all of this 

 has been absorbed, the bone 

 formed round the cartilage (the 

 periosteal bone) is attacked by 

 burrowing processes from in- 

 side and outside, which hollow 

 out long channels running in 

 the long axis of the bone. 

 These are -the Haversian 

 spaces (Fig. 14). Round the 

 inside of these, calcification 



occurs, spreading inwards in layers, and enclosing connective 

 tissue cells, until, at length, only a small canal is left, an 

 Haversian canal, containing some connective tissue, blood 

 vessels, lymphatics, and nerves, with layer upon layer of 

 bone concentrically arranged around it. This constitutes an 

 Haversian system. In this way the characteristic appearance 

 of the shaft of a long bone is produced (Fig. 14), with layers 

 of calcified fibrous tissue, the bone lamellae arranged as shown 

 as Haversian, interstitial, peripheral, and medullary lamellae. 



FIG. 14. Cross Section through part 

 of the shaft of an adult long bone to 

 show the arrangement in lamellae 

 distributed as Haversian (1), inter- 

 stitial (2), peripheral (3), and medul- 

 lary (4). 



