48 



VETERINARY PHYSIOLOGY 



while the active changes extend further and further from the 

 centre to the ends of the shaft. The centre, which was once 

 formed by the embryonic cartilage, is thus changed 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 ; but after all of this has been absorbed, 

 the bone formed outside the cartilage in the fibrous tissue is 

 attacked by burrowing processes from inside and outside, 



Fig. 15.— Cross section through part of the shaft of an adult long bone to 

 show the arrangement in lamellte distributed as Haversian (1), inter- 

 stitial (2), periphei'al (3), and medullary (4). 



which hollow out long channels running in the long axis of 

 the bone. These are the Haversian sioaces (fig. 15). 

 Round the inside of each, 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 fibrous tissue, blood-vessels, lymphatics, and 

 nerves, with layer upon layer of bone concentrically arranged 

 around it, Tliis constitutes an Haversian system. In this 

 way the characteristic appearance of the shaft of a long bone 

 is produced, with layers of calcified fibrous tissue, the bone 



