BONE 



519 



the Haversian canal which they surround, by fine canaHculi. 

 The centre of a bone often has the lamellae irregularly arranged, 

 with large spaces containing marrow ; it is called cancellated, 

 while that with the regular Haversian systems is said to be hard, 

 or compact. 



Bone is covered with a periosteum. The outer part of this is 

 a connective tissue sheath, but below this is a highly vascular 



Fig. 404. — Diagram to show the lamellar structure of bone. The Haversian systems 

 are represented greatly out of scale to show their arrangement in a long bone. 

 —From Le Gros'Clark, The Tissues of the Body, 3rd edition, 1952. Clarendon 

 Press, Oxford. 



He, Haversian canals ; PI, peripheral lamellae. 



layer with many osteoblasts, cells which form bone partly by 

 liberating a phosphatase enzyme which precipitates calcium 

 phosphate from a calcium hexose phosphate circulating in the 

 blood. As the periosteum is on the outside of the bone, the latter 

 must grow chiefly on the surface. There is change of shape of 

 a bone during hfe, and growth is assisted by the erosion of 

 material already formed, and in this the chief agent is probably 

 a special type of cell called an osteoclast. Some bones take the 

 place, topographically and functionally, of embryonic cartilage, 

 and are called cartilage bones, while others are formed in 



