326 THE TISSUES. 



demonstration of the fact that bone is constantly undergoing an 

 active disassimilation and repair. Indeed, one side of an Haversian 

 space may be becoming the seat of a new Haversian rod of bone, 

 while the opposite is undergoing further enlargement from ab- 

 sorption. 



The contents of the Haversian canals ^TQ first, the vessels; se- 

 condly, the nerves of bone ; and, thirdly, in case of the larger canals, 

 a small quantity of marrow surrounding the former. 



Differences in the Compact and the Cancellated Forms of Bone-structure. 



All bones contain both the cancellated and the compact forms of 

 structure; the former constituting an external layer of varying 

 thickness, and the latter the internal portions. In the shafts of the 

 long bones, however, the entire thickness is formed of compact 

 bone-substance ; and the same is true of the thinnest portions of 

 some of the flat bones. The extremities of the long bones, however, 

 like all the short bones, consist of a thin layer of compact substance 

 externally, and cancellated substance within. 



In both forms, however, the bone-tissue is arranged in the form 

 of lamina? or plates, formed of granules or hyaline substance (or 

 both), as already described (p. 321); and the manner in which these 

 laminae are arranged, determines the two forms of bone-substance 

 now to be described. 



1. Cancellated Bone-structure. The cancellated bone-structure 

 consists of an aggregation of cavities, each of which is called a can- 



Fig. 215. 



Spherical canceling (diagrammatic). Its walls consist of three lamellse, with their lacunje and pores. 



