308 ELEMENTARY PHYSIOLOGY less. 



spacious medullary cavity filled by a reddish, highly 

 vascular mass of connective tissue, abounding in fat cells, 

 called the medulla or marrow ; and a longitudinal 

 section shows that this medullary cavity extends through 

 the shaft, but in the articular ends becomes subdivided 

 by bony partitions and bi'eaks up into smaller cavities, 

 like the areohie of connective tissue. These cavities are 

 termed cancelli, and the ends of the bone are said to 

 have a cancellated structure. The walls of the medullary 

 cavity in the shaft are very dense, and exhibit no cancelli 

 and appear at first to be solid throughout. But on 

 examining them carefully with a magnifying glass it will 

 be seen that they are traversed by a meshwork of narrow 

 canals, varying in diameter from 20/i to lOOfi or more. 

 The long dimensions of the meshes lie parallel with the 

 axis of the shaft. These are the Haversian canals. 

 This system of Haversian canals opens by short communi- 

 cating branches on the one hand upon the periosteal and 

 on the other upon the medullary surface of the wall of the 

 shaft ; and in a fresh bone, minute vascular prolongations 

 of the periosteum and of the medulla respectively, may be 

 seen to pass into the comnmnicating canals and become 

 continuous with the likewise vascular contents of the 

 Haversian canals. Moreover, at one part of the shaft 

 there is a larger canal through which the vessels which 

 supply the medulla pass. This is the so-called nutritive 

 foramen of the bone. At the two ends of the bone the 

 cavities of the Haversian canals open into those of the 

 cancelli ; and the vascular substance which fills the latter 

 thus further connects the vascular contents of the 

 Haversian canals with the medulla. 



Thus the bone may be regarded as composed of (i) an 

 internal, thick, cylinder of vascular medulla ; (ii) an 

 external, hollow, thin, cylindrical sheath of vascular peri- 

 osteum, completed at each end by a plate of articular 

 cartilage ; (iii) of a fine, roopilar, long-meshed vascular 

 network which connects the sides of the medullary 



