662 THE SKELETAL SYSTEM 



channels or canaliculi which course through the lamellae. Some of the 

 canaliculi join larger channels within the bony substance which contain blood 

 vessels. Bony substance in the living animal, therefore, is living tissue, con- 

 structed of the following features (fig. 314): 



( 1 ) Bony layers or lamellae are present, composed of a ground substance 



of fibrous and amorphous materials infiltrated with mineral salts, par- 

 ticularly the salts of calcium (fig. 314A, B); 



(2) between the bony layers are small cavities or lacunae, each contain- 

 ing a bone cell or osteocyte (fig. 314B); 



(3) coursing through the lamellae and connecting the various lacunae, are 

 small channels, known as canaliculi, into which extend processes from 

 the osteocytes (fig. 314B); and 



(4) the canaliculi make contact in certain areas with blood vessels which 

 lie within small canals coursing through the bony substance or in 

 larger spaces, called marrow cavities (fig. 314A, B). 



b. Types of Bone 

 From these fundamental structural features, two types of bone are formed: 



( 1 ) spongy and 



(2) compact. 



The difference between these two types of bone rests upon the proportion 

 of bony substance to blood-vessel area or marrow cavity present, and is not 

 due to a difference in the character of the bony substance itself. 



c. Characteristics of Spongy Bone 



Spongy bone differs from compact bone in that large marrow cavities or 

 spaces are present between an irregular framework of compact bone. The 

 bony substance present is in the form of a meshwork of irregular columns 

 or trabeculae between the marrow-filled spaces (fig. 314A). 



d. Compact Bone 



Compact bone (fig. 314A, B, D) lacks the widespread, marrow-filled cavi- 

 ties of the spongy variety, the marrow spaces being reduced to a minimum. 

 This is accomplished by the utilization of a structural unit known as the 

 Haversian system, named after Clopton Havers, an English anatomist who 

 discovered the system during the latter part of the seventeenth century while 

 investigating the blood supply of bone. The bony walls of the shafts of long 

 bones are composed largely of many Haversian systems, associated side by 

 side as shown in figure 314D. Irregular layers (lamellae) lie between the 

 various systems. 



The Haversian system is composed of a very narrow canal or lumen, the 

 Haversian canal, around which are placed concentrically arranged bony plates 



