194 



THE ORGANS 



bones. Compact bone forms the bulk of the shafts of the long bones 

 and the outer layers of the Hat and the short bones. 



In compact bone the layers or lamellae of bone tissue have a defi- 

 nite arrangement into systems, the disposition of which is largely 

 dependent upon the shape of the bone and upon the distribution of 

 its blood-vessels. 



In spongy hone (Fig. 115) there is no arrangement of the bone tissue 

 into systems. The trabeculae consist wholly of bony tissue laid down 



ft ' , • • 



Fig. 116. — Longitudinal Section of Hard (Undecalcified) Bone: Shaft of Human Ulna. 

 X90. (Szymonowicz.) Haversian canals, lacunae, and canaliculi in black. 



in lamellae. These trabeculae anastomose and enclose spaces which 

 contain marrow and which serve for the passage of blood-vessels, 

 lymphatics, and nerves. 



On examining a longitudinal section of compact bone (Fig. 116) 

 there are seen running through it irregular channels, the general 

 direction of which is parallel to the long axis of the bone. These 

 channels anastomose by means of lateral branches, and form a com- 

 plete system of intercommunicating tubes. They are known as 

 Haversian canals, contain marrow elements, and serve for the trans- 

 mission of blood-vessels, hmphatics, and nerves. They anastomose 

 not only with one another, Init arc in communication with the sur- 



