HISTOLOGY 



157 



All external surfaces of bone are covered by a membrane, the peri- 

 osteum (Fig. 77), of dense fibrous connective tissue well supplied with 

 blood vessels. Most bones, notably the long bones of the appendages, 

 have internal cavities (Fig. 208) occupied by a more or less vascular soft 

 tissue the marrow. The ''yellow marrow" of long bones contains much 

 fat. "Red marrow" is very highly vascular and contains little fat. 

 Marrow has a framework of reticular tissue in whose spaces occur marrow 

 cells (myelocytes) and various types of blood cells. 



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Fig. 120. — Section, highly magnified, of compact bone from the shaft of the human 

 humerus. The section, cut transversely to the long axis of the bone, shows four 

 Haversian systems with their central canals, concentric lamellae of bone, lacunae between 

 adjacent lamellae, and canaliculi extending between lacunae. (From Bremer, Text- 

 book of Histology; after Sharpey.) 



Blood vessels from both the periosteum and the marrow enter and 

 branch throughout the bone. From these vessels substances necessary 

 for the metabolism of the bone cells diffuse through the system of con- 

 nected lacunar spaces. 



In long bones the larger blood vessels lie approximately parallel to the 

 long axis of the bone. Around such a vessel the bone lamellae are arranged 

 in concentric order (Figs. 119 and 120) forming a so-called Haversian 

 system. The central space, occupied by the blood vessel, is called a 

 Haversian canal. These concentric systems are much less prominently 

 developed in flat bones. 



The matrix of bone consists of commingled organic and inorganic 

 materials. Collagenous and other protein substances constitute the 



