CELLULAR DIFFERENTIATION 



57 



Salts of lime may be deposited in the intercellular substance, giving it some of 

 the qualities of bone. 



78. Osseous or Bony Tissue. — These tissues are found only in vertebrates, and 

 are the most complicated of the supportive tissues. The firm matrix which is 

 secreted by the bone cells consists of a mixture of organic substance and inorganic 

 matter, especially the salts of lime. The cells with their fine filamentous branches 

 occur more or less regularly between thin plates or lamella of the bony material. 

 A cross-section of one of the long bones shows the typical condition. The perios- 

 teum is a superficial fibrous membrane about the bone, well supplied with blood 

 vessels. Its inner layer of cells is capable of producing bone. Within this is a 

 region of firm bone, in which a series of lamellae are parallel with the surface of 

 the periosteum. Between the lamellae occur the spaces {lacuna) occupied by the 

 bone-cells which have been left behind as the matrix was deposited. Deeper in 



Pig. 26. 



Fig. 26. Bony Tissue, A, portion of cross-section of a bone, the upper portion of the figure 

 representing the outer surface of the bone, just beneath the periosteum. The open spaces, A, are 

 Haversian canals; I, lacuna, occupied in life by bone cells. The minute canals through the bone 

 connecting the lacunse are canalicuU, B, a portion of one Haversian system much magnified, h. 

 Haversian canal, containing artery (a), vein (0), lymphatic spaces, nutritive cells; c, canalicuU; I, 

 lacuna; la, plate of bony intercellular substance. 



Questions on the figures. — How does bone compare in appearance and structure 

 with the. other supportive tissues? What is the really living part oJE bone? How 

 is its intercellular substance laid down? How are the celts in the bone nourished? 

 How do they come to lie in the solid bone? What changes occur in this type of 

 tissue with age? What is the function of the Haversian canal? 



the bone the lamellas and cells are in concentric layers about the numerous blood 

 vessels (occupying spaces known as Haversian canals) which penetrate the bone, 

 chiefly in a longitudinal direction. The included bone-cells communicate with 

 each other and with the blood vessels by processes which occupy. minute catials 

 {canaliculi) in the intercellular substance (Fig. 26). Within this region and imme- 

 diately surrounding the central cavity of the bone is often a mass of spongy bone 

 in which the regularity of arrangement of the cells is lost. Bone may be formed 

 by replacing cartilage, or wholly independent of it. 



