CELLULAR DIFFERENTIATION. 



49 



the matrix. Cartilage is bounded on its free surfaces by a fibrous mem- 

 brane, the pcrichondrium. This membrane assists in the growth of the 

 cartilage. There are no blood capillaries in cartilage. 



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

 some of the qualities of bone. 



FIG. 23. 



FIG. 23. Cartilage. A, Hyaline cartilage; B, fibrous cartilage. In the latter a large 

 portion of the intercellular substance is conspicuously fibrous. The cells occur in 

 pockets (p) in the matrix; /, intercellular fibres. 



Questions on the figure. What are the points of similarity and of 

 difference in the two types of cartilage? In what manner do the multi- 

 cellular pockets arise? What is the nature and origin of the intercellular 

 substance in each case? 



75. Osseous or Bony Tissue. These tissues are found only in verte- 

 brates, and are the most complicated of the supportive tissues. The firm 

 matrix which is secreted by the bone cells cpnsists 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 periosteum 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) occu- 

 pied by the bone-cells which have been left behind as the matrix was 

 deposited. Deeper in the bone the lamellae 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 canals (canaliculi) 

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