50 ZOOLOGY. 



in the intercellular substance (Fig. 24). 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. 



FIG. 24. 



FIG. 24. 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, h, are Haversian canals; I, lacuna, occupied in life by bone cells. The 

 minute canals through the bone connecting the lacunae are canaliculi. B, a portion 

 of one Haversian system much magnified, h, Haversian canal, containing artery (a), 

 vein (f), lymphatic spaces, nutritive cells; c, canaliculi; /, lacunae; la, plate of bony 

 intercellular substance. 



Questions on the figure. How does bone compare in appearance and 

 structure with the other supportive tissues? How is its intercellular sub- 

 stance laid down? How are the cells 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? 



Dentine and enamel, though differing in structure from bone, are to 

 be looked upon as belonging to the same class of tissues. They differ 

 chiefly in the fact that no cellular elements are included in the secretion. 

 They are thus harder and denser than bone. 



76. We find all stages of transition between the more sim- 

 ple and more complex supportive tissues, and it may be seen 

 furthermore that there is a fundamental embryological 

 sequence. In the development of the organism the simpler 

 connective tissues give place, by transformation or substitu- 

 tion, to the more complex. The cellular connective tissue of 

 early life is replaced, for example, by cartilage, and this may 

 be transformed into bone in adult life. 



