SUPPORTING TISSUES 



79 



ceded by cartilage. In infancy, the 

 bones of the skull are soft and flex- 

 ible, because they are largely carti- 

 lage. One can observe the soft 

 texture of certain parts of an in- 

 fant's skull until the child is about 

 eighteen months old. As growth 

 takes place, this cartilage becomes 

 hard and rigid by the deposit of 

 mineral matter. This compact sub- 

 stance is then called bone. A child's 

 back should be carefully supported be- 

 cause its backbone is largely cartilage 

 and is, therefore, very elastic, and 

 may bend so much that the organs of 



CctruaX 



:>-cartilafie 





.Cartal 



b-periosteum 



Magnified cross section of bone showing the arrange- 

 ment of cells and canals through which blood vessels 

 run. The outer covering, periosteum, and the center 

 filled with marrow can be seen in the lower diagram. 



Certain cells, cartilage cells, occur 

 in pairs. They deposit a thick, tough 

 extracellular material called a matrix. 



the body will be injured. 

 When a bone is broken, 

 the new part first de- 

 velops as cartilage and 

 is replaced, afterward, by 

 true bone. The type of 

 cartilage studied in the 

 laboratory exercise is 

 hyaline cartilage. It is 

 found chieflv on the ends 

 of bone. This is not the 

 only kind of cartilage 

 found in the body. 

 There are other tj^pes 

 with different functions. 

 One function of cartilage 

 is to give ease to the 

 motion of joints, and by 

 means of its tough elas- 



