264 ADVANCED PHYSIOLOGY 



much softer than bone and can be readily cut with a knife. 

 It differs from bone in that it is not supplied with blood 

 vessels. Under the microscope a thin piece of cartilage 

 appears as in Figure 4, page 14. The cells composing it are 

 far apart, separated by much intercellular substance. 



When, in early life, certain cartilage masses begin to turn 

 into bone, the change does not take place throughout the 

 cartilage uniformly, but at certain points only, called centres 

 of ossification. 



Cartilage is not so readily repaired as bone, but on the 

 other hand, it is not so easily broken. A broken rib is quickly 

 mended in a few weeks and is as good as ever, but if the injury 

 breaks or tears the cartilage, its mending may take a long 

 time. 



JOINTS 



When two bones are fitted together in such a way that 

 there is no movement between them, as for example, the 

 bones of the cranium, the line of joining is generally called 

 a suture joint; where movement is possible, a joint is said to 

 occur. Of the true joints there are two kinds, imperfect and 

 perfect. 



Imperfect joints are those in which the bones concerned do 

 not actually glide over one another although a certain amount 

 of movement is possible because of the flexibility of the elastic 

 cartilage between them. Such joints are noticed where the 

 front ends of the ribs approach the sternum; movement takes 

 place at every breath, though it involves only bending the 

 cartilages which occur there. The bending and slight torsion 

 which may take place between any two vertebrae also illus- 

 trate the action of imperfect joints. Perfect or movable 

 joints are the sort generally thought of as joints and occur 

 where the end of one bone actually turns on some part of the 

 of another. 



