CHAPTER XXXVIII 



JOINTS 



651. Kinds of joints. The union of two bones is called 

 a joint. Some bones grow together and form a single 

 rigid bone, while others are joined together only by loose 

 fibrous tissue which permits the joints to bend freely. 

 Between these two extremes, joints possess all gradations 

 of movement. 



652. Inflexible joints. Some bones of the skull are 

 joined together by cartilage during childhood. Later in 



life, when growth ceases, the cartilage 

 becomes bone and unites the two bones 

 into a single one. 



Other bones of the skull are dove- 

 tailed with each other, so that while 

 they can move slightly, they cannot be 

 separated. The thick bones of the top 

 of the skull are united in this way. 



Between the separate bones of the 

 pelvis and between the vertebrae there 

 are large pads of fibrous tissue, almost 

 like cartilage. These pads permit slight 

 movements between the bones and so 

 prevent jarring during violent move- 

 They are usually stronger than bone 

 itself, so that, by pulling or bending, the bones will be 

 torn apart rather than the pads. 



364 



Hinge joint of the elbow. 



z humerus. 2 ulna. 



ments of the body. 



