230 MOTION AND COORDINATION 



ARTICULATIONS 



Any place in the body where two or more bones meet 

 is called an articulation, or joint At the place of meeting 

 the bones are firmly attached to each other, thereby secur- 

 ing the necessary coherence of the skeleton. The large 

 number of bones, and consequently of articulations, are 

 necessary for the different movements of the body and 

 also on account of the manner in which the skeleton de- 

 velops, or grows. Articulations are classed with reference 

 to their freedom of motion, as movable, sligJitly movable, 

 and immovable articulations. 



Most of the immovable articulations are found in the 

 skull. Here irregular, tooth-like projections from the dif- 

 ferent bones enable them to interlock with one another, 

 while they are held firmly together by a thin layer of con- 

 nective tissue. The wavy lines formed by articulations of 

 this kind are called sutures (Fig. 100). 



The best examples of joints that are sligJitly, but not 

 freely, movable are found in the front of the spinal column. 

 The cartilaginous pads between the vertebrae permit, by 

 their elasticity, of a slight bending of the column in differ- 

 ent directions. These movements are caused, not by one 

 bone gliding over another, but by compressions and exten- 

 sions of the cartilage. Between the vertebrae in the back 

 of the spinal column, however, there is a slight movement 

 of the bone surfaces upon one another. 



Structure of the Movable Joints. By far the most nu- 

 merous and important of the joints are those that are freely 

 movable. Such joints are strongly constructed and endure 

 great strain without dislocation, and yet their parts move 

 over each other easily and without friction. The ends of 

 the bones are usually enlarged and have specially formed 



