184 BIOLOGY 



is usually attached at the ends to two separate bones, the 

 term origin being applied to the attachment nearest to the 

 center of the body, and insertion to the attachment the farthest 

 from the center; muscles pull in the direction of their origin. 

 Since these muscles are numerous and attached to the bones 

 at different places, they pull upon the bones in different direc- 

 tions and produce a great variety of movements. Figure 89 

 shows the chief muscles of the frog. The names given to them 

 are the same as those applied to the corresponding muscles 

 in man. 



Joints or Articulations. Where two bones come together 

 they form a joint. In some cases the bones are so rigidly grown 

 together that there is no motion between them, thus forming 

 the fixed joints, like those that are between the bones which 

 form the skull. In other places the bones are freely movable, 

 forming the movable joints. All the movements of the body 

 are produced at the joints. The bones at these joints are so 

 connected that, while they are held firmly together, they are 

 at the same time freely movable. The ends of the bones are 

 generally more or less rounded, the end of one bone fitting 

 into a rounded depression on the other. The ends of the bones 

 are also covered by a layer of cartilage, which is quite smooth 

 so as to prevent friction. This structure makes it possible 

 for one bone to move upon the other without difficulty. All 

 friction is eliminated, and movement of the bones is rendered 

 easier, by a secretion of fluid which is poured into the joint 

 from the synovial glands. This is called the synovial fluid. 

 To prevent the bones from being pulled apart they are held 

 together by bands of white connective tissue called ligaments. 

 These are tough but flexible, and are attached to the two bones 

 that form the joint. They are long enough to make the mo- 

 tions of the bones free, but short enough to hold them in posi- 

 tion and prevent their being pulled away from each other by 

 slight strains. The bones are held firmly in position by the 

 muscles. The muscles which move the bones usually have 



