THE FROG 185 



their origin on the bones above the joint, and their insertion 

 on the bone below. The muscles end in bands of connective 

 tissue called tendons that extend down over the joint to the 

 insertion on the lower bone. The muscles are always tightly 

 stretched in the body and always pulling upon the tendons. 

 As a result the tension upon the tendons holds the two bones 

 of the joint in firm contact. Outside of the muscles and tendons 

 is the skin. The joint thus consists of smoothly moving bones, 

 which are moistened by synovial' fluid, held in position by 

 tightly drawn tendons, prevented from being pulled apart by 

 ligaments that protect them against strains, and moved by 

 muscles. 



The freedom of motion in the different joints varies with 

 the shape of the bones at the joints. In some of the articula- 

 tions, one bone ends in a ball which fits into a rounded socket 

 of the other bone. In this type, the ball-and-socket joint, 

 the bones are freely moved in any direction. The joint at 

 the hip and that of the shoulder are examples of this type. 

 In other joints the form of the bones is such that motion is 

 possible only backward or forward. These are called hinge 

 joints, and are illustrated by the joints at the elbow, the knee, 

 the wrist, and by the separate joints of the fingers and toes. 

 In some joints one bone moves around the other as on a pivot. 

 No good examples of this are found in the frog, but in the human 

 body the motion of turning the head, or turning over the hand 

 so that the back or the palm is uppermost, are excellent illus- 

 trations. It is evident that the movements of the body are 

 dependent upon the free motion of the bones at the joints, 

 and that the growing of the bones together at a joint, anchy- 

 losis as it is called, will destroy all power of motion. 



Alimentary Canal. The wide mouth (oral opening) leads 

 into a very large cavity, the buccal cavity. There are teeth 

 on the maxilla,. premaxilla, and vomer (Fig. 88 C, D), which 

 are of use for holding, but not for masticating food. On the 

 floor of the mouth is, a large muscular tongue, attached to 



