378 



HUMAN PHYSIOLOGY 



bone ; move the leg in as many different directions as possible ; notice that 

 it may be flexed or bent ; that it may be extended to its original position ; 

 that it may be rotated; that it may be moved to and from the midline 

 of the body; that, with the knee held stiff, the whole limb may be made to 

 describe the arc of a circle. 



These same movements are possible in the leg of a man. This move- 

 ment between bones is obtained by means of joints. If, in the frog, we care- 

 fully separate the muscles of the thigh to the bone, we 

 find that they are attached to the bone by white, glis- 

 tening tendons. Careful examination shows that the 

 bones themselves are held together by very tough white 

 bands or cords; these are the ligaments. We find, too, 

 that one end of the large thigh bone fits into a socket 

 in the hip bone or pelvic arch. It is thus easy to see 

 how such free movement is obtained in the leg. 



Ball and Socket Joint. — Such a joint as just 

 described is called a hall and socket joint. In 



Ball and socket joint. ,i irxT_i • ii.*j" 



man the movement oi the leg is obtained in ex- 

 actly the way described for the hind leg of the frog. The two 

 best examples of a ball and socket joint are found between the 

 long bone in the arm and the shoulder, and between the bones of 

 the hip and the long bone of the leg. 



Hinge Joints. — The second kind of joint, in the leg of the frog, is found 

 between the thigh and the shank. Notice that movement here occurs 

 freely in only two directions, backward and forward ; hence 

 this is called a hinge joint. In man the best examples of 

 a hinge joint are found in the knee and elbow; others are 

 in the fingers and toes. 



Gliding Joints. — Another form of joint, best seen in 

 the skeleton of man, is a sliding or gliding joint. Here 

 the range of movement is slight. Gliding joints are found 

 between the vertebrse or bones of the vertebral column 

 (backbone). 



Pivot Joints. — Another rather unusual joint is the 

 pivot joint. This is best seen between the skull of man 

 and the topmost bone of the vertebral column. The skull 

 is held in place by means of two small knobs which pro- 

 ject downward and rest in cavities in the bone directly 

 beneath them. 



Levers in the Body. — It is evident that movement 

 of a joint is caused by muscles which act in cooperation 

 with the bones to which they are attached ; the latter 

 form true levers. A lever is a structure by which either greater work power 

 or greater range of motion is obtained. In this apparatus, the lever works 



Hinge joint, show- 

 ing muscle (a) 

 and its tendon 

 (6). 



