A STUDY OF THE SKELETON 181 



tween the more or less spherical head of a long bone and a 

 cup-shaped cavity in another bone. The best examples of 

 this sort of joint are found at the shoulder and at the hip. 

 Great freedom of movement is possible in a ball-and-socket 

 joint. The arm can be moved upon the shoulder blade 

 (1) backward and forward in a quarter circle at the side of 

 the body, i.e. flexed and extended, (2) outward or inward in 

 a circle in front (ab-duct'ed and ad-duct- ed), (3) the whole arm 

 when held stiff may be made to describe a cone (cir'cum- 

 duct'ed), and (4) it may be twisted in the socket (ro'ta-ted). 

 A much greater range of movement than that just de- 

 scribed is made possible for the arm by the fact that the 

 bones of the pectoral girdle are loosely attached to the 

 rest of the skeleton, and hence can move with the arms. 

 The hip joint allows all four of the movements enumerated 

 above, but the amount of motion is less, since the hip socket 

 is much deeper, and since the pelvic bones are closely at- 

 tached to the sacrum. Other ball-and-socket joints are 

 found between the metacarpals and phalanges of the hand 

 and between the metatarsals and phalanges of the foot. 



We have become familiar with the structure of a hinge 

 joint in our study of the leg of a sheep. Motion is possible 

 in but two directions. When the joint is moved so that the 

 two bones form an angle with each other, the joint is said to be 

 flexed; if the bones are made to form a straight line, the joint is 

 extended. Following is a list of the hinge joints of the body: 



REGION OF JOINT JOINT OCCURS BETWEEN 



elbow humerus and ulna -f- radius, 



wrist carpals and metacarpals. 



fingers the different phalanges, 



knee femur and tibia, 



ankle tibia -f- fibula and tarsals. 



toes the different phalanges, 



head * lower jaw and temporal, 



neck skull and atlas. 



i This joint also belongs to the class of gliding joints. 



