THE SKELETON 



267 



Pelvic Girdle 



of ligaments vary in them, and no other joint has a bone in 

 its ligaments like the knee cap. But they all have the same 

 smooth, rounded surfaces, the synovial membranes and 

 fluids, the ligaments and muscles to complete the joint, 

 and are all, of course, bound together on the outside by 

 the skin. 



Ball-and-Socket Joints. There are only two typical ex- 

 amples of ball-and-socket joints, one at the shoulder and the 

 other at the hip. As the name indicates, one bone in such a 

 joint ends in a rounded, ball-like head, while the other pre- 

 sents a concave socket 

 into which the ball fits. 

 In an arrangement of 

 this kind the motions 

 of the bones are not 

 confined to one direc- 

 tion, giving greater free- 

 dom of motion, but less 

 strength, than the hinge 

 joint. 



Three bones enter in- 

 to the shoulder joint, 

 though only two of them 

 are of much importance. 

 The humerus, the upper 

 bone of the arm, has at 

 its upper end a good sized, rounded head fitting into a 

 socket made by a concave part of the scapula; Fig. .127 

 This cavity is very shallow but in the living body there is 

 a little rim of cartilage around its edge, making the socket 

 deeper, and the joint, therefore, somewhat more secure. 



It will be noticed from the figure that two little projections 

 of the scapula hang over this socket; while these do not form 

 a part of the socket proper, they protect it from injury above 

 and in front. It is evident that when the arm bone is lifted, 



femur 



FIG. 135. THE HIP JOINT 



