LIGAMENTS 325 



edges. The hollow of the cup articulates with a spheroidal 

 surface furnished by the humerus : the lip of the cup, 

 with a concave depression on the side of the ulna. 



The large lower end of the radius bears the hand, and 

 has, on the side next the ulna, a concave surface, which 

 articulates with the convex side of the small lower end of 

 that bone. 



Thus the upper end of the radius turns on the double 

 surface, furnished to it by the pivot-like ball of the humer- 

 us, and the partial cup of the ulna ; while the lower end 

 of the radius can rotate round the surface furnished to it 

 by the lower end of the ulna. 



In supination, the radius lies parallel with the ulna, 

 with its lower end to the outer side of the ulna (Fig. 101, 

 A). In pronation, it is made to turn on its own axis 

 above, and round the ulna below, until its lower half 

 crosses the ulna, and its lower end lies on the inner side 

 of the ulna (Fig. 101, B). 



The ligaments which keep the mobile surfaces of bones 

 together are, in the case of ball and socket joints, strong 

 fibrous ca])sulcs which surround the joint on all sides. In 

 hinge-joints, on the other hand, the ligamentous tissue is 

 chiefly accumulated, in the form of lateral ligaments, 

 at the sides of the joints. In some cases ligaments are 

 placed within the joints, as in the knee, where the bundles 

 of fibres which cross obliquely between the femur and the 

 tibia are called crucial ligaments (Fig. 97, I) ; or, as 

 in the hip, where the round ligament passes from the 

 bottom of the socket, or acetabulum of the pelvis to 

 the ball furnished by the head of the femur (Fig. 98). 



Again, two ligaments pass from the apex of the odontoid 

 peg to both sides of the margin of the occipital foramen, 

 i.e. the large hole in the base of the skull, thi'ough which 

 the spinal cord passes to join the brain ; these, from their 

 function in helping to stop excessive rotation of the skull, 

 are called check ligaments (Fig. 102, a). 



In one joint of the body, the hip, the socket or aceta- 



