THE SKELETON 



6 9 



sac - 



irregular lines known as sutures. A crayon box is so joined, 

 but the teeth in the wood are much more regular than in 

 the joints of the skull. 



115. A Movable Joint. Let us take the knee joint, for 

 example. The two bones which join are the femur and the 

 tibia. They present to each other an enlargement, or head, 

 which serves to increase the strength of the joint and increase 

 the surfaces applied to each other. The two heads instead 

 of being formed altogether of rigid bone are covered by 

 cartilage, which by its elasticity and smoothness provides 

 for the gliding of one bone upon the other. Strong 

 fibrous bands called ligaments bind 

 together the ends of the two bones. 

 Surrounding the ends of the bones, 

 like a collar, is found a liga- 

 ment (the capsular ligament) 

 inclosing the space of the 

 joint in a closed 

 sac. This closed sac 

 is called the capsule. 

 A thin membranous 

 sac, called the syno- 

 via I membrane, lines 

 the capsule. It se- 

 cretes a slimy fluid 

 which resembles the 

 white of an egg and 

 is called the synovial 

 fluid. This lubri- 

 cates the joint, and 

 is deposited continu- 

 ally, but only so fast 

 as used up in exercise. As the sac has no opening, air is 

 excluded, and atmospheric pressure aids in holding the 

 bones in place. The hip joint (Fig. 65) and other joints 



fem,- 



isc 



FIG. 65. The Right Hip Joint. The Hip Bone 

 sawed through so as to show the Cup of the Joint. 



fern, femur; acet, cup, or acetabulum; caps, capsule; 

 r. L, round ligament. 



