68 THE HUMAN BODY 



are known as fontanelles. At them a pulsation can often be felt 

 synchronous with each beat of the heart, which, driving more blood 

 into the brain, distends it and causes it to push out the skin where 

 bone is absent. Another good example of an articulation admit- 

 ting of no movement is that between the rough surfaces on the 

 sides of the sacrum and the innominate bones. 



We find good examples of the second class of articulations 

 those admitting of a slight amount of movement in the vertebral 

 column. Between every pair of vertebrae from the second cervical 

 to the sacrum is an elastic pad, the intervertebral disk, which ad- 

 heres by its surfaces to the bodies of the vertebrae between which it 

 lies, and only permits so much movement between them as can 

 be brought about by its own compression or stretching. When 

 the back-bone is curved to the right, for instance, each of the 

 intervertebral disks is compressed on its right side and stretched 

 a little on its left, and this combination of movements, each in- 

 dividually but slight, gives considerable flexibility to the spinal 

 column as a whole. 



Joints. Articulations permitting of movement by the gliding of 

 one bone over another are known as joints, and all have the same 

 fundamental structure, although the amount of movement per- 

 mitted in different joints is very different. 



Hip-joint. We may take this as a good example of a true joint 

 permitting a great amount and variety of movement. On the 

 os innominatum is the cavity of the acetabulum (Fig. 39), which is 

 lined inside by a thin layer of articular cartilage which has an ex- 

 tremely smooth surface. The bony cup is also deepened a little by 

 a cartilaginous rim. The proximal end of the femur consists of a 

 nearly spherical smooth head, borne on a somewhat narrower neck, 

 and fitting into the acetabulum. This head also is covered with 

 articular cartilage; and it rolls in the acetabulum like a ball in a 

 socket. To keep the bones together and limit the amount of move- 

 ment, ligaments pass from one to the other. These are composed 

 of white fibrous connective tissue (Chap. IV) and are extremely 

 pliable, but quite inextensible and very strong and tough. One is 

 the capsular ligament, which forms a sort of loose bag all round the 

 joint, and another is the round ligament, which passes from the 

 acetabulum to the head of the femur. Should the latter rotate 

 above a certain extent in its socket, the round ligament and one 



