^4 THE HUMAN BODY. 



blood into the brain, distends it and causes it to push out 

 the skin where bone is absent. Another good example 

 of an articulation admitting of no movement, is that between 

 the rough surfaces on the sides of the sacrum and the in- 

 nominate bones. 



We find good examples of the second class of articula- 

 tions 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 adheres by its surfaces to the 

 bodies of the vertebrae between which it lies, and only per- 

 mits 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 move- 

 ments, each individually 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 permitted 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 move- 

 ment. On the os innominatum is the cavity of the aceta- 

 bulum (Fig. 40), which is lined inside by a thin layer of 

 -articular cartilage, which has an extremely 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 movement, ligaments pass from one to the 

 other. These are composed of white fibrous connective 

 tissue (Chap. VIII.) and are extremely pliable but quite 

 inextensible and very strong and tough. One is the cap- 

 sular ligament, which forms a sort of loose bag all round 



