92 THE HUMAN BODY. 



unite the majority of the cranial bones afford a good example. 

 Except the lower jaw, and certain tiny bones inside the tem- 

 poral bone belonging to the organ of hearing, all the skull- 

 bones are immovably joined together. This union in most 

 cases occurs by means of toothed edges which fit into one 

 another and form jagged lines of union known as sutures. 

 Some of these can be well seen in Fig. 29 between the 

 frontal and parietal bones (coronal suture) and between the 

 parietal and occipital bones (lambdoidal suture)', while an- 

 other lies along the middle line in the top of the crown 

 between the two parietal bones, and is known as the sagittal 

 suture. In new-born children where the sagittal meets the 

 coronal and lambdoidal sutures there are large spaces not yet 

 covered in by the neighboring bones, which subsequently 

 extend over them. These openings are known asfontanelles, 

 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 admitting 

 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 interqer- 

 tebral disk, which adheres by its surfaces to the bodies of the 

 vertebras 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 individually but 

 slight, gives considerable flexibility to the spinal column as a 

 whole. 



Joints. Articulations permitting of movement by the glid- 

 ing 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 movement. 

 On the os innominatum is the cavity of the acetabulum (Fig. 

 43), which is lined inside by a thin layer of articular carti- 



