72 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. 



Joint Motions. The wide variety of motions possible to the 

 body group themselves within a small number of classes : flexion, 

 the bending of a joint as at elbow or knee; extension, the straighten- 

 ing of a joint, the opposite of flexion; abduction, the movement of a 

 part away from the axis of the body, as in moving the arm out to 

 the side nt right angles, or the thumb and fore finger in spreading 

 Jin 1 h:md; adduction, the opposite of abduction; rotation, the rolling 

 movement seen when the hand is turned from the palm up to the 

 palm down position, or when one ankle is placed on the opposite 

 knee. There are a few movements, such as the sliding of the jaw 

 from side to side in chewing, that do not fall in any of these classes, 

 but the great majority of joint motions belong either to one of 

 these groups or are combinations of two or more. Thus flexion 

 and abduction of the hip can occur together, or extension and 

 rotation. 



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

 permitting a great amount and variety of movement. On the 



