16 



THE HUMAN MECHANISM 



FIG. 8. Sixth thoracic 

 vertebra 



Seen from above 



upper thoracic region, the curvature is slightly convex ; in 

 the thoracic region it is quite concave ; in the lumbar region 

 slightly convex ; and in the sacral-coccygeal region again 



concave. It may well be asked how 

 these separate vertebrae, piled, as it 

 were, one above another, maintain 

 their proper relative positions. This 

 is partly due to the shape of the 

 individual vertebrae, partly to the 

 ligaments (p. 17) which pass from 

 one vertebra to another and limit 

 the movements of each, and partly 

 to the action of muscles which are 

 placed upon opposite sides of the 

 vertebrae and by their antagonistic 



action hold them in place. The action of muscles and liga- 

 ments upon the bones may be illustrated by two blocks of 

 wood held together by two rubber bands (w, m', Fig. 11) 

 slightly stretched ; so long as each 

 pair of opposite bands pulls with 

 the same force, the blocks are kept 

 iii what we may call their resting 

 position. Here the rubber bands 

 represent two of the antagonistic 

 muscles, which, by maintaining a 

 steady and equal pull on the oppo- 

 site sides of the vertebree, keep 

 them in place. Should one pull 

 harder than its antagonist, as when 

 a muscle contracts (see Chap. IV), 

 the antagonist will be stretched 

 and the bones become inclined 

 toward one another, as shown in right portion of Fig. 11. 

 This principle of muscular antagonism is quite general in 

 the maintenance of the proper relative positions of bones in 



FIG. 9. Sixth thoracic 

 vertebra 



Seen from the side 



