390 THE MECHANICAL FUNCTIONS. 



tightly braced by connecting ligaments, is 

 allowed but a very small degree of flexion at the 

 point of junction. This slight flexion at each 

 single joint, however, by becoming multiplied 

 along the series, amounts to a considerable 

 degree of motion in the whole column. 



The broad basis of each bone is connected 

 with the next, not by a joint, but by a plate of 

 equal breadth (M, M, Figures 178 and 179), com- 

 posed of a peculiar substance, intermediate in 

 its texture to ligament and cartilage, and pos- 

 sessing in a remarkable degree the qualities of 

 toughness and adhesion, united with compres- 

 sibility and elasticity. By yielding for a certain 

 extent to a force tending to bend it to either side, 

 it diminishes the quantity of motion which would 

 otherwise have been required in each individual 

 joint ; and by acting at the same time as a 

 spring, it softens all the jars and concussions 

 incident to violent action : for we find that how- 

 ever the spine may be bent, no chasm is left by 

 the flexions of the vertebrae upon one another, 

 nor is the continuity of the column in the 

 smallest degree interrupted. 



The motions of the vertebrae upon each other 

 are further regulated by the mode in which their 

 articular processes, which are the pieces that 

 project obliquely on each side, play upon each 

 other. These processes, which are seen at A, A, 

 in the preceding figures (177 and 178), are of 



