272 THE MECHANICAL FUNCTIONS. 



accommodate itself to various kinds of flexions, and twist- 

 ings of the trunk. While these objects are provided for, 

 car^ust at the same time be taken that the spinal marrow 

 it encloses shall, amidst all these motions, remain secure from 

 pressure; for so delicate is its structure that the least degree 

 of compression would at once interrupt its functions, and 

 lead to the most fatal consequences. A safe passage is like- 

 wise to be afforded to the nerves, which issue from the spi- 

 nal marrow, at certain intervals, on each side throughout 

 its whole length. 



No where has mechanical art been more conspicuously 

 displayed than in the construction of a fabric capable of ful- 

 filling these opposite, and apparently incompatible functions. 

 The principal difficulty was to combine great strength with 

 sufficient flexibility. This we find accomplished, first, by 

 the division -of the column into a great number of pieces, 

 each of which being locked in with the two adjoining 

 pieces, and tightly braced by connecting ligaments, is al- 

 lowed but a very small degree of flexion at the point of junc- 

 tion. This slight flexion at each single joint, however, by 

 becomino- multiplied along the series, amounts to a consider- 

 able 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,) composed of a peculiar substance, intermediate 

 in its texture to ligament and cartilage, and possessing in a 

 remarkable degree the qualities of toughness and adhesion, 

 united with compressibility 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 -con- 

 cussions incident to violent action: for we find that, how- 

 ever the spine may be bent, no chasm is lefLby the flexions 

 of the vertebrae upon one another, nor is tlie continuity of 

 the column in the smallest degree interrupted. 



The motions of the vertebrae upon each other are farther 

 regulated by the mode in which their articular processes, 



