92 SKELETON. 



chanism of the human body disposes us to approach surround- 

 ing objects in that direction; and the musdes of the abdomen, 

 besides their intrinsic strength, act most advantageously in pro- 

 ducing it, by being removed to a great distance from the centre 

 or line of motion. In this position the inter-vertebral carti- 

 lages are diminished or compressed in front, and thickened be- 

 hind, the anterior vertebral ligament is in a state of relaxation, 

 while the posterior vertebral ligament and those which connect 

 the spinous processes are in a state of proportionate tension. 



2. The motion of extension, on the contrary, is much more 

 limited from several causes. The muscles which act in this 

 case, by arising either from the posterior face of the pelvis, or 

 from the transverse processes, and going upwards to be insert- 

 ed either into the ribs, the transverse or the spinous processes, 

 are much less advantageously placed than the abdominal mus- 

 cles, in regard to the length of the lever which they employ. 

 Moreover, mechanical obstruction is opposed to this motion by 

 the spinous processes of the back and neck, being very near to, 

 and overlapping each other. The abdominal muscles also af- 

 ford a strong resistance to its being carried beyond a certain 

 point as any one may assure himself of, by the tension commu- 

 nicated to these muscles from placing a large billet of wood un- 

 der the loins of a subject; and, when they are cut through tran- 

 versely, the immediate consequence is, a great increase in the 

 posterior flexion of the spine, through the agency of the lower 

 dorsal and the lumbar vertebras. The anterior vertebral and 

 the inter-vertebral ligaments, likewise, oppose the extension of 

 the spine, much more than the elastic and the inter-spinous li- 

 gaments do its flexion.* 



3. The lateral inclination of the spine is a motion of consi- 

 derable extent, and is obtained both by the very advantageous 

 position of the muscles on the side of the trunk and neck, and 

 by the little mechanical resistance to it from the shape and ar- 

 rangement of the parts concerned. A principal impediment to 

 this motion being carried beyond a* certain point, is presented 

 by the ribs striking against each other. The transverse pro- 

 cesses of all the vertebrae are so far apart, particularly in the 

 loins, that they scarcely deserve to enter into the estimate of 



