VERTEBRAL COLUMN. 93 



resistances. As the muscles of the one side produce the late- 

 ral curvature, so their resistance on the other limit it to a cer- 

 tain extent, as may be readily ascertained by cutting them 

 through. 



4. The circumduction of the spine is that motion in which 

 the trunk is caused to describe a cone, the base of which is 

 above, and the apex below. It is performed on the lower dor- 

 sal and the lumbar vertebrae, and is a succession of the move- 

 ments already described. 



5. The rotation of the spine is a very limited motion. It is 

 performed almost entirely on the lower dorsal and the upper 

 lumbar vertebrae, and presents in its analysis a series of minute 

 and oblique slidings of the body of one vertebra upon another, 

 the pivot being the oblique processes. The action occurs by 

 the lateral yielding of the inter-vertebral substance; it must, 

 therefore, be almost inconceivably small in any individual sub- 

 stance, particularly when the latter has been hardened and ren- 

 dered more fibrous by old age. In the very young subject it 

 is more appreciable. 



Of the Motions peculiar to each Class of Vertebra. 



1. The cervical vertebrae, as a whole, enjoy a considerable 

 share of flexion, extension, lateral inclination: and of circumduc- 

 tion, as the result of the other motions. Their rotation, or the 

 oblique sliding of one vertebra upon the other, is very limited. 

 The apparent facility with which they are twisted upon each 

 other, when the face is turned to the shoulders alternately, is 

 almost wholly the motion of the first vertebra upon the second, 

 the participation of the other vertebrae being very inconsidera- 

 ble. The possibility of the dislocation of these vertebrae, with 

 the exception of the first, is very stoutly denied by authorities 

 of the first standing in anatomy, on the score that too great a 

 resistance to this accident is afforded by the inter-vertebral and 

 yellow ligaments, by the inter-spinal and inter- transverse mus- 

 cles, by the inter-locking of the bodies of the vertebrae through 

 their reciprocal concavities and convexities, and by the shape 

 and extent of their oblique processes. 



