THE SPINAL COLUMN OF BIRDS. 151 



yet nothing more effectual ; the canal whicli tra- 

 verses each vertebra, is not of the same uniform 

 diameter throughout, but is much larger at 

 each end of the bone, whence it contracts 

 gradually to the centre ; its diameter at the 

 centre is just adapted to the size of the cord 

 and its sheath ; but at each extremity it is 

 much larger than would be needed, were no 

 flexures of the whole column to be made. The 

 annexed rough diagram supposes a section of 



two cervical vertebras, a, the vertebrae ; b, 

 the spinal canal ; C, the spinal cord. Now, 

 the central narrowness and the gradual ex- 

 pansion of the canal through each vertebra, 

 tend to accompUsh the following object — 

 namely, the susceptibility of a great flexure of 

 any two given vertebrsB at their joint, without 

 compressing the spinal cord, and indeed with- 

 out bending it in an equal ratio ; in fact-, the 



