564 THE MECHANICAL FUNCTIONS. 



of very considerable flexion, without reducing 

 the diameter of the canal beyond that of the 

 narrow portion, and therefore without producing 

 compression of the spinal marrow. Mr. Earle 

 found t that vertebrae united in this manner may 

 be bent backwards to a right angle, and laterally 

 to half a right angle, without injury to the en- 

 closed nervous substance. The design of this 

 structure is further evident from its not existing 

 in the dorsal and lumbar portions of the spine, 

 which admit of no motion whatever, and where 

 there is no variation in the diameter of the spinal 

 canal. 



A plan entirely different is followed in the 

 vertebrae of the back and loins. For the jDurpose 

 of ensuring the proper actions of the wings, the 

 great object here is to prevent motion, and to 

 give all possible strength and security ; and ac- 

 cordingly the whole of this portion of the spine, 

 together with the sacrum, is consolidated into 

 one piece. All the processes are largely deve- 

 loped, and pass obliquely from one vertebra to 

 the next, mutually locking them together : and, 

 in order most effectually to preclude the possi- 

 bility of any flexion, the spinous processes, and 

 sometimes even the bodies of the dorsal vertebrae 

 are immoveably soldered together by ossific mat- 

 ter, so as to form one continuous bone. 



The sacrum (v, Fig. 224) consists of the union 

 of a great number of vertebrae, as many as twenty 



f In the paper already quoted, p. 278. 



