490 



THE MECHANICAL FUNCTIONS. 



leaves of the first cranial vertebra, and assimilat- 

 ing it more to a hinge joint. In birds, however, 

 where, as we have just seen, the most extensive 

 lateral motions are required, the plan of the ball 

 and socket joint is again resorted to; and the occi- 

 pital bone is made to turn upon the atlas by a 

 single pivot. So great is the freedom of motion in 

 this joint, that the bird can readily turn its head 

 completely back upon its neck, on either side. 



As spinous, or transverse processes of any length 

 would have interfered with the flexions of the 

 neck, we find scarcely a trace of these processes 

 in the cervical vertebrae of birds. But another, 

 and a still more important consideration was to be 

 attended to in the construction of this part of the 

 spine. It must be recollected that the spinal mar- 

 row passes down along the canal formed by the 

 arches of the vertebrae, and that any pressure ap- 

 plied to its tender substance would instantly para- 

 lyze the whole body, and speedily put an end to 

 life. Some extraordinary provision was therefore 

 required to be made, in order to guard against the 

 possibility of this accident occurring during the 

 many violent contortions into which the column is 

 liable to be thrown. This is accomplished in the 



simplest and most effec- 

 tual manner by enlarging 

 the diameter of the canal 

 at the upper and lower 

 part of each vertebra, 

 while at the middle it re- 

 mains of the usual size ; 

 so that the shape of the 

 cavity, as is well seen in 

 Fig. 225, which shows a 



225 



