383 



THE MECHANICAL EUNCTIONS. 



ticular surfaces, one on each side of the spinal canal, formed 

 on' processes corresponding to the leaves of the first cranial 

 vertebra, and assimilating 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 occipital bone is made to 

 turn upon the atlas by a single pivot. So great is the free- 

 dom 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 scarce- 

 ly 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 marrow passes down 

 along the canal formed by the arches of the vertebrae, and 

 that any pressure applied to its tender substance would in- 

 stantly paralyze the w^hole 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 accom- 

 plished in the simplest and most effectual manner, by en- 

 larging the diameter of the canal at the upper and lower part 

 225 ^ of each vertebra, while, at the 



middle, it remains of the usual 

 size, so that the shape of the ca- 

 vity, as is well seen in Fig. 225, 

 which shows a vertical section 

 of one of the cervical vertebrae 

 of the ostrich, resembles that of 

 an hour glass.* Thus, a wide 

 space is left at the junction of 

 each successive vertebra, allow- 

 ing of very considerable flexion, 



• For the specimen from which this engraving wa,s made, I am indebted 

 to the kindness of Mr. Owen. 



