Mr . Earle on the 
380 
these various ends, this beautiful structure is admirably adapt- 
ed. The broad horizontal planes afforded by the bodies of the 
vertebrae, the mechanical locking of the articular processes, 
and the powerful ligamentous bands which unite them, so con- 
nect the whole as to form one column, whilst the numerous 
articulations into which it is subdivided, which are separated 
by masses of highly elastic matter, at once interrupt the 
effect of concussion, and allow a slight extent of yielding of 
one vertebra upon the other. The motions of the individual 
vertebra are obscure and limited, but the aggregate of the 
whole is considerable. The extent of motion varies in each 
region ; in the back, every thing conspires to limit it ; but in 
the neck and loins it is much greater; and in sawing open 
the spinal canal, we find a very similar provision to that 
which I have before described in birds, namely, an exact cor- 
respondence between the extent of motion permitted, and the 
size and form of the canal. Thus, in the dorsal division, 
where motion hardly exists, its calibre is less ; it is of a 
rounded form, and it is more closely adapted to the size of 
its contents. In the superior cervical vertebrae, where the 
extent of motion is greater, the canal is of a triangular form, 
and is considerably larger in proportion to the spinal cord. 
In the lumbar vertebras it is also triangular, and much more 
capacious than in the dorsal. Obviously, with the same in- 
tention, the theca is very loosely connected with the bony 
canal, and a considerable space is left between it and the other 
membranes, to allow of a sufficient play of one surface on the 
other, so that at the greatest extent of natural curve, no per- 
ceptible stretching of the marrow can take place, which would 
be liable to continual pressure, if, closely enveloped in its 
