Mr, Earle on the 
278 
freedom of motion, appears to be intended, at the same time, 
to guard against the possibility of any undue pressure on the 
spinal marrow. This is very readily demonstrated by re- 
moving the ligamentum nuchas and membrane which closes 
the above mentioned opening. The spinal marrow, enveloped 
in its membranes, will immediately come in view. Its outer 
membrane is very vascular, and of a delicate structure, and 
is connected with the canal by a fine filamentous cellular 
substance ; it is larger than the inner membrane, with which 
it is but loosely connected. 
When the spinal canal has been thus exposed, the indivi- 
dual vertebrae may be bent backward to a right angle, and 
laterally to an angle of 45 0 , without in the least compressing 
the marrow which occupies so small a space of the whole 
calibre of the canal at each articulation, as to be quite secured 
from any injury from this motion. The design, in this struc- 
ture, becomes even yet more obvious, on viewing the whole 
extent of the spinal cord. It is nearly of the same size 
throughout, diminishing very gradually from above down- 
wards, and completely occupies the narrow central part of 
the canal of each cervical vertebra, where no motion can 
affect it. The same may be observed in that part of the 
spine which corresponds to the dorsal and lumbar divisions, 
which in birds do not admit of motion ; for here we find 
no variation, either in the spinal canal or the marrow, except 
where the numerous branches are given off to form the great 
sciatic plexus, to supply the lower extremities, where it swells 
out into a bulbous shape, corresponding to the cavity in the 
bone. 
Before quitting the subject of the spine in birds, it will be 
