474 UNION OF VERTEBRA INTO SPINAL COLUMN. 
completely check the movements of the dorsal vertebras ; 
thereby giving to the trunk that firmness which is required 
for the attachment of the muscles of the wings. The portions 
of bone which pass backwards from the body of each vertebra 
to its transverse processes, and thus form the side-wall of the 
spinal canal, are called the arches of the vertebrae. These are 
the parts first formed. On the under edge of each there is 
a notch which corresponds with one in the upper side of the 
next, in such a manner that, when two vertebrae are placed 
together, a complete foramen or aperture is formed, which 
serves for the passage of the nerves that are given-off from 
the Spinal Cord (§ 457). 
631. The vertebral column of Man is disposed in a double 
curve, as seen in fig. 224 ; the effect of this is to diminish 
the shock that would be produced by a sudden “jar,”—such 
as when a man jumps from a height upon his feet. If the 
vertebral column had been quite straight, this jar would have 
been propagated directly upwards from the pelvis to the head, 
and would have produced very injurious effects upon the 
brain; but by means of the double curvature, and the elasti¬ 
city of the ligaments, &c. which hold together the vertebrae, 
it is chiefly expended in increasing for a moment the curves 
of the spine, which thus acts the part of a spring. The 
constant pressure of the head and upper part of the trunk 
has a tendency to increase these curves permanently, and 
thus to diminish the height of the body. The elasticity of 
the intervertebral substance, however, causes it to recover, 
during the time when the body is in the horizontal posture, 
the form it had lost by pressure in the upright position; and 
thus a man is taller by half an inch or more when he rises 
in the morning, than he was when he lay down the night 
before. 
632. The first vertebra of the neck, termed the atlas, is 
much more movable than the rest, and differs considerably 
from them in its form. It is destitute of body ; but it has a 
broad smooth surface on either side, on which rest the “ con¬ 
dyles 99 of the occipital bone of the skull (§ 618), in such a 
manner that the head is free to nod backwards and forwards. 
The atlas itself turns upon a sort of pivot, formed by an 
upward projection from the next vertebra, which is termed 
the axis; this projection, called from its form the processus 
