474 UNION OF VERTEBRAE IXTO SPINAL COLUMN. 



completely check the movements of the dorsal vertebrae ; 

 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 vertebras 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" 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 



