The posterolateral tunnel on the superior surface of the first cervical (at
las) vertebra is of normal occurrence in monkeys and other lower animals, b
ut its presence in the form of a tunnel-like canal, for the passage of the
third part of the vertebral artery over the posterior arch of the human atl
as vertebra is not reported. The aim of the present study was to detect the
presence of such a canal, in addition to other types of ponticuli (little
bridges) reported by earlier investigators, in macerated atlas vertebrae an
d routine cadaveric dissections. The posterolateral tunnel was detected in
1.14%, and the posterior and lateral ponticuli in 6.57 and 2% of vertebrae.
Probably the bony roof of the posterolateral tunnel serves the purpose of
additional lateral extension for the attachment of the posterior atlanto-oc
cipital membrane in quadrupeds, where the load of the head is supported by
the extensor muscles of the neck, ligaments and posterior atlanto-occipital
membrane. In man, where the weight of the head is borne by the vertical lo
ading of the superior articular process of the atlas, the roof of the tunne
l has disappeared.