Intentional Cranial Deformation: A Disappearing Form of Self-mutilation.
Neurosurgery. 37(3):374-382, September 1995.
Gerszten, Peter C. M.D.; Gerszten, Enrique M.D.

Abstract
Of the forms of human self-mutilation that have been recorded, few have been
so widespread and long lasting as intentional cranial deformation. The
earliest known record of the practice is from Iraq and dates back to 45,000
BC. The custom, which was practiced in many areas of the world,

Os incae: normal radiological variant of a separate interparietal bone in
the skull which is of no clinical significance. The name is derived from the
frequent finding of this bone in Peruvian mummies. The inca bone is bounded
laterally by the lambdoid sutures and inferiorly by a transverse suture
joining the right and left mendosal sutures. Normal variation in suture
pattern gives the inca bone a variable appearance it may be triangular,
rectangular (Fig.1), diamond-shaped or M-shaped. Rarely there may be a
persistent superior median fissure running vertically and dividing the bone
into two to give it a bifid appearance occasionally it may be seen as two
separate laterally placed bones.
http://www.medcyclopaedia.com/library/topics/volume_vii/o/os_incae.aspx

The Inca bones, or interparietal bones, occasionally occur as a result of
incomplete fusion of the ossifying nuclei during the developmental course.
http://linkinghub.elsevier.com/retrieve/pii/S1344622302000299

The New World populations have generally high frequencies of the Inca bone,
whereas lower frequencies occur in northeast Asians and Australians.
Tibetan/Nepalese and Assam/Sikkim populations in northeast India have more
Inca bones than do neighbouring populations.
In Central and West Asia as well as in Europe, frequency of the Inca bone is
relatively low.
Relatively high frequencies of the Inca bone in Subsaharan Africans indicate
that this trait is not a uniquely eastern Asian regional character.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=684
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