By N.S. Gill
Chichén Itzá Sacrifice at the Ball Game.receoin
Although the head or heart were selected for the sacrifice as valuable objects, some skulls may have been used for the interior of the rubber balls to make them lighter. The rubber was then wrapped around the skull.
[Source: www.ballgame.org/sub_section.asp?section=2&sub_section=3 "The Mesoamerican Ball Game]