The San Giovanni Eleemosynario Nativity Relief: Detail, the manger
The ox and ass feed appear to feed on the child himself, a reference to medieval interpretations that make them symbolic of Christians nurtured by the Eucharist.
View the entire relief.
Read more about the Nativity.
Source: Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.