tefilla - What does "חפש כל חדרי בטן" mean?

Immediately before the "על חטא" prayer on יום כפור, we say a paragraph that says God knows everything. It includes this passage:

אתה חפש כל חדרי בטן ובחן כליות ולב

In my own translation:

You search all the abdomen's [or: uterus's] rooms and check kidneys and heart.

I know the kidneys are considered the seat of counsel and the heart the seat of feelings and sometimes of thoughts. (See almost any chapter of Mishle, for example.) But what does the abdomen/uterus (בטן) represent, or why do we say God searches it?

Answer

The phrase, "חפש כל חדרי בטן" comes from Proverbs 20:27:

נר ה' נשמת אדם חפש כל חדרי בטן

"The spirit of man is the lamp of the LORD, searching all the inward parts." (JPS 1917)

The word, בטן, is often used simply to mean "belly" (and is therefore associated with pregnancy), but in this context it means the innermost aspects of the human being. Thus, ibn EZra understands it to refer to the heart, writing "והוא רמז ללב שהוא חדר המחשבות" - "and this alludes to the heart, which is the chamber of thoughts".