Charak Samhita
नाभिस्तनान्तरं जन्तोरामाशय इति स्मृतः। अशितं खादितं पीतं लीढं चात्र विपच्यते ॥१७॥
[Description of Amashaya(stomach) Surface anatomy of amashaya] It is the part of the human body- the amashaya - situated between umbilicus and the breast line wherein all that is eaten, masticated, drunk and licked up is digested.
english translation