Vatthu: 'physical base', i.e. the 6 physical organs on which the mental process is based, are the 5 physical sense-organs and, according to the Com., the heart hadaya-vatthu, as the 6th. This 6th vatthu must not be confounded with the 6th Ayatana which is a collective name for all consciousness whatever.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.