http://www.accesstoinsight.org/tipitaka/dn/dn.22.0.than.html
[4] "Furthermore... just as if a sack with openings at both
ends were full of various kinds of grain — wheat, rice, mung beans,
kidney beans, sesame seeds, husked rice — and a man with good
eyesight, pouring it out, were to reflect, 'This is wheat. This is
rice. These are mung beans. These are kidney beans. These are
sesame seeds. This is husked rice,' in the same way, monks, a monk
reflects on this very body from the soles of the feet on up, from
the crown of the head on down, surrounded by skin and full of
various kinds of unclean things: 'In this body there are
head hairs, body hairs, nails, teeth, skin, flesh, tendons, bones,
bone marrow, kidneys, heart, liver, pleura, spleen, lungs, large
intestines, small intestines, gorge, feces, bile, phlegm, pus,
blood, sweat, fat, tears, skin-oil, saliva, mucus, fluid in the
joints, urine.'
"In this way he remains focused internally on the body in &
of itself, or focused externally... unsustained by anything in the
world. This is how a monk remains focused on the body in & of
itself.
