...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.'
Maha-satipatthana Sutta-The Great Frames of Reference