Garudapurana 1.156.5

1.156.5

bālyapravāhiṇī tāsāmantramadhye visarjinī /
bāhyāsaṃvaraṇe tasyā gudādau bahiraṅgule //

Among them, the inner is pravāhiṇī, in the middle of the bowel is visarjinī, / and the outer is saṃvaraṇī, at the anus, extending outward by a finger-breadth;