Astanga Hrudaya
पक्वाशयाद्गुदोपस्थं वायुस्तीव्ररुजः प्रयान् । तूणी प्रतूणी तु भवेत्स एवातो विपर्यये ॥ ६२ ॥
Vata produces severe pain, travelling from pakvasaya (large instance) to the Guda (rectum) and upastha (penis ) is known as Tuni; travelling in the revere direction (from rectum and penis) pain travelling to the large intestine is known as pratituni.
english translation
pakvAzayAdgudopasthaM vAyustIvrarujaH prayAn । tUNI pratUNI tu bhavetsa evAto viparyaye ॥ 62 ॥
hk transliteration by Sanscript