Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 22
Sanskrit:
आस्तृतायाममार्गोऽयं रसनां प्रतिगर्जति ।एषामन्तर्गतं ध्वान्तमेतदप्यन्तराननम् ॥ २२ ॥
ITRANS:
āstṛtāyāma-mārgo ’yaṁrasanāṁ pratigarjatieṣāṁ antar-gataṁ dhvāntametad apy antar-ānanam
Translation:
In length and breadth the animal’s tongue resembles a broad traffic-way, and the inside of its mouth is very, very dark, like a cave in a mountain.