SB 10.12.22
astrtayama-margo ’yam
rasanam pratigarjati
esam antar-gatam dhvantam
etad apy antar-ananam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.