रसमात्रमभूत्तस्मादम्भो जिह्वा रसग्रह: ॥ ४१ ॥
rasa-mātram abhūt tasmād
ambho jihvā rasa-grahaḥ
rūpa-mātrāt — which evolves from the subtle element form; vikurvāṇāt — undergoing transformation; tejasaḥ — from fire; daiva-coditāt — under a superior arrangement; rasa-mātram — the subtle element taste; abhūt — became manifested; tasmāt — from that; ambhaḥ — water; jihvā — the sense of taste; rasa-grahaḥ — which perceives taste.
By the interaction of fire and the visual sensation, the subtle element taste evolves under a superior arrangement. From taste, water is produced, and the tongue, which perceives taste, is also manifested.
The tongue is described here as the instrument for acquiring knowledge of taste. Because taste is a product of water, there is always saliva on the tongue.