rÅ«pa-mÄtrÄd vikurvÄṇÄt
tejaso daiva-coditÄt
rasa-mÄtram abhÅ«t tasmÄd
ambho jihvÄ rasa-grahaḥ
rÅ«pa-mÄtrÄt - which evolves from the subtle element form; vikurvÄṇÄt - undergoing transformation; tejasaḥ - from fire; daiva-coditÄt - under a superior arrangement; rasa-mÄtram - the subtle element taste; abhÅ«t - became manifested; tasmÄt - from that; ambhaḥ - water; jihvÄ - the sense of taste; rasa-grahaḥ - which perceives taste.
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.