Charak Samhita
दद्यान्नासापुटे स्नेहं तर्पणं बुद्धिमान् भिषक्। अनवाक्शिरसो नस्यं न शिरः प्रतिपद्यते॥१००॥
If the head is not lowered at all, the medication does not reach the head. If the head is excessively lowered the nasya medication may reach into the brain.
english translation
dadyAnnAsApuTe snehaM tarpaNaM buddhimAn bhiSak। anavAkziraso nasyaM na ziraH pratipadyate॥100॥
hk transliteration by Sanscript