Charak Samhita
तरुणाः कृष्णसर्पास्तु गोनसाः स्थविरास्तथा । राजिमन्तो वयोमध्ये भवन्त्याशीविषोपमाः ॥१३६॥
॥ Characters of snakes of different ages॥ Black snakes in young age, gonasa in old age, rajimana in middle age are like a sivisha (dreadful snakes with poisoned vision and breadth).
english translation
taruNAH kRSNasarpAstu gonasAH sthavirAstathA । rAjimanto vayomadhye bhavantyAzIviSopamAH ॥136॥
hk transliteration by Sanscript