Sūtra 4·1·54
स्वाङ्गाच् चोपसर्जनाद् असंयोगोपधात्
svāṅgāc copasarjanād asaṃyogopadhāt
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And the feminine affix ṅīṣ comes optionally after what ends with the name of a part of the body, when the word is a subordinate member in a compound, and has not a conjunct for its penultimate letter (i. e. the final a is not preceded by a double consonant).
Padapāṭha svaaṅgāt caupasarjanāt asaṃyogaupadhātस्वअङ्गात् चौपसर्जनात् असंयोगौपधात्
