7.3.50 ठस्येकः
संधि विच्छेद : ठस्य इकः
अनुवृत्ती : अङ्गात्
Language meaning : of ठ, इक्, at अङ्ग
physical (लौकिक) meaning : At what goes to outside(अङ्ग), instruction of foundation (ठ) becomes inward stone (इक्)
commentary
ठ includes following प्रत्यय
ठक् = unsderside stone
ठञ् = underside surface
ठण् = push underside
ठन् underside pull