181. In the interior of a Sanskrit word

Hence, in the interior of a Sanskrit word, the dental s is not usually found after any vowel save a and ā, but, instead of it the lingual ṣ. But—

a. A following r prevents the conversion: thus, usra, tiaras, tamisra. And it is but seldom made in the forms and derivatives of a root containing an r-element (whether r or ṛ), whatever the position of that element: thus, sisarti, sisṛtam, sarīsṛpá, tistire, parisrút. To this rule there are a few exceptions, as viṣṭír, viṣṭārá, níṣṭṛta, víṣpardhas, gáviṣṭhira, etc. In ajuṣran the final ṣ of a root is preserved even immediately before r.

b. This dissimilating influence of a following r, as compared with the invariable assimilating influence of a preceding r, is peculiar and problematic.

c. The recurrence of ṣ in successive syllables is sometimes avoided by leaving the former s unchanged: thus, sisakṣi, but siṣakti; yāsisīṣṭās, but yāsiṣīmahi. Similarly, in certain desiderative formations: see below, 184e.

d. Other cases are sporadic: RV. has the forms sisice and sisicus (but siṣicatus), and the stems ṛbī́sa, kīstá, bísa, busá, bṛ́saya; a single root pis, with its derivative pesuka, is found once in ÇB.; MS. has mṛsmṛṣā́; músala begins to be found in AV.; and such cases grow more numerous; for puṁs and the roots niṅs and hiṅs, see below, 183a.