What's the meaning of ず in 通ず? (Not a negative)


It's from the Japanese saying, 読書百遍意自ずから通ず(Repeated reading makes the meaning
clear). In this case, the ず ending doesn't make the verb negative. What is the
function and nuance of this usage? Is this a verb form from the old days
reserved for use in Japanese proverbs and perhaps some other specific and
narrowly defined instances? Another example where this is used in the same way
is 窮すれば通ず [きゅうすればつうず] (Necessity is the mother of invention). Is there another
example with another verb that uses the ず ending in the same way?




I'm guessing it derives from `通{つう}ずる` (literary form `通{つう}ず` [according to
Daijisen](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E9%80%9A%E3%81%9A%E3%82%8B&dtype=0&dname=0na&stype=0&pagenum=1&index=12293000)),
which is classified by edict as a "zuru verb alternative form of -jiru verbs"
(an alternative form of `通{つう}じる`).

