The Collaborative International Dictionary of English v.0.48:

Heartstring \Heart"string`\ (h[aum]rt"str[i^]ng`), n.
   A nerve or tendon, supposed to brace and sustain the heart.
   --Shak.
   [1913 Webster]

         Sobbing, as if a heartstring broke.      --Moore.
   [1913 Webster]