  For M and N finite module categories over a finite tensor category C, the
category Rex_C(M,N) of right exact module functors is a finite module category
over the Drinfeld center Z(C). We study the internal Homs of this module
category, which we call internal natural transformations. With the help of
certain integration functors that map C-C-bimodule functors to objects of Z(C),
we express them as ends over internal Homs and define horizontal and vertical
compositions. We show that if M and N are exact C-modules and C is pivotal,
then the Z(C)-module Rex_C(M,N) is exact. We compute its relative Serre functor
and show that if M and N are even pivotal module categories, then Rex_C(M,N) is
pivotal as well. Its internal Ends are then a rich source for Frobenius
algebras in Z(C).
