FORTRAN with R

Hello, I am currently an undergrad working under a professor at our university and I was asked to find a way to run a subroutine in FORTRAN by using R.  What he wants is to run the commands through R and have R do the work and then send a command to FORTRAN (since R is FORTRAN based) asking it to run a subroutine, so that in essence all of the number crunching goes faster!  If there is anyone out there that knows of a way to do this, has some example code, really anything I would appreciate it because I have looked and looked and cannot figure it out!

Many Thanks,
Josh