Location of Internal Code

Hello,

This seems like a fairly elementary question, but I couldn't seem to
find the answer anywhere online.

Where can I find code which is called with .Internal? ?Specifically,
the R function colSums() calls an internal function with the same name
(I presume a C function), and I'd like to see how it works.

I've searched through the header files in R-2.x.x/include/ (Windows
version), but to no avail.

Thanks,

Robin

Robin Evans
Statistics Department
University of Washington
www.stat.washington.edu/~rje42