[R] how to study the code of R

[rongguiwong]

i want to study R programming by studying the existing code from R itself,but 
i don't know how to read the code,can any one give me some guide?
my R is installed in /usr/lib/R/

[ronggui at mylinux ronggui]$ /usr/lib/R/
afm      bin      doc      etc      include  library  modules  share


> version
         _
platform i586-mandrake-linux-gnu
arch     i586
os       linux-gnu
system   i586, linux-gnu
status
major    1
minor    9.1
year     2004
month    06
day      21
language R
>