[R-sig-Geo] Can I read only a portion of a raster?

[ben harrison]

I thought I could do something like this:

library(raster)
my.ext <- extent(xmn, xmx, ymn, ymx)
my.r <- raster("raster.tif", ext=my.ext)

But it still reads in the entire file. How can I read in only the 
portion I need?