Not forgotten just omitted in this particular example. You can see its impression in the flexor muscles of the forearm, and you can also see now without the aponeurotic extension clearly the continuance of the bachialis and the pronator teres muscles as they extend towards their insertion points.
wow these are one of the most easily understood anatomy sketches ive ever seen you wouldnt happen to know what the muscles in the forearm looks like when u twist ur wrist would you? or rather where i might acquire that information
You could use your imagination while looking at your own arm and noting where the muscles are currently as appose to where they were. If that doesn't work my best advice would be to get an anatomy book, one that has the pose you are looking for or that will give you a the knowledge of how to sketch the orientation of the arm you are looking for.
im moderately fat but its been on and off for a while thats its evenly spread so i cant see my muscles i get a basic idea but... and the poses i look for a are hard to find coz for some reason i like to torture myself