Most of the deep muscles of forearm are longitudinal, with exception of the pronator quadratus. Thus, most of them extend along the full length of the ulna and radius, from which they originate.
In this deep muscles layer, you can distinguish those lying on the anterior aspect of forearm right in front of the radius and ulna, hugging them, and those located posteriorly (behind these bones).
Anterior aspect: pronator quadratus, pronator teres, flexor pollicis longus, and flexor digitorum profundus. The first two ones, the pronators, pronates the forearm, that is to say, they rotate the forearm and hand; while the third and fourth ones flex the thumb and fingers respectively when you want to grasp something.
Posterior aspect: the abductor pollicis longus, which extends (abducts) the thumb at the wrist; extensor pollicis brevis; extensor pollicis longus, which extends the thumb; supinator, which supinates the forearm, and extensor indicis, which extends (stretches out) the index finger.
Diagram of transverse section cut across the middle of right forearm. Those that make up the deep layer are labeled with their names. The pronator quadratus is not shown here, because it lies in the distal (lower) end of forearm.
