A nerve plexus is a branching network of nerves which serves a part of the body. Nerves which go to and from the central nervous system bunch up in certain places. Between each vertebra is a plexus of nerves going in and coming out of the spinal cord. Another plexus is underneath each arm. That is a plexus of nerves and also blood vessels which serve every part of the arm, hand and fingers. Under the arm is much the safest place to have these things because of the damage they would get if too exposed. This also explains why arteries are deeper under the surface than veins.