Could you help me with the meaning of the phrase “bound together by some necessity of nerve that excluded him” in the extract below? What does "nerve" mean here? Does it mean "courage" or "nervous system?" Please help me.
“...The boy shrugged and got up. He walked off, across the room to the screen door where he stood looking out. At once Bishop climbed down off his chair and started after him, putting on his hat as he went. Tarwater stiffened when the child approached but he did not move and Rayber watched as the two of them stood there side by side, looking out the door --the two figures, hatted and somehow ancient, bound together by some necessity of nerve that excluded him. He was startled to see the boy put his hand on Bishop's neck just under his hat, open the door and guide him out of it...”