Jerry stood with his back to the wall waiting for the doctor.

The Sargent had barked "strip off and stand against the wall," pointing to which wall he meant. Then he left the room, leaving Jerry on his own.

So, Jerry stripped off and pushed his back against the wall.

Then, nothing. Nobody else entered the room. The sergeant didn't re-enter the room.

Jerry was left standing naked with his back against the wall. 

The room was empty. The walls were white.

After a while all Jerry felt was cold feet.