[4] The plaintiff's account of the first incident in evidence in chief was as follows. In dismounting from the bin on the dog trailer, he had difficulty in obtaining a firm foothold on a ladder attached to the end of the bin because the limited distance between the rungs of the ladder and the bin wall permitted him to put only the front part of his boots on the rungs. It was thus necessary, to obtain greater support, to position the soles of his boots on an angle to the rungs. As he was attempting to place one foot on a rung, his other foot, which was on the top rung, slipped. He thought he was going to fall sideways across the dog trailer's drawbar and attempted to obtain a footing on the drawbar but his foot slipped on it. Trying to land clear of the drawbar, he pulled himself away from it, landed on his feet on the ground, lost his footing and, in trying to stop himself falling to the ground, he "... put (his) hands on the ground as (he) gathered (himself) and stopped ... at the time (he) felt something in (his lower) back go."