Sentence:  The driver ... neither knew any thing, said any thing, or did any thing but watch a dozen equine ears, and keep six reins taut in his hands, and coax the off <WSD>wheel</WSD>[-]<WSD>horse</WSD> with the belly of his whip-lash every two minutes, invariably accompanying the stroke with a tremendous solitary cluck. 
Meaning of the word marked with <WSD> tags:  One of a team of horses which is nearest to the wheels of a carriage, as opposed to a leader or forward horse.