NP - Cormac McCarthy - Who's Will

[Lealdo de Góis Andrade]

This is from McCarthy's All The Pretty Horses:

"He’d ride sometimes clear to the upper end of the laguna before the horse
would even stop trembling and he spoke constantly to it in Spanish in
phrases almost biblical repeating again and again the strictures of a yet
untabled law. Soy comandante de las yeguas, he would say, yo y yo sólo ...
While inside the vaulting of the ribs between his knees the darkly meated
heart pumped of who’s will and the blood pulsed and the bowels shifted in
their massive blue convolutions of who’s will and the stout thighbones and
knee and cannon and the tendons like flaxen hawsers that drew and flexed
and drew and flexed at their articulations and of who’s will all sheathed
and muffled in the flesh and the hooves that stove wells in the morning
groundmist and the head turning side to side and the great slavering
keyboard of his teeth and the hot globes of his eyes where the world
burned."

Would any enlightened soul please help me understand what McCarthy's is
trying to convey here by 'who's will'? This left me puzzled.