Student or Learner
Turjan sat in his workroom, legs sprawled out from the stool, back against and elbows on the bench. Across the room was a cage; into this Turjan gazed with rueful vexation. The creature in the cage returned the scrutiny with emotions beyond conjecture.
(The Dying Earth; Jack Vance)
Would you be so kind as to tell me whether it means 'the rear part of the human body' and if so, why there is no 'his' before it?
There is no need for "his". It's clear whose legs, back and elbows it refers to.
The back is the part of the body which runs from the base of the neck to the top of the buttocks and down which the spine runs.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.