The posterior aspect of the rectum exposed by removing the lower part of the sacrum and the coccyx. (Puborectalis not labeled, but levator ani labeled at bottom right, and sphincter ani ext labeled at bottom center.)
Left Levator ani from within. (Puborectalis not labeled, but 'levator ani' labeled at center, and anal canal labeled at bottom center.)
They meet with the corresponding fibers of the opposite side around the lower part of the rectum, and form for it a strong sling. Relaxation increases the angle between rectum and anus, allowing defecation in conjunction with relaxation of the internal and external sphincters.
Puborectalis is part of the levator ani group of muscles.
^Wallner C, Maas C, Dabhoiwala N, Lamers W, Deruiter M (2006). "Evidence for the innervation of the puborectalis muscle by the levator ani nerve.". Neurogastroenterol Motil18 (12): 1121–2. doi:10.1111/j.1365-2982.2006.00846.x. PMID17109696.