Fig. 7. Models of trigone formation. (A) Old model of trigone formation, showing the trigone to be continuous with the ureters (green), formed in large part from ureteral fibers that fan out across the surface generating the inter-ureteric ridge and Bell's muscle. Note that the trigone has been considered to form independently of the bladder. (B) Current model of trigone formation, showing a small contribution from ureteral fibers (green) and the bulk of the structure derived from bladder muscle and the space around the ureter that functions as a tunnel.