The anatomists have described the human urinary bladder as a pear-shaped structure made up of an external longitudinal layer of muscle fibers, a middle circular, and an inner longitudinal layer extending from the ureteral orifices down into the urethra. These fibers can be traced to the area about the verumontanum in the male bladder and to the vicinity of the external sphincter in the female. This layer of muscle fibers is easily dissected free from the underlying muscle fibers and is usually called the trigone. We have found the two remaining layers not to be distinct but to consist of freely interlacing muscle fibers, and we prefer to designate them as the detrusor muscle of the bladder.
Scher1 has recently described "right and left lateral muscle bands which are more distinct and separate than the trigone muscle." These bands are described as originating in the posterior wall of the
Van Duzen RE, Duncan CG. ANATOMY AND NERVE SUPPLY OF URINARY BLADDER. JAMA. 1953;153(15):1345-1347. doi:10.1001/jama.1953.02940320017005