This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
On the interior of the base of the bladder the ureters open about 2.5 cm. (1 in.) posterior to the urethral orifice, and the same distance (or more if the bladder is distended) from each other. The included triangular space is called the trigone. Its mucous membrane is without the rugae possessed by the rest of the bladder and, if it is distended, is not quite so pale in color. The ureters pass obliquely through the walls a distance of 1.25 cm. (1/2 in.) and cause slight elevations of the mucous membrane called the plicae uretericae or ureteric folds. Joining the two ureteral orifices is a fold of mucous membrane called by Kelly the interureteric ligament. The part immediately posterior to this fold is the postprostatic Pouch or bas-fond. It becomes enlarged in prostatics, and then contains residual urine. Calculi also tend to lodge there (Fig. 452).
The bladder is composed of a muscular wall covered externally by the peritoneum and internally by the submucous and mucous coats. In the undistended bladder blood-vessels can be seen in the mucous membrane, which is in folds. These folds and vessels diminish or disappear when the bladder is distended. The membrane at the trigone is more firmly connected to the muscle beneath than elsewhere in the bladder, hence its smoothness and increased color. The muscular coat is composed of two longitudinal layers with one more or less transverse layer between. The external layer is continuous with the ureters, and over the prostate to be attached to the lower posterior part of the pubes under the name of pubovesical muscle. The circular fibres are continued around the opening of the urethra, forming the internal sphincter. The openings of the ureters are not closed by muscular action, but by the interior pressure. When the bladder is distended, if the ends of the ureters are thickened they do not close as the urine accumulates, but allow it to back up and distend the ureters and pelvis and even cause the kidney itself to become enlarged. Thus infection ascends from the bladder to the kidney and the ureters become distended until they may equal in size the small intestine.
The fibres of the muscular coat pass in various directions, more or less in the form of bundles. When these bundles become hypertrophied they can be seen as distinct ridges on the interior of the bladder. In sounding they can be felt and recognized by the tip of the sound. Such a condition is called a ribbed bladder. If the bladder becomes hyperdistended the fibres become separated and the mucous membrane bulges out, forming a sac. It is then called a sacculated bladder. These sacs are favorite lodging places for vesical calculi. From diseases of the prostate and urethra the muscular coat becomes thickened. It is then called a hypertrophied bladder; such a one is usually contracted.
The bladder walls ordinarily are quite thin, about 3 mm. (1/8 in.) thick. When hypertrophied they are three or four-times as thick. When the bladder is viewed in life in abdominal operations it usually appears as a somewhat flaccid sac. It does not assume the globular form until considerably distended and must contain a moderately large amount of urine before showing above the symphysis. The commonly flaccid condition of the bladder leads one to think that its emptying is largely favored by the pressure of the intestines compressing it against the floor of the pelvis, and that it is mainly in case of considerable distention or the pressure of irritation or disease that its own muscular coat is utilized for, the purpose. This view is strengthened by the increase in flow when coughing and by the occurrence of bladder troubles (prolapse, etc.) so soon as the integrity of the pelvic floor is injured, as occurs in rupture of the perineum from childbirth. The laxity of the bladder walls allows it to spread side-wise to the neighborhood of the inguinal rings, and it has frequently been found in the inguinal canal and has been wounded in operating for hernia. The urethral orifice in the male is about 6.25 cm. (2 1/2 in.) from the surface at the upper margin of the symphysis in a downward and backward direction; with the body in a vertical position it might be said to lie on a level with the middle of the symphysis if the bladder is empty, lower if the bladder is distended, and slightly higher if the rectum is distended. It is therefore within easy reach of the finger inserted through a suprapubic incision.