The =urinary bladder= (Vesica urinaria) (Figs. 270, 271, 272) differs in
form, size, and position according to the amount of its contents. When
empty and contracted, it is a dense, pyriform mass, about the size of a
fist, lies on the ventral wall of the pelvic cavity, and does not reach
to the inlet. When moderately filled, it is ovoid in form, and extends a
variable distance along the ventral abdominal wall. Its physiological
capacity varies greatly, but may be estimated approximately at about
three or four quarts.
The anterior rounded blind end is termed the =vertex=;[140] on its
middle is a mass of cicatricial tissue (Centrum verticis), a vestige of
the urachus, which in the fœtus forms a tubular connection between the
bladder and the allantois. The =body= or middle part (Corpus vesicæ) is
rounded, and is somewhat flattened dorso-ventrally, except when
distended. It presents two surfaces, dorsal and ventral, the former
being the more strongly convex, especially in its posterior part in
front of the entrance of the ureters.[141] The posterior narrow
extremity, the =neck= (Collum vesicæ), joins the urethra.
The =relations= of the bladder vary according to the degree of fullness
of the organ, and also differ in important respects in the two sexes.
The ventral surface lies on the ventral wall of the pelvis, and extends
forward on the abdominal wall as the bladder fills. The dorsal surface
in the male is related to the rectum, the urogenital fold, the terminal
parts of the vasa deferentia, the vesiculæ seminales, and the prostate;
in the female it is in contact, instead, with the body of the uterus and
the vagina. The vertex of the full bladder is related to coils of the
small intestine and small colon, and to the pelvic flexure of the large
colon.
=Fixation.=—Displacement of the bladder is limited chiefly by three
peritoneal folds, termed the middle and lateral ligaments (Figs. 257,
272). The =middle ligament= (Plica umbilicalis media) is a median
triangular fold, formed by the reflection of the peritoneum from the
ventral surface of the bladder on to the ventral wall of the pelvis and
abdomen. In the new-born animal it is extensive and reaches to the
umbilicus; in the adult it is usually much reduced in length relatively.
It contains elastic and muscular fibers in its posterior part. The
=lateral ligaments= (Plicæ umbilicales laterales) stretch from the
lateral aspects of the bladder to the lateral pelvic walls. Each
contains in its free edge a round, firm band, the =round= or =umbilical
ligament= (Lig. umbilicale s. teres); this is the remnant of the large
fœtal umbilical artery, the lumen of which in the adult is very small.
The retroperitoneal part of the bladder is attached to the surrounding
parts by loose connective tissue, in which there is a quantity of fat.
It is evident that the posterior part of the bladder has a definite
fixed position, while its anterior part is movable.
=Structure.=—The wall of the bladder consists of a partial peritoneal
investment, the muscular coat, and the mucous lining.
The =serous coat= (Tunica serosa) covers the greater part of the dorsal
surface, from which it is reflected in the male to form the urogenital
fold; in the female it passes on to the vagina, forming the
vesico-genital pouch. Ventrally the peritoneum covers only the anterior
half or less of the bladder, and is reflected posteriorly on to the
pelvic floor.
The =muscular coat= (Tunica muscularis) is relatively thin when the
bladder is full. It is unstriped, pale, and not clearly divided into
layers, but has rather a plexiform arrangement. Longitudinal fibers
occur on the dorsal and ventral surfaces, but laterally they become
oblique and decussate with each other. A distinctly circular arrangement
is found at the neck, where the fibers form a =sphincter= (Annulus
urethralis).
The =mucous coat= (Tunica mucosa) is pale and thin. It is in general
attached by a highly elastic submucosa to the muscular coat, and forms
numerous folds when the organ is empty and contracted. It is modified
dorsally in the vicinity of the neck over a triangular area, termed the
=trigonum vesicæ=; the angles of this space lie at the orifices of the
two ureters and the urethra, which are close together. Here the mucous
membrane is closely attached and does not form folds. From each
=ureteral orifice= (Orificium ureteris) a fold of mucous membrane (Plica
ureterica) passes backward and inward, uniting with its fellow to form a
median crest (Crista urethralis) in the first part of the urethra. The
ureteral orifices are a little more than one inch (ca. 3 cm.) apart. The
terminal part of the ureter, after piercing the muscular coat of the
bladder, passes for a distance of about an inch (ca. 2 to 3 cm.) between
the muscular and mucous coats before piercing the latter; this
arrangement constitutes a valve which prevents absolutely the return of
the urine from the bladder into the ureter. The =internal urethral
orifice= (Orificium urethæ internum) lies at the apex of the trigonum,
and is about an inch and a half (ca. 4 cm.) behind the ureteral
orifices. The mucous membrane is covered with transitional epithelium
like that of the ureter and renal pelvis. It contains lymph nodules.
=Blood-supply.=—This is derived chiefly from the vesico-prostatic branch
of the =internal pudic artery=. It is also supplied by small twigs from
the =obturator= and =umbilical arteries=. The =veins= terminate chiefly
in the =internal pudic veins=. They form plexuses posteriorly.
=Lymph Vessels.=—These form plexuses on both surfaces of the muscular
coat. They go to the internal iliac and lumbar glands.
=Nerves.=—The nerves are derived from the =pelvic plexus= (sympathetic
and ventral branches of third and fourth sacral nerves). They form a
plexus in the submucosa which presents microscopic ganglia.
In the fœtus and new-born animal the bladder is situated chiefly in
the abdomen. It is long, narrow, and fusiform. Its abdominal end lies
at the umbilicus, through which it is continued by the urachus to the
extra-embryonic part of the allantois. As the pelvis increases in size
and the large intestine grows, the bladder retracts into the pelvis
and changes its form.
The urethra will be described with the genital organs.