562 THE URINARY ORGANS OF THE HORSE 
folds, termed the middle and lateral ligaments (Figs. 352, 370). The middle liga- 
ment (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 (Plicze um- 
bilicales laterales) stretch from the lateral aspects of the bladder to the lateral pelvie 
walls. Each contains in its free edge a round, firm band, the round ligament (Lie. 
teres vesice); this is the remnant of the large foetal 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 genital 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 pelvie floor. The muscular coat 
(Tuniea 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. Longi- 
tudinal 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 vesicea. 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 con- 
tracted. It is modified dorsally in the vicinity of the neck over a triangular area, 
termed the trigonum vesice; 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 an 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 8 em.) 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 urethre internum) les at the apex of the tri- 
gonum, and is about an inchand a half (ca. 4 em.) behind the ureteral orifices. The 
mucous membrane is covered with transitional epithelium like that of the ureter 
and renal pelvis. It contains lymph nodules. 
Vessels and Nerves.—The arteries are derived chiefly from the internal pudic, 
but branches also come from the obturator and umbilical arteries. The veins 
terminate chiefly in the internal pudic veins. They form plexuses posteriorly. 
The lymph-vessels form plexuses on both surfaces of the muscular coat. They 
go to the internal iliac and lumbar glands. 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 foetus 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 econ- 
tinued by the urachus to the extra-embryonic part of the allantois. The lateral ligaments also ex- 
tend to the umbilicus and each contains in its edge the large umbilical artery. As the pelvis in- 
creases 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. 
