562 THE UHINARY ORGANS OF THE HORSE 



folds, termed the middle and lateral ligaments (Figs. 352, 370). The middle liga- 

 ment (Plica umbilicus 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 (Plicse um- 

 bihcales 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 rotmd ligament (Lig. 

 teres vesicae) ; this is the remnant of the large foetal umbilical artery, the lumen of 

 which in the adult is verj^ 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 refiected 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 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. Longi- 

 tudinal fibers occur on the dorsal and ventral surfaces, but laterally they become 

 oblique and decussate with each other. A distinctljr circular arrangement is found 

 at the neck, where the fibers form a sphincter vesicag. 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 vesicae ; 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 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 urethrse internum) lies at the apex of the tri- 

 gonum, 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. 



Vessels and Nerves. — The arteries are derived chiefiy 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 (sj'mpathetic 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-bom animal the bladder is situated chiefly in the abdomen. It is 

 long, narrow, and fusiform. Its abdominal end Ues at the umbilicus, through which it is con- 

 tinued by the urachus to the extra-embryonic part of the allantois. The lateral Ugaments 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 ita 

 form. 



The urethra will be described with the genital organs. 



