476 THE URINARY ORGANS OF THE HORSE 



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 ; ^ on its middle is a mass 

 of cicatricial tissue (Centrum verticis), a vestige of the urachus, which in the fcetus 

 forms a tul)ular connection between the bladder and the allantois. The body or 

 middle jmrt (Corpus vesicae) 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 en- 

 trance of the ureters.-' The posterior narrow extremity, the neck (C'ollum vesica?), 

 joins the urethra. 



The relations of the bladder vary accorcUng 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 vcsiculse 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 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 al)domen. In the new-born animal it is extensive and reaches to the um- 

 bilicus; in the adult it is usually much reduced in length relatively. It contains clas- 

 tic and muscular fibers in its posterior part. The lateral ligaments (Plicne 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 fo'tal 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 i)eri- 

 toneum covers only the anterior half or less of the l)ladder, and is reflected i)osteri- 

 orly on to the pelvic floor. 



The muscular coat (Tunica muscularis) is relatively thin when the bladder is 

 full. It is unstri])ed, pale, and not clearly divided into layers, but has rather a 

 plexiform arrangement.^ Longitudinal fil)ers occur on the dorsal and ventral sur- 

 faces, but laterally they become oblique and decussate Avith each other. A dis- 

 tinctly 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 vesicae ; the angles of this 

 space lie at the orifices of the two ureters and the urethra, which are close together. 



' This is commonly termed the fundus by veterinarians, but is not the homologue of the 

 fundus of the human l)ladder. 



^ This would correspond to the fundus vesica; of man. 



