Urinary Bladder and Urethra
The urinary bladder is a hollow, muscular organ that varies in size and position depending on the volume of its contents. In the horse and ox the bladder lies within the pelvic cavity, but in the dog and cat it extends into the abdomen.
The wall of the bladder comprises an inner transitional epithelium, three muscle layers and an outer serosa that is actually peritoneum. In the horse and ox the bladder is retroperitoneal, but as it fills with urine it becomes intraperitoneal.Three vestiges of embryonic structures are visible in the outer wall of the bladder. These are the paired lateral vesical folds, originally the umbilical arteries, and the median vesicle fold, originally the urachus. The lateral vesicle folds become the round ligaments of the bladder.
The paired ureteric openings are located near the neck of the bladder and form a triangle with the exit of the urethra; the arrangement is called the trigone.
The urethra of the female lies ventral to the reproductive tract and opens at the junction of the vagina and vestibule. The urethra is lined by a mucous membrane, and its wall comprises circular and longitudinal smooth muscle. The urethra of the mare is short and that of the cow opens in association with a suburethral diverticulum. In the bitch the urethra opens on a hillock that can be a problem when inserting a catheter.
The urethra of the male consists of three sections. The first of these passes through the prostate gland, where it receives the paired deferent ducts. The middle section is the membranous portion and the distal section is the cavernous (penile) portion.
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