THE FEMALE URETHRA
The female urethra runs caudally on the pelvic floor below the reproductive tract. It passes obliquely through the vaginal wall to open ventrally at the junction of vagina and vestibule (Figure 5-35).
Its length and breadth vary considerably among species; it is conspicuously short and wide in mares. In some animals, such as the cow and sow, it opens together with a suburethral diverticulum (Figure 5-32Z72') and in others, such as the bitch, on a hummock. Both arrangements create difficulties when catheterization of the bladder is attempted.When a diverticulum is present, it is enclosed within the urethralis, which surrounds the urethra along most of its length. The cranial fascicles of this muscle encircle the urethra, while the caudal ones support it within U-shaped loops that arise and end on the vaginal wall. Contraction of this part of the muscle closes the urethra by pressing the two organs together; it also narrows the vagina. The urethralis obtains a somatic innervation through the pudendal nerve, but sympathetic and parasympathetic involvement is also described.
The urethral submucosa contains many veins that constitute a form of erectile tissue that may contribute to continence by assisting mucosal apposition. These features apart, the structure of the urethra continues that of the bladder.