The Urinary Bladder and Pelvic Urethra

The urinary bladder is a muscular, expandable sac in the lower abdomen that serves as a temporary storage space for urine produced by the kidneys.  The urinary bladder has an apex, a body, and a neck. Three connecting peritoneal folds (ligaments) are reflected from the bladder on the pelvic and abdominal walls. The median ligament of the bladder leaves the ventral surface of the bladder and attaches to the abdominal wall as far cranial as the umbilicus. In the fetus it contains the urachus and umbilical arteries. These degenerate and leave no ligamentous remnant in the adult. The lateral ligaments of the bladder pass from the left and right sides of the bladder to the pelvic wall and often contain an accumulation of fat along with the ureter and umbilical arteries (in the fetus, or if still patent in the adult).  The round ligaments of the bladder, along the cranial free edges of the lateral ligaments of the bladder, are the adult remnants of the fetal umbilical arteries once they are no longer patent.  All of the structures of fetal circulation will be covered in the lab over placentas.

Bundles of smooth muscle on the surface of the bladder pass obliquely across the neck of the bladder and the origin of the urethra. These bundles, collectively called the detrusor m., are innervated by the pelvic nerve (sacral parasympathetic neurons).

Recall that the ureters lie opposite each other near the neck of the urinary bladder. The trigone of the bladder is the dorsal triangular area located within the line drawn between the two ureteral openings and the lines connecting each ureteral opening with the urethral exit from the bladder.

No gross anatomical sphincter is present in the neck of the bladder, but a physiological sphincter of smooth muscle is innervated by the hypogastric nerves, which supply sympathetic visceral efferent innervation. This smooth muscle is called the internal urethral sphincter and is continuous with the detrusor muscle.  It’s responsible for the involuntary control of urine flow from the bladder through the internal urethral orifice, which is the opening between the urinary bladder and pelvic urethra.  

The male urethra is composed of a pelvic part (pelvic urethra) within the pelvis and a penile part within the penis (penile urethra).   The prostate gland, a male accessory sex gland that contributes to the production of semen,  completely surrounds the neck of the bladder and the beginning of the pelvic urethra.   The subsection of pelvic urethra which passes through the prostate gland, which will be identified later, in males is sometimes called the prostatic urethra.  Females, lacking both a penis and prostate gland, possess only a pelvic urethra.

FYI – Not far into the pelvic urethra there is a ridge of tissue projecting from the dorsal wall, the colliculus seminalis. Ejaculatory ducts have openings on this ridge, and provide a common outflow duct for the ductus deferens and vesicular gland (in those animals which possess vesicular glands – carnivores do not). On the lateral sides of the colliculus seminalis, there are numerous openings of the prostatic ducts of the compact portion of the prostate (stallion, bull, boar).

The urethral muscle (urethralis m) is striated and is confined to the pelvis, where it surrounds (in the male) the pelvic urethra and serves as a voluntary sphincter to retain the urine.  In the female, the urethralis m. originates from the sides of the vagina and forms a sling ventral to the urethra and also serves as a voluntary sphincter.  The pelvic urethra is continuous with the penile urethra in the male, which begins caudally at the level of the ischiatic crest, where the root of the penis is located.  It DOES NOT extend around the penile urethra in the male.   The urethralis muscle may also be called the external urethral sphincter. The urethralis m./external urethral sphincter  is innervated by the pudendal nerve (sacral somatic efferent neurons).

Urine exits the urethra through the external urethral orifice.  In the male, the external urethral orifice is located at the distal end of the penile urethra.  The surrounding penile structures and their relationship with the penile urethra will be studied in depth during the dissection of the penis.  The female animal, quite obviously, does not possess a penile urethra.  In the female domestic species, the pelvic urethra empties into the vestibule through the external urethral orifice, which will be studied in relation to the female genitalia later in this unit.

Perineum and Pelvic Diaphragm Muscles

The perineum is the region of the body superficial to the pelvic outlet.  It extends ventrally from the base of the tail to the base of the scrotum in the male, and to below the vulva in the female.  Laterally the perineum extends to the level of the ischiatic tuberosities.  Its deep boundary is the pelvic outlet.  Structures of the perineum include the anus, anal sphincter mm, the vulva in the female and the root of the penis in the male.

The levator ani and coccygeus muscles of each side form a pelvic diaphragm through which the genitourinary and digestive tracts open to the outside.

The levator ani muscle lies medial to the coccygeus muscle. It is a broad, thin muscle originating on the medial edge of the body of the ilium and the dorsal surface of the pubis and the pelvic symphysis. It covers the cranial part of the internal obturator. The muscle appears caudal to the coccygeus, where it inserts on caudal vertebrae 3 to 7.  The coccygeus muscle lies lateral to the levator ani muscle. It is shorter and thicker and arises from the ischiatic spine and inserts on the transverse processes of caudal vertebrae 2 to 4.  These muscles, in addition to the sacrotuberous ligament (in the dog) and the sacrosciatic ligament (in the ungulate), form the lateral soft-tissue boundaries of the pelvic cavity.

Recall that the the colon consists of the ascending, transverse, and descending colon.  The rectum continues the descending colon through the pelvis. It begins at the pelvic inlet. The anal canal is a continuation of the rectum to the anus. Just inside the anus, at clock positions 8 and 4, respectively, you should find the openings to the paranal sinuses (anal sacs). The anal canal is surrounded by both a smooth internal anal sphincter muscle and a striated external anal sphincter muscle. The external opening of the anal canal is the anus.

The external anal sphincter muscle receives its nerve supply from the caudal rectal nerve (branch of the pudendal n.) and its blood supply from the caudal rectal a. (branch of the internal pudendal artery).  The caudal rectal nerve provides voluntary control over defecation.

In the wall of the paranal sinus are microscopic glands, the secretion of which accumulates in the lumen. The secretion is held in the paranal sinus and is discharged through the duct of the paranal sinus into the anal canal during normal defecation.   The microscopic glands are properly called the anal glands.  In a clinical setting, it is sometimes necessary to manually express the paranal sinuses, which are often erroneously called the anal glands.

The internal anal sphincter muscle is an enlargement of the smooth circular muscle coat of the anal canal. It is not as distinct as the external anal sphincter.

The rectococcygeus muscle continues the longitudinal smooth muscle of the rectum to the ventral surface of the tail.