Comparative Aspects of the Pelvis

Question 1

Pelvis/ Pelvic Cavity/Perineum

Answer 1

• Pelvis- the caudal region of the trunk within which lies within the pelvic girdle.
• Pelvic cavity- accommodates parts of the urinary (bladder & urethra) & reproductive tracts as well as the terminal part of the digestive tract/ rectum.
• Perineum- the region surrounding the terminations of the digestive and urogenital tracts i.e. “around the birth canal”.

Question 2

Boundaries of the Pelvic Cavity

Answer 2

• Pelvic inlet - the pelvic cavity is continuous with the abdominal cavity at the pelvic inlet, bounded by the terminal line from the sacrum along the wing of the ilium to the brim of the pubis. - Gap between them is obturator foramen
• Pelvic outlet – demarcated by a caudal vertebra & the caudal edge of the sacrosciatic ligaments & the ischial arch. Closed by pelvic diaphragm which separates the pelvic cavity from the perineum.
• Roof- formed by s_acrum & variable number of caudal vertebrae._
• Floor- formed by pelvic bones.
• Lateral walls - formed by wing of ilium & the sacrosciatic ligaments

Question 3

Species Differences- Pelvis

Answer 3

• Horse – pelvic inlet relatively wide, pelvic canal straight, floor of pelvis flat & ischial spine & tubers small
• Ruminants - pelvic inlet relatively narrow, floor concave & ischial spine and tubers larger than in mare.
• Dog - sacrosciatic ligament (sheet-like) reduced to cord-like **sacrotuberous ligament (absent in cat).

Most common cause of dystocia in ruminants–> oversized foetus

Question 4

Pelvic Diapraghm

Answer 4

Muscle sheet separating pelvic cavity from perineum:

• levator ani m. & coccygeus m. more laterally
• Originating from os coxae ventrally & run dorsocaudally to insert on the caudal vertebrae.
• Pelvic diapragm: are the muscles that somewhat separate the pelvic cavity cranially and perineum caudally
• levator ani m. also inserts on external anal sphincter to draw the anus cranially to prevent the anus being pulled caudally with the movement of faeces
• Breakdown/ weakening of these muscles can–> abnormal passage of organs (usually the rectum) from the pelvic cavity–> the perineum constituting a perineal hernia

Question 5

Perineum

Answer 5

• Diamond-shaped area which can be subdivided into:
• Anal Triangle- containing the anus & ischiorectal fossa either side
• Urogenital Triangle- In ♀ contains the vulva, In ♂- contains 2 variable parts e.g. parts of penis (crura & bulb) & urethra and scrotum in some species (tom cats & boars)

Boundaries: the c_audal vertebrae in the root of the tail dorsally,_ the caudal edges of the sacrosciatic or sacrotuberous ligaments dorsolaterally and the i_schial tuberosity and ischial arch ventrolaterally._

Question 6

Blood Supply- Internal Iliac Artery

Answer 6

aorta branch & major supply to pelvis.

Terminal branches are:

• Caudal gluteal a. (mainly parietal)
• Internal pudendal a. (mainly visceral)- has many branches to the various organs within the pelvic cavity and perineal branches which continue into the perineum giving rise to various branches including arteries to the penis.

Veins – satellite to arteries & drain to caudal vena cava.

Question 7

Pudendal Nerve

Answer 7

formed from ventral rami of sacral spinal nerves.

Forms the:

• Superficial perineal nerve- supplies skin of perineum including caudal parts of scrotum & mammary

glands

• Deep perineal nerve- supplies striated muscles in the perineum.

Question 8

Caudal rectal Nerve

Answer 8

formed from same nerves as pudendal n.

-Supplies sensation to rectum, anal canal, skin around anus & striated muscles in dorsal part of perineum.

Question 9

Pelvic Nerve

Answer 9

also formed from ventral rami of sacral spinal nerves.

• Parasympathetic fibres from sacral segments of spinal cord to pelvic plexus on lateral wall of rectum.
• Sympathetic input to the pelvic plexus is from the caudal mesenteric ganglion via the hypogastric nn. The pelvic plexus gives rise to autonomic fibres supplying the pelvic organs

Question 10

Bladder in the Pelvic Cavity

Answer 10

• An organ changes its position somewhat is the urinary bladder, as it expands it projects further more cranially into the abdominal cavity.
• In some species, even when full it mainly lies in the pelvic cavity (large animals), in dog it can move into the abdominal cavity to be palpated.
• Being a pelvic organ means it has the same blood and nerve supply as the other pelvic organs

Question 11

Os coxae

Answer 11

• Os coxae is often filled with large sheet of ligamentous tissue is LARGE animals–> this ligament is called the sacrosciatic ligaments
• -much is contributed by the ischium

-ischium and sciatic can be quite interchangable. Sciatic nerve… really referring to ischium