anatomy of the rectum, anus and associated glands

Question 1

what is the rectum

Answer 1

the most distal part of the large bowel (stores feces)

Question 2

what is the anus

Answer 2

distensible short section terminating in a valved opening

Question 3

is there any anatomical difference between the rectum and large colon

Answer 3

no, arbitrarily delineated. just the terminal end of large bowel
- begins at the pelvic inlet and ends at anal canal
- straight rectangle

Question 4

what is the difference between adventitia and serosa and where will you find these along the rectum

Answer 4

• visceral peritoneum = serosa, covers most of cranial rectum
• adventitia = connective tissues. surrouns caudal rectum - = retroperitoneal

Question 5

what are peritoneal pouches

Answer 5

where the peritoneum turns around within the pelvic canal

Question 6

what is mesorectum

Answer 6

• an extension of the mesocolon
• mesenteric support
• wider cranially
• tapers away

Question 7

discuss rectal histology

Answer 7

• same as large intestine
• no villi
• solitary lymphoid aggregates
• columnar epithelium
• more goblet cells

Question 8

discuss vascular supply of the rectum

Answer 8

• cranial rectal artery = branch off aorta, caudal mesenteric artery branches into left colic and cranial rectal artery
• middle and caudal arteries branch off internal pudendal artery

Question 9

discuss venous drainage of the rectum

Answer 9

• cranial rectal artery drains into caudal mesenteric then hepatic portal vein
• middle and caudal rectal drain into internal pudendal artery then internal iliac to enter systemic circ

Question 10

explain why rectal suppositories are effective

Answer 10

• middle and caudal rectum drains into systemic circulation
• bypasses heptic portal vein
• doesnt get metabolised by liver

Question 11

what is the pelvic diaphragm

Answer 11

• tent of muscles plus internal and external sphincter
• supports rectum
• made of coccygeus and levator ani muscles
• sacrotuberous ligament supports muscles and makes “tent”

Question 12

discuss coccygeus muscle

Answer 12

• originates at ischiatic spine
• insters on transverse process of caudal vertebrae 2-4
• innervated by branches of sacral plexus and pudendal nerve
• function is to compress rectum during defecation and pressing tail against anus

Question 13

discuss levator ani muscle

Answer 13

• originates from medial ilium and pelvic symphysis
• inserts on tendons caudal to vertebrae 3-7
• innervated by branches of sacral plexus and pudendal nerve
• function to compress rectum during defecation and press tail against anus

Question 14

what types of muscle are the anal sphincter muscles

Answer 14

• internal = smooth muscle
• external = skeletal muscle (striated)

Question 15

the external anal sphincter muscle functions to do what

Answer 15

• cover anal sacs and compresses them during defecation
• conscious control

Question 16

what muscles make up the internal anal sphincter

Answer 16

fibres of rectal circular muscles are organised caudally to form the smooth muscle of internal sphincter

Question 17

list the zones of the anus

Answer 17

Question 18

compare the zones of the anus

Answer 18

Question 19

list glands and sacs around the anus

Answer 19

• anal glands
• circumanal glands
• anal sacs

Question 20

what are anal glands and where are they located

Answer 20

• cranial to the anocutaneous line
• fatty secretion
• contained scattered apocrine sweat glands around the anus in the cutaneous zone

Question 21

what are circumanal glands

Answer 21

• present in dogs
• all around anus in cutaneous zone
• subcutaneous
• sebaceous secretion

Question 22

what are anal sacs and where are they located

Answer 22

• paired diverticula present in carnivores
• located within the lamina propria/submucosa
• lined by apocrine paranal sinus gland
• paired ducts open into anal canal
• vets call them anal glands but they are sacs

Question 23

how do the anal sacs get empied

Answer 23

• embedded between internal and external anal sphincter muscles
• normally compressed during defecation to coat feces in scent

Question 24

discuss anal sacs histologically

Answer 24

• cornified stratified epithelium
• coiled apocrine tubules contained within wall of sacs
• secrete foul secretion

Question 25

what is tenesmus

Answer 25

straining = forced expiration against a closed glottis which raises intra-abdominal and intra thorcic pressure

Question 26

whatis dyschezia

Answer 26

painful straining

Question 27

discuss innervation of the internal anal sphincter

Answer 27

- sympathetic = excitatory = constricts - parasympathetic - inhibitory = relaxes - in resting state, there is high internal sphincter tone

Question 28

discuss innervation of the external anal sphincter

Answer 28

- innervated by branches of pudendal nerve - allows for conscious contraction

Question 29

discuss the defecation reflex

Answer 29

The process of defaecation is normally a combination of both voluntary and involuntary processes As faecal material enters the rectum, it distends – if the wall is sufficiently distended the anorectal reflex produces concurrent contraction of the rectal wall, relaxation of the internal sphincter and (mostly) relaxation of the external anal sphincter. Untrained animals, or those without behavioural constraints, will defaecate at this point.

Question 30

discuss the reflex for defecatio after eating

Answer 30

- There is often an urge to defaecate after eating (especially a large meal) Notable in young carnivores - This is related to distension of the stomach which initiates the gastrocolic reflex (gastrin release) causing onward passage (emptying) of faecal material from the colon into the rectum thereby initiating the anorectal reflex

Question 31

list common clinical conditions of the bum

Answer 31

- anal sac impaction/abscess - anal furunculosis - hepatoid adenomas - adenocarcinomas - rectal prolapse - perineal hernia -

Question 32

what is an anal sac impaction

Answer 32

- inflammation of anal sac duct - impaction of anal sac secretion - secondary infection - abscess ruptures to skin surface

Question 33

what is anal furunculosis

Answer 33

- immune mediated fistula - can be seen in combination with anal sac abscess

Question 34

what is a hepatoid adenoma

Answer 34

- most common anal tumor in the dog (mostly in intact older males) - testosterone dependant benign masses - found in external region of outer cutaneous zone - single or multiple - malignant form of tumor rare - rare in cats

Question 35

what is an anal adenocarcinoma

Answer 35

circumanal gland adenocarcinoma - malignant lesion of perianal sebaceous glands - rare - occur in same areas as adenomas - can diffusely infiltrate anal areas - often adherent to deeper tissues - rapidly grow - do not respond to castration or anal sac (apocrine) adenocarcinoma - predominently affects older bitches - small discrete nodules in wall of either sac - pananeoplastic syndrome often accompanies - secretes PTH like susbstance - hypercalcemia causes pu/pd, depression, weakness, weight loss - aggressive, 50% metastisied at presentation

Question 36

what is rectal prolapse

Answer 36

- associated with endopatasites/enteritis in young animals and tumours or perineal hernias in middle aged/older animals - incomplete prolapse =mucosa only - complete = all layers of rectal wall - few mm to many cm - everted tissue is oedematous, excoriated and can be bleeding