Anatomy of rectum and anus

what is the rectum

• the most distal part of large bowel

what is the anus

• distensible short section terminating in a valved opening

what is the function of the:

1. rectum

2. anus

1. faecal storage

2. faecal continence

where does the rectum start and end?

1. begins at pelvic inlet

2. ends at the anal canal

what are the 2 junctions of the rectum?

• colorectal junction

• anorectal junction

what is visceral peritoneum

• the serosa/serosal layer

where do we find most of the visceral peritoneum?

• covered most of cranial rectum

what is the caudal rectum surrounded by?

• connective tissue = adventitia

• we call it retroperitoneal

what do we mean by peritoneal pouches around the rectum?

• where the peritoneum turns around within the pelvic canal

• these peritoneal reflections make pouches

what is the mesenteric support of the rectum?

• an extension of the mesocolon

• wider cranially

• tapers away at the 2nd coccygeal bone

what are the 2 mesenteric areas on the rectum?

• mesocolon

• mesorectum

outline the histology of the rectum

• same as large intestine

  • no villi

  • solitary lymphoid aggregates

  • additionally have small cratered lymphoid nodules

what is the structure of the rectal mucosa?

• no villi

• columnar epithelium

• long, tall intestinal glands

• more goblet cells

• contain lymphoid follicles

what is the main longitudinal muscle of the rectum, in which direction are the fibres organised?

• rectococcygeus

• dorsocaudally

what are the two key circular muscles of the rectum?

• internal and external anal sphincter muscle

in which direction are the fibres of the internal anal sphincter arranged?

• caudocranially

what are the 3 rectal arteries?

• cranial

• middle

• caudal

what is the origin of the cranial rectal muscle

• aorta → caudal mesenteric → left colic and cr. rectal

what is the origin of the middle and caudal rectal arteries?

• internal pudenal artery

what are the 2 veins the venous rectal blood drains into?

• hepatic portal vein

• internal iliac (systemic)

what rectal arteries drain into:

1. hepatic portal vein

2. internal iliac

1. cranial rectal → caudal mesenteric

2. middle and caudal rectal -. int. pudenal → internal iliac

what is the pelvic diaphragm

• two paired muscles either side of the rectum that closes off open spaces b/w sacrum/tail and ilium/ischium

what makes up the pelvic diaphragm?

• sacrotuberous ligament

• coccygeus m.

• levator ani m.

what is the function of the pelvic diaphragm?

• compresses pelvic contents, it helps mediate rectal function during defecation

what muscle makes up:

1. internal anal sphincter

2. external anal sphincter

1. smooth

2. skeletal

how do anal glands get expressed during defecation?

• external anal sphincter which contracts during defecation compresses them → expression

how does the tone of the internal and external anal sphincter differ?

• high tone maintains involuntary continence in internal sphincter

• low tone in external sphincter

what muscle is the main constrictor of the anus?

• external anal sphincter

under what control is the external anal sphincter?

• conscious

what kind of junction do we have in the anus?

• mucocutaneous - b/w mucosa and integument

how does cell type differ from rectum to anus?

• transition (sudden) from columnar epithelium to squamous epithelium

what are the 3 glands?

• anal

• circumanal

• anal sacs

where are the anal glands located in dogs and cats?

• cranial to anocutaneous line

function of anal glands in dogs and cats?

• contain apocrine and eccrine glands for secretions onto faeces as it passes - scent

where are circumanal glands?

• all around anus in cutaneous zone

describe the anal sacs in dogs and cats

• paired diverticula in the lamina propria/submucosa

• lined by apocrine paranal sinus glands

• paired ducts open into anal canal

how should anal sacs empty?

• when compressed during defecation

what are anal sacs lined with?

• cornified, stratified epithelium

• coiled apocrine tubules within wall of sacs

issues with anal sacs?

• can get occluded in dogs, common

how can we treat anal sacculitis?

• cannulate the ducts for antibiotic administration

  • uses spruell’s needle

what is tenesmus?

• straining

what is dyschezia?

• painful straining

what is strainful?

• act of forced expiration against a closed glottis - raised intra-abdominal and thoracic pressure

why do animals have a fixed stance during defecation?

• as more force can be generated this way

what is the rectum innervated by?

• autonomic NS → initiates reflex contraction upon distension

what triggers defecation?

• increased distension of the rectum

is defecation simple?

• no - complex and synchronised event with 1+ reflexes

in terms of defecation behaviour what do large herbivores do?

• tend to go anywhere

  • alpacas use faecal small for defining herd area

what do small furies do with defecation, in terms of behaviour

• tend to have special toilet areas - species dependent

what do carnivores use defecation for?

• marking territory

• scent patterns

do fundamental neural pathways change in the domestic species?

• no

how does the degree of control of defecation change b/w species

• highly variable

what happens if defecation is not acted on

• material in rectum often returns to colon by reverse peristalsis

• rectal wall tone falls allowing distension of rectum and internal sphincter constricts

how high does rectal pressure have to exceed for the anal sphincters to relax and defecation to occur

• 55mmHg

when is there a particularly strong urge to defecate?

• after eating

why is there a strong urge to defecate after eating?

• distension of stomach → gastrocolic reflex → forward passage of faecal material of colon into rectum → anorectal reflex

outline the defecation/anorectal reflex

• as faecal material enters rectum → distension → anorectal reflex

  • concurrent contraction of rectal wall + internal sphincter relaxes + relaxation of external anal sphincter

what kind of receptors are there involved in the anorectal reflex?

• stretch receptors

as it’s the anorectal reflex, which part of the CNS does itinvolve?

• reflex → just spinal cord

• control of the external anal sphincter involves some conscious control (therefore from cerebral cortex)

what neurons are inhibited in order to allow the external anal sphincter to relax?

• voluntary motor neurones

how do the following affect the internal anal sphincter?

1. SNS

2. PSNS

1. excites → constriction

2. inhibits → relaxation

via what nerves do the following act on the internal anal sphincter

1. SNS

2. PSNS

1. hypogastric nerve

2. sacral outflow of PSNS

outline the muscles involved in defecation and how they act

1. pelvic diaphragm (cocygeus + levator ani) compress rectum

2. rerococcygeus elongates rectum

3. sphincters relax

4. smooth muscle of rectum contracts

5. straining against a closed glottis

what is colitis?

• irritation of rectum

what is the classic clinical sign of colitis?

• repeated attempts to defecate even though rectum may be empty

how does a mother encourage defecation in pups and kits?

• licking anus

how can we mimic the mothers stimulating defecation if she’s ill?

• wiping anus with a cotton bud

4 causes of anal sac impaction/abscess

1. inflammation of the ducts

2. impaction of secretion

3. secondary infection

4. abscess ruptures to skin surface

what is anal furunculosis

• immune mediated fistula

• seen in combination with anal sac abscessation

Perianal gland adenomas:

1. are these common? In what animals

2. what are they dependent on

3. where?

4. how many?

5. malignant?

6. rare in what species?

• most common anal tumour the dog - of older intact males

• they’re testosterone dependent benign masses

• in the external region of the outer cutaneous zone

• one+

• rarely malignant

• rare in cats

what else can we call perianal gland adenomas?

• hepatoid or circumanal

circumanal gland adenocarcinomas:

1. what are they

2. common?

3. what area?

4. where can they go?

5. what do they often adhere to?

6. growth rate?

7. does castration help?

1. malignant lesions of perianal sebaceous glands

2. rare

3. external region of outer cutaneous zone

4. can infiltrate anal areas diffusely

5. deeper tissues

6. rapid

7. no

Anal sac (apocrine) adenocarcinomas

1. in what species

2. appear as?

3. what often accompanies it?

4. what does the tumour secrete

5. how does hypercalcaemia affect it?

6. benign or metastasised?

1. older bitches

2. small discrete nodule on wall of either sac

3. paraneoplasic syndrome

4. PTH-like-substance

5. causes pu/pd, depression, weakness, weight loss

6. aggressive, 50% metastasised at presentation

what are rectal prolapses associated with in young animals?

• endoparasites

• enteritis

what are rectal prolapses associated with in middle/older aged animals?

• tumours or perineal hernias

what does an incomplete prolapse involve?

• mucosa only

what does a complete prolapse involve?

• all layers of rectal wall in entire circumference

How does the everted tissue appear?

• oedematous

• excoriated

• can be bleeding

what cat breed is especially prone to prolapse?

• manx

what is a perineal hernia?

• degeneration of pelvic diaphragm

• where we have a separation of the muscles involved

what are the 2 kinds of perineal hernias? which is more common?

• anal sphincter and levator ani - most common

  • levator ani and coccygeus - less common