Perineum - anus (dave's notes)

Question 1

Describe the anus

Answer 1

The anus is the last four centimetres of the GI tract

Question 2

What did the anus develop from?

Answer 2

The antorectal canal and proctodeum

Question 3

Describe the muscle layers of the anus

Answer 3

There are two layers of muscle fibres; internal (visceral) and external (skeletal). Both of these are circular.

Question 4

Where is the junction between the rectum and the anal canal?

Answer 4

At the pelvic floor. Puborectalis angles the gut forward.

Question 5

If the anus is a tube within a funnel, what structures make up these components?

Answer 5

Levator ani down to the external sphincter is the funnel, and the inner sphincter (a continuation of rectal muscle) is the inner tube.

Question 6

What does the rectal end of the external sphincter blend with? What is this line of fusion called?

Answer 6

Puborectalis, the anorectal ring

Question 7

Describe the shape and attachments of the middle part of the external anal sphpincter

Answer 7

The middle part is elliptical, attatched to the coccyc and perineal body

Question 8

Where is the lower part of the external anal sphincter found?

Answer 8

Subcutaneous, cuffing around the internal anal sphincter

Question 9

Regarding the external anal sphincter, what occurs at the anorectal junction?

Answer 9

The outer longitudinal layer fuses with puborectalis

Question 10

Describe the internal anal sphincter

Answer 10

This is a thickened downward continuation of the inner circular muscle of the rectum

Question 11

Describe the mucous membrane of the upper third of the anus

Answer 11

The upper third has up to 12 longitudinal ridges - anal columns which are joined together

Question 12

What is found at the base of the anal columns? How else is this area known?

Answer 12

The anal valves or sinuses. This is the pectinate or dentate line. Here are mucous secreting anal glands.

Question 13

What is found below the dentate line?

Answer 13

A band of pecten to the interphincteric groove - (Hilton’s white line) - and then cutaneous skin

Question 14

What does the upper part of the mucous membrane develop from? What does the lower part develop from?

Answer 14

Upper - cloaca, lower - proctodeum

Question 15

What is the blood supply of the anus?

Answer 15

The superior rectal artery. The muscle wall is from supplied by the middle rectal and median sacral arteries. The lower end and mucous membrane is supplied by the interior rectal artery.

Question 16

Describe the venous drainage of the anus

Answer 16

Venous drainage corresponds with blood supply, and is continuous with the rectal venous plexus. The upper end drains via the portal system, and the lower end drains via the internal iliac vein.

Question 17

Describe the lymphatic drainage of the anus

Answer 17

Lymphatic drainage shows a watershed corresponding with the vascular pattern.

Question 18

What is the nerve supply of the external anal sphincter?

Answer 18

The interior rectal branches of the pudendal nerve supply the external sphincter.

Question 19

What is remarkable about the nerve supply of the external anal sphincter

Answer 19

There is a high proportion of slow twitch fibres that are almost always firing.