USMLE Step 1 Review 04 12 Anatomy Anus 2

USMLE Step 1 Review 04 12 Anatomy Anus 2

 

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Play USMLE Audio MP3 04 12 Anatomy Anus 2 Below

 

Begin 04 12 Anatomy Anus 2 Transcription

About halfway down the anatomical anal canal, there is a circular groove running around the canal.

What is this called?

  • This is the intermuscular groove.

And what is the other name for it?

  • The white line of Hilton.

There is controversy in terms of the meaning of the white line of Hilton or the intermuscular groove, in terms of the type of epithelium above and below it. According to some experts, the epithelium of the anal canal above the intermuscular groove or white line of Hilton is non-keratinized stratified squamous epithelium.

And according to those who hold this view, what type of epithelium is below the white line?

  • Keratinized pigmented stratified squamous epithelium. One thing that there does seem to be good agreement upon is the embryological origin of the epithelium above the dentate line and below the dentate line.

What is the embryological origin of the mucosa above the dentate line?

  • This is from endoderm.

And the epithelium lining the anal canal below the dentate line is derived from?

  • Ectoderm.

There is also agreement of course upon the sphincter muscles of the anus that are divided by the white line.

What is the anal sphincter muscle above the white line?

  • This is the internal anal sphincter muscle.

And what is the sphincter muscle below the white line?

  • This is the subcutaneous external anal sphincter muscle.

Now above the white line again is the internal anal sphincter muscle and this is an extension of what muscular layer in the rectum?

  • The internal anal sphincter muscle is an extension of the circular muscle of the rectum.

Does the outer longitudinal muscle layer of the rectum form an anal sphincter?

  • Not directly although it does fuse with other muscles.

What is the first muscle that it fuses with on the outside?

  • The deep external anal sphincter muscle.

Further down, it runs on the outside of the internal anal sphincter muscle, which is of course above the white line, and below what sphincter?

  • The superficial external anal sphincter muscle. The longitudinal layer then wraps under the internal anal sphincter muscle at about the level of the white line and then comes back up as a thin muscular layer deep to the internal anal sphincter muscle and superficial to the submucosa of the anal canal above the white line.

As these longitudinal fibers wrap under the internal anal sphincter muscle at about the level of the white line, what anal sphincter lies inferior?

  • The subcutaneous external anal sphincter muscle which is the sphincter muscle below the white line. Now let’s go back up and out a little bit.

What muscle comes down from above and joins the longitudinal muscle layer where it merges with the deep external anal sphincter muscle and helps suspend the entire anal structure?

  • Levator ani muscle. Okay, now please remember back to where I was talking the fibers of the longitudinal muscle coming all the way down under the internal anal sphincter muscle and then reflecting back up and running as a thin layer between the internal anal sphincter muscle and the submucosa of the anal canal above the white line.

What is this vertical reflection called?

  • This is the musculus submucosa ani.

And what thin layer of smooth muscle runs under the mucosa of the rectum all the way down to join the musculus submucosa ani and the region of the anal valves?

  • The muscularis mucosa of the rectum. The muscles most on the outside above the white line: levator ani and the deep external anal sphincter muscle and the superficial external anal sphincter muscle, all of these have an outer layer of fascia.

And what is this outer layer of fascia called?

  • This is the inferior fascia of the pelvic diaphragm and levator ani as it comes down to join with the longitudinal muscle layer of the rectum has an internal fascial layer.

And what is this fascial layer called?

  • The superior fascia of the pelvic diaphragm.

Between the superior fascia of the pelvic diaphragm which is on the inside of levator ani, and the outside of the rectum, which is covered of course by the rectal fascia, what do we have between these two fasciae?

  • Fat.

What do we have on the outside of the inferior fascia of the pelvic diaphragm?

  • More fat.

Where the inferior fascia of the pelvic diaphragm runs down to the base of the superficial external anal sphincter muscle, the base of which is at about the level of the white line, there is a fascial plane. A horizontal plane that both goes laterally and in toward the longitudinal muscle layer which is passing between the superficial external anal sphincter muscle and the internal anal sphincter muscle.

What is this horizontal plane of fascia?

  • This is the transverse fibrous septum of ischiorectal fossa. Now again to review.

On the outside, coming down from above is? Levator ani muscle. Then, there are three external sphicters. These are all called external anal sphincter muscles.

And what are these three, starting at the top and going to the bottom?

  • At the top, underneath the levator ani is the deep external anal sphincter muscle. Then, below that is the superficial external anal sphincter muscle and then, below the level of the white line is the subcutaneous external anal sphincter muscle.

On the inside then, again to review, above the white line is?

  • The internal anal sphincter muscle.

And again, between the internal anal sphincter muscle and the submucosa of the anal canal above the white line is?

  • The musculus submucosa ani.

Which of the three external anal sphincter muscles is not separated from the anal canal by another layer of muscle?

  • The subcutaneous external anal sphincter muscle.

Which is, in relation to the white line?

  • Below the white line. Finally there are some thin strips of muscle extending from the longitudinal muscle fibers as they curl under the internal anal sphincter muscle at the level of the white line. These thin strips come off of that, travel down through the subcutaneous external anal sphincter muscle and attach to the underside of the skin surrounding the anal verge.

What is the name for these fibers coming down?

  • Corrugator cutis ani muscle.

****END OF TRANSCRIPTION****

 

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