Lectures

Anal Canal – Perineum and External Genitalia

by James Pickering, PhD
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    00:01 Okay, now let's look at the anal canal.

    00:05 The anal canal is particularly important because it's the terminal part of the gastrointestinal tract.

    00:11 Direct continuation of the rectum and we have spoke about the rectum in the previous lectures and we can see the transverse folds of the rectum here.

    00:20 We can see the rectal ampulla which stores the faeces prior to defecation.

    00:25 But we then had a junction. A junction between the anus and the rectum.

    00:30 The so called anorectal junction and this juction is in line with the levator ani.

    00:39 Where levator ani is coming down from its lateral attachments that creates a junction known as the anorectal junction.

    00:49 Everything beneath that line is the anal canal.

    00:55 Everything above it is the rectum. And which part of the levtor ani goes around the rectum? The puborectalis.

    01:04 Puborectalis. So the anal canal is the distal portion of the gastrointestinal tract.

    01:13 And importantly for this lecture it is located in the perineum.

    01:16 It extends from the superior limit from the anal columns.

    01:21 So here we can see some anal columns that are really starting at the anorectal junction.

    01:27 And it runs down to the anus which we can see here, this opening.

    01:33 This anorectal junction, as I have mentioned marks where pubicrectalis sweeps around and this constricts the lumen helping to prevent the faeces to passing out.

    01:46 The anal columns: these contain the terminal branches of those superior rectal vessels.

    01:58 Also the middle rectal blood vessels as well if they are present.

    02:01 But these are we coming down the superior rectal blood vessels that are supplied the rectum above. The anal columns are containing those terminal branches of the superior rectal blood vessels.

    02:14 Which means beneath them, this region is going to be supplied by the inferior rectal arteries. And remember the inferior rectal we just saw came from the internal pudendal which is a branch of the internal iliac.

    02:30 Inferior to the anal columns we find we have some anal valves and we have some anal sinuses. These sinuses release mucus which is important with defaecation and when the faeces passes through they become compressed. They release the mucus and they support the faeces from exiting the gastrointestinal tract.

    02:51 The inferior anal valves form this pectinate line and this pectinate line is important because it separates the superior and inferior aspects of the anal canal.

    03:03 And importantly this is where the skin creeps in and the mucusa becomes keratinized.

    03:09 You go from this digestive membrane that we had in the gastrointestinal tract and actually changes as we get into the inferior anal canal and the skin become keratinized.

    03:23 And at this region, its important, because we really have different lymphatic drainage from this region as well. We have different nerve supply where we have parasympathetic and sympathetic above and we have somatic below. So this pectinate line really does mark an important transition.

    03:43 And here we can see that in more detail. The anal canal superiorly and inferiorly. We can see we have the difference in the arterial supply superior rectal arteries and we have got inferior rectal arteries coming from different origins. We have superior rectal veins which from the superior aspect drain into the portal system.

    04:04 Inferiorly they drain into the systemic system and we spoke about these portal systemic anastomosis.

    04:10 We will come to the lymphatic drainage in an another lecture and we will look at the nerve supply too.

    04:16 in an another lecture. But this is a good reference table to remind you of the differences between the superior and inferior aspect of the anal canal.


    About the Lecture

    The lecture Anal Canal – Perineum and External Genitalia by James Pickering, PhD is from the course Pelvis.


    Included Quiz Questions

    1. Levator ani
    2. Coccygeus
    3. Piriformis
    4. Obturator internus
    5. Gluteus minimus
    1. Superior rectal
    2. Inferior rectal
    3. Internal pudendal
    4. Inferior gluteal
    5. Superior gluteal
    1. Superficial inguinal lymph nodes
    2. Internal iliac lymph nodes
    3. Deep inguinal lymph nodes
    4. External iliac lymph nodes
    5. Aortic lymph nodes

    Author of lecture Anal Canal – Perineum and External Genitalia

     James Pickering, PhD

    James Pickering, PhD


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