Okay, now let's look at the anal canal.
The anal canal is particularly important because
it's the terminal part of
the gastrointestinal tract.
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.
We can see the rectal ampulla which
stores the faeces prior to defecation.
But we then had a junction. A junction
between the anus and the rectum.
The so called anorectal junction
and this juction is in
line with the levator ani.
Where levator ani is coming down
from its lateral attachments
that creates a junction known
as the anorectal junction.
Everything beneath that
line is the anal canal.
Everything above it is the
rectum. And which part of the
levtor ani goes around the
rectum? The puborectalis.
Puborectalis. So the anal canal is the distal
portion of the gastrointestinal tract.
And importantly for this lecture
it is located in the perineum.
It extends from the superior
limit from the anal columns.
So here we can see some anal columns that are
really starting at the anorectal junction.
And it runs down to the anus which
we can see here, this opening.
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.
The anal columns: these contain
the terminal branches
of those superior rectal vessels.
Also the middle rectal blood vessels
as well if they are present.
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.
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.
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.
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.
And importantly this is
where the skin creeps in
and the mucusa becomes keratinized.
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.
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.
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.
Inferiorly they drain
into the systemic system
and we spoke about these
portal systemic anastomosis.
We will come to the lymphatic
drainage in an another lecture
and we will look at
the nerve supply too.
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.