remained in an abnormally low position. Now,
let’s look at the branchial arches.
What I’ve done is I’ve drawn a red line which
shows the plane of section of the diagram
on the right-hand side. You can see that I’ve
cut through the frontonasal prominence.
So there’s a little piece that’s separated
where the opening of the mouth will be.
And it happens that we’ve sectioned that through
the optic vesicles. Then we can see the four
arches, and these are represented by grooves
on the outside of the head and neck, and on
the inside, the corresponding grooves are
known as pouches. Each groove and pouch is
numbered from the arch in front. So, groove
one and pouch one come immediately after the
first arch, and so on through the rest of the
arches. So, numbering is in this particular
way, and we’ll use this consistently all the
way through. Now, we don’t need to bother
out the detail in this image. All I’m going
to do is to make the general point here, that
each arch contains a supply of blood vessels
and nerves, there is muscle and there is cartilage
and this reflects as evolutionary origin where
this is involved in being gill arches.
In human embryos, the gill has never become open.
There’s always a connection between the
layers so that it never opens to the outside,
unlike fish of course. But because we are
no longer using them as gills, it means that
they are then freed in evolutionary history
to take on other functions. You could describe
it as a kind of general toolkit of cartilage
muscle, blood vessel, and nerve which can
give rise to a whole range of structures in
the developing head and neck, and in the face.
And this is a summary of some of the derivatives
of those branchial arches. Here we can see
that I’ve numbered what I’ve called somitomeres.
Now, these are analogs to the somites, the
segmental blocks of structure, which runs
down in either side of the neural tube. And
we’re looking in the very forepart of the
neural tube which will mostly give rise to
the brain. Again, back in the evolutionary
history, these were segmented and we’re
looking at the relics, the descendants of
these segmented structures later on. What
we can see is that there are muscles which
are derived from these somitomeres. So, for
instance, the superior, inferior, and medial
rectus muscles derived from the first and
second somitomeres, and so on down the list.
Key ones we will emphasize are those from
the fourth somitomere, which will provide
the large muscles of mastication, and those
from the sixth somitomere, which will provide
the muscles of facial expression and those
were associated with arch one and arch two,
respectively. The other muscles are relatively
small, although important in their function
of course. At this point, we can also note
that the cartilage elements arising from arches
one and two represent mainly the malleus and
incus of the inner ear bones, of the middle
ear bones, and the core of the mandible. Arch
two will give rise to the stapes, the styloid
process and the upper part of the hyoid bone.
If we are to continue down the sequence, we
can see the first of the somites which follow
caudally from the somitomeres, and you can
see the first somites contributing to the
laryngeal muscles. This is part of the fourth
arch, and it will also give rise to one of
the cranial nerves associated with them.