i am so glad that you guys are not five
years old if you were all in like
kindergarten the only way i could teach
you about your skeletal system would be
to sing you that stupid song
you know the toe bones connect to the
foot bone the foot bones connected to
the ankle bone
the ankle bone's connected to the leg
bone like apparently there's one leg
bone for the purposes of teaching
college level anatomy and physiology
let's just say that that song is
reductive insufficient for that song to
be at all useful for remembering all 206
bones to make up your skeletal structure
would have to begin with something like
the distal phalanx is connected to the
middle phalanx and then
end with something like the frontal bone
meets the parietal bone along the
coronal suture and it would probably
take like two and a half hours to sing
the whole thing but there is way
more to know about your skeletal system
than just the names of the bones in
addition to bones there's
cartilage and fibrous connective tissue
that allows them to work together
and maybe more importantly if you're
going to talk about the physiology of
your skeleton you have to talk about
joints joints are the meeting places
between two or more
bones and even though it might not sound
mathematically possible you actually
have more joints in your body than you
have
bones in a lot of places like your hands
and feet each individual bone is part of
more than one or two joints and then
there's the matter of what all that
bone cartilage connective tissue and the
joints that they form actually work
together to do
and that is move body movements happen
when muscles contract across
joints moving one bone toward another
and studying the different types of
movements your body is capable of
is not only pretty fun it's also one of
the best practical ways to understand
your bones and joints if you ask me all
that is
infinitely more interesting than
memorizing what goes where and
pretending like a backbone is really an
anatomical structure so
aren't you glad you're a grown-up
don't worry it's not anyone you knew
it's plastic
and it's a little bit smaller than the
average adult skeleton but other than
having me stand in front of a giant
x-ray machine for ten straight minutes i
can't think of a better way to walk you
through the major anatomical structures
of your skeleton now if you managed to
retain our introduction to human bones
last week you'll recall that anatomists
typically divide the skeleton into two
major parts
the axial and the appendicular the axial
structure includes all 80
midline bones that form your skull
vertebral column and thoracic cage
starting at the top here your skull's
eight cranial and 14 facial bones form
your body's most complex bony structure
coming together to do some pretty great
things like protect your brain help you
see smell hear eat and you know have a
face the 33 irregular bones in your
vertebral column
start with your atlas vertebra the one
that holds up your skull and is named
for the greek god atlas
who held the world on his shoulders as a
punishment i'm not saying you got a huge
head or anything i'm just saying that's
what it's called the vertebra run down
from your skull to your pelvis providing
the central support for your upper body
and completing the enormously important
job of protecting your spinal
cord the main communication line between
nearly all of your body and your brain
most of your vital organs are protected
by the 12 pairs of ribs and dagger-like
sternum that together comprise your
thoracic
cage which also provides attachment
points for your back chest
shoulder and neck muscles now the
appendicular skeleton includes your
upper and lower limb appendages and
pectoral and pelvic girdles that attach
to the axial skeleton
at the shoulder and the thigh and even
though your arms and legs clearly serve
different functions unless you spend a
lot of time walking around on your hands
or picking stuff up with your feet
both sets of limbs share a similar setup
they're each composed of three major
segments a common arrangement we see in
loads of animals all the way back to
fish which anatomists refer to as
one bone two bones lots of bones and
digits
we can't all be dr seuss and that's just
about enough anatomy for this plastic
skeleton to pay for itself i'd rather
focus on the physiology of the skeleton
and in order to do that
we got to talk about joints as is often
the case in anatomy we classify joints
both by what they're made of and by what
they do
because form follows function we can't
really talk about one without talking
about the other so the structural
classification of your bones is all
about what kind of material binds those
bones together like
is it a fibrous tissue or cartilage or a
special fluid-filled joint cavity but
the functional classification
focuses on how much that joint can move
so for example you have joints in your
body that don't move at all
these include say the joints between the
bones that make up your cranium
these non-moving joints are called
synarthroses but you also have joints
that move only slightly like the spot
where your two pubic bones meet to form
your pelvis
that joint exists mainly to absorb shock
from walking and running but it also has
proven very handy during things like
childbirth these partly moving joints
are called amphiarthroses finally there
are the diarthroses which are fully
movable like your classic knee and elbow
joints
they're mostly found in your limbs but
if you eventually find yourself in the
business of examining and treating
people's joints and there are a lot of
doctors who do
then it'll be especially useful to know
the structural classifications
structurally speaking you've got fibrous
cartilaginous
and synovial joints fibrous joints
connect bones with dense
fibrous connective tissue and are mostly
immovable like those sutures between
your skull bones as my guess your
cartilaginous joints unite bones using
cartilage and are similar to their
fibrous brethren and that they don't
move very much and they lack a joint
cavity but the family of joints that
make all the sports and the entire art
of breakdancing and yoga possible are
the freely movable
synovial joints most joints in your body
fall into this category although they do
make use of cartilage and fibrous
connective tissues like ligaments
they're different in that the bones they
join are separated by a fluid-filled
joint cavity the cool thing about these
cavities is that they contain a bit of
viscous egg-white-like synovial fluid
that acts like grease on a hinge
without that lubricant just running down
the street could cause enough friction
not only to wear out your joint surfaces
but actually
overheat your joints enough to
essentially cook the surrounding tissue
and leave your legs smoking like a
desperate looney tunes character
these freely movable synovial joints
come in six different configurations
that together allow pretty much
any movement your body can make from a
subtle head nod to a vigorous jumping
jack all of which you will find in
action at your typical dance party
seriously though dance parties are an
excellent place to study skeletal
physiology
i mean let's just take this from my
perspective i'm at my high school prom
i'm wearing a tux that is really
uncomfortable and i'm feeling kind of
shy
standing on the edge of the crowd maybe
i want to test the waters out a little
bit with a little hand dancing
just a hand going waving my hand from
side to side it's a type of gliding
movement one that occurs when
one flat bone surface glides over
another all these motions use gliding or
plane joints like the one between the
distal ends of your radius and ulna and
the carpal bones of your wrist
but say after a while i feel like these
gliding movements just aren't enough to
fully express myself
i have to enlist the more versatile
angular movements these are the types of
motion that either
increase or decrease the angle between
two bones across
any plane in the body like if i bob my
head or kick my leg up or
raise the roof if people still do that
if that bending motion decreases the
angle of the joint and brings those
bones together
like when i bend my arm together that
movement is called flexion when i bend
it back
i'm increasing that angle and it's
called extension and if i continue that
motion beyond my normal anatomical
positioning and it's
somewhat dangerous that is called
hyperextension these motions enlist the
hinge joints
similarly my oval shaped condylar
knuckle joints allow my fingers to bend
and ball up for a good fist pump now i'm
getting super into it
am i like raise my arms up over my head
well the act of raising my arms away
from my body is called
abduction while moving them back down
toward the body is called adduction
and this here like lasso movement that's
circumduction a combination of all
five of those movements that allow the
forearm to move in a circle while the
elbow joint stays relatively stable
which really makes you want to give
credit to those cowboys now if you want
to get really serious about that
lassoing and expand it to include the
entire arm the move eventually will
morph from an angular movement to a
rotational one
a motion that turns the bone around its
axis in this case the humerus
hip and shoulder joints use a ball and
socket design that allow rotational
movement but the more flexible a joint
is the more unstable and fragile it is
which is one reason why you see so many
dislocated shoulders and hip
replacements and it's worth pointing out
that some movements are just
special and unique and weird and only
occur in a few select joints
especially in your hands feet and jaw
for example your ability to touch your
thumb to your fingertips or give a
thumbs up is thanks to
opposition movement facilitated by your
saddle joint which makes your thumbs
opposable the pivot joint between your
radius and ulna allow you to rotate your
palm forward or anteriorly in a motion
called supination
and turning it backward or posteriorly
is called pronation so that's what i
like to think is the more grown-up
approach to teaching you about your
skeleton
it did not involve any singing but yes a
little dancing
and it was worth it because you learned
about the basic structure of your
skeletal system including the
arrangements of both the axial and the
appendicular skeleton and we went over
the structural and functional
classifications of your joints along
with the major types of movement that
they make possible special thanks to our
headmaster of learning thomas frank for
his support for crash course and free
education
this episode was co-sponsored by greg
overbush fatma iqbal
and ricky d shields thank you to all of
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monthly contributions
if you like crash course and want to
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you can check out patreon.com crash
course crash courses filmed in the dr
cheryl c
kinney crash course studio this episode
was written by kathleen yale edited by
blake divestino and our consultant is dr
brandon jackson
our director is nicholas jenkins the
editor and script supervisor is nicole
sweeney our sound designer is michael
laronda and the graphics team is thought
cafe
you