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Professor Dave again, let’s look at the
human skeleton.
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Now that we’ve learned about the structure
of bones, we are ready to take a look at how
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they are assembled in the body.
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The skeletal system is comprised mainly of
bones, around two hundred and six of them
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in an adult to be specific, but there is also
a good amount of cartilage, joints, and ligaments,
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which all together make up around twenty percent
of a person’s body mass.
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We will get to joints a little bit later,
first let’s check out all the different
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bones in the body.
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As we recall, there are two sections to the
human skeleton, those being the axial skeleton,
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made of the skull, vertebral column, and thoracic
cage, and the appendicular skeleton, made
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more or less of just the limbs.
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Let’s go through the axial skeleton first,
starting at the top with the skull.
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The skull is a fascinating structure, made
of twenty two different bones.
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Cranial bones are the ones that protect the
brain, and facial bones are the ones that
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give structure to the face.
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Most of the bones in the skull are flat bones,
and in the cranium these are connected at
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serrated lines called sutures.
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The cranium is made of a vault, as well as
a base, and we should note that the base is
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divided into the anterior, middle, and posterior
cranial fossae.
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Together, these produce the cranial cavity,
where the brain sits.
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There are also ear cavities and nasal cavities,
as well as orbits, which house the eyes.
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All together there are eight cranial bones.
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There is the frontal bone, two large parietal
bones, the occipital bone, two temporal bones,
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the sphenoid bone, and the ethmoid bone.
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The cranial bones are connected, as we said,
by sutures, and those have specific names
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as well.
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These are the coronal, sagittal, lambdoid,
squamous, and occipitomastoid sutures.
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We should also mention the foramina, which
are holes that nerves and arteries and veins
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pass through, most notably the foramen magnum
at the base of the skull through which the
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spinal cord passes.
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Moving on to the facial bones, of which there
are fourteen, we can start with the mandible,
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which is the lower jawbone.
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Then there are maxillary bones, which form
the upper jaw and part of the face.
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Next we have two zygomatic bones which are
the cheekbones, nasal bones which make up
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the bridge of the nose, lacrimal bones, palatine
bones, the vomer, and inferior nasal conchae.
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Lastly, technically not part of the skull,
there is also the hyoid bone, which sits just
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below the mandible, and does not connect with
any other bone.
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Next up in the axial skeleton is the vertebral
column, also called the spinal column, or
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simply the spine.
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This is comprised of twenty six irregular
bones that come together to form a flexible
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structure in a curvy S-shape, and this supports
everything from the skull to the pelvis.
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The spine can be divided into five sections.
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At the top we have the cervical vertebrae,
which are the first seven.
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The next twelve are called the thoracic vertebrae.
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The remaining five are called the lumbar vertebrae.
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We should note that the vertebrae get larger
as we go down, in order to support more and
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more weight.
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Below the vertebrae we can find the sacrum,
which is actually five vertebrae fused together,
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and lastly, below the sacrum there is the
coccyx, otherwise known as the tailbone, which
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is made of a few tiny vertebrae fused together.
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Of course there is much more to the spine
than just the vertebrae.
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There are lots of ligaments keeping everything
together.
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The main ones are the anterior and posterior
longitudinal ligaments, running down the front
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and back of the column from the neck to the
sacrum.
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There are also shorter ligaments that connect
adjacent vertebrae, as well as intervertebral discs.
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These are cushiony pads made of a nucleus
pulposus, which is the more elastic part,
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surrounded by an anulus fibrosus, with lots
of collagen.
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These are found in between each vertebra,
acting as shock absorbers when we run and jump.
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Now let’s look a little closer at an individual vertebra.
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These all have a body and a vertebral arch.
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The hole is called the vertebral foramen,
and the spinal cord passes through here, which
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we will discuss later.
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The vertebral arch is made of two pedicles
and two laminae, and from these project various
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processes.
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These are the spinous process, two transverse
processes, as well as the superior and inferior
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articular processes.
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The vertebrae vary slightly depending on where
they are found in the column.
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Cervical vertebrae have a spinous process
that is very short, a vertebral foramen that
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is large, and an additional transverse foramen
to accommodate vertebral arteries.
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Thoracic vertebrae have a spinous process
that is long and points down, and they also
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exhibit structures called demifacets which
connect to the ribs.
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Lumbar vertebrae, being much larger, have
pedicles and laminae that are short and thick,
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as well as other slight discrepancies.
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The last part of the axial skeleton is the
thoracic cage.
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This is essentially comprised of the sternum
and the ribs, as well as a lot of costal cartilage.
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The sternum is a flat bone right in the middle
of the thorax, and it is made from three smaller
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bones that have fused together.
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From top to bottom these are the manubrium,
the body, and the xiphoid process.
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Then there are twelve pairs of ribs that project
from the vertebrae.
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The first seven pairs attach directly to the
sternum via sections of costal cartilage,
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and these are called true ribs.
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Then there are five pairs of false ribs, three
of which attach to the sternum indirectly,
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with costal cartilage joining the cartilage
from ribs above, and then the last two are
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called floating ribs, because they don’t
attach to the sternum at all.
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Ribs are flat bones that get longer going
from pair one to seven, and then shorter again
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from eight to twelve.
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With the axial skeleton complete, let’s
move on to the appendicular skeleton.
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While this is mainly just our limbs, there
are other components to mention as well.
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Let’s start with the pectoral girdle.
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This is comprised of the clavicle, or collarbone,
and the scapula, or shoulder blade, which
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together give structure to the shoulder, thereby
attaching the upper limbs to the axial skeleton.
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The clavicle has a sternal end where it attaches
to the manubrium, and an acromial end, which
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joins the scapula.
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The scapula is a thin, flat bone, roughly
triangular, and it has three borders, the
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superior, the medial or vertebral, and the
lateral or axillary.
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From here, we move on to the upper limb.
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This consists of the arm, forearm, and hand.
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Though colloquially we think of this whole
thing as an arm, when speaking in terms of
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anatomy, it is just this upper portion that
we call the arm, so let’s start there.
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In the arm we find the humerus, a typical
long bone, with its greater and lesser tubercle,
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radial groove, medial and lateral epicondyle,
radial and coronoid fossa, trochlea, and capitulum.
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Moving on to the forearm, we now see two bones,
the radius and the ulna.
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These are connected all the way down by the
interosseous membrane, a flexible ligament.
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The ulna is slightly longer, with its olecranon
and coronoid process.
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The radius goes from wide to thin the other
way, with a thin head, the radial tuberosity,
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and a radial styloid process.
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From there we see the hand, which has many
separate bones.
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The carpus, or wrist, is made of eight short
bones called carpals.
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These are the scaphoid, lunate, triquetrum,
pisiform, and then the trapezium, trapezoid,
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capitate, and hamate.
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Next we see the five metacarpals, which make
up the palm of the hand, and they are simply
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named one through five, from thumb to pinky.
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These connect to the phalanges, which are
the bones that make up your fingers.
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There are fourteen of these bones per hand,
three per finger, which are the distal, middle,
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and proximal phalanges, except the thumb which
has two, as it has no middle phalanx.
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Moving back over to the torso, we see the
pelvic girdle.
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This attaches the lower limbs to the axial
skeleton just like the pectoral girdle did
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for the upper limbs, although this one has
far less mobility and far more stability than
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the other.
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This girdle starts at the sacrum we described
earlier, and continues with two hip bones.
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These are made of three separate bones at
birth, which fuse to become one by adulthood,
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but we still describe the regions of the hip
bone as being the ilium, ischium, and pubis.
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Lastly, the lower limb contains very thick
bones, allowing us to run and jump effectively.
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The thigh is made of a single bone just like
the arm, and this one is called the femur,
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which is the largest bone in the body.
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Here we see the head, with a small pit called
the fovea capitis.
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Then the greater and lesser trochanter, the
intertrochanteric crest, the gluteal tuberosity,
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linea aspera, medial and lateral condyles,
and epicondyles, intercondylar fossa, and patella.
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From there, we go to the leg, which like the
forearm, contains two bones, the tibia and
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the fibula.
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Again, we see an interosseous membrane between them.
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In the larger tibia, we see the medial and
lateral condyles, the intercondylar eminence,
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tibial tuberosity, anterior border, medial
malleolus, and fibular notch.
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The fibula is much thinner, with its head
and lateral malleolus.
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Then we get to the foot, which is similar
to the hand.
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We see the tarsus, made of seven bones called tarsals.
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The biggest two, the talus and calcaneus,
make up the ankle.
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Then there is the cuboid, the navicular, and
the medial, intermediate, and lateral cuneiform bones.
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Next we see the metatarsus, with five long
metatarsals, again numbered one through five.
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Also like the hand, we see fourteen phalanges,
three per toe, except two for the big toe,
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also known as the hallux.
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So that wraps up our basic tour of the human
skeleton, at least from the standpoint of
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the bones, which are the primary component.
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But there are other structures that are critical
to the function and mobility of the skeleton,
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and these are called joints, so let’s move
forward and learn about these now.
