The Skeletal System

The Skeletal System


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

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