Introduction to the skeletal system

Definitions & Key takeaways

The skeletal system serves as the frame for the body, providing support and protection. It consists of bones, joints, cartilage, and ligaments. Bones consist of living tissue that undergoes constant remodeling as it matures and ages, whereas joints are the points where two bones meet and can allow for movement. Cartilage is a tough yet flexible connective tissue that lines the joint surfaces and helps to cushion them, whereas ligaments attach bone to bone generally to stabilize joints.

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Introduction0:00–0:26

What comes to your mind when you hear the word “skeleton”? Maybe last halloween?
While you may associate the skeletal system with the dead, it’s actually an integral part of our living bodies. It is made up of 206 bones that provide form and support, protect the inner organs, and together with muscles, bones also help in performing different types of movements.
Now, our skeleton can be divided into an axial and an appendicular skeleton. Let’s start with the bones of the axial skeleton, which consists of the bones of the head, neck and trunk.

Skeleton0:26–0:56

These bones include the skull or the cranium, a small bone in the neck called the hyoid bone, the vertebral column, and the ribs and sternum.
The appendicular skeleton, on the other hand, consists of the bones of the upper and lower limbs. Now, bones of the axial and appendicular skeletons are further classified based on their shape as long, short, flat, sesamoid, and irregular bones.

Bone classification0:56–2:02

Let’s start with long bones, which are tubular in shape, have a long axis and usually two ends that articulate with other bones.
An example is the humerus of the upper arm. Short bones, on the other hand, are cuboidal in shape, and examples include the carpal bones of the wrist.
Next are flat bones, which are flat, thin, and possibly curved bones that usually protect internal organs. An example of flat bones are the cranial bones that protect the brain.
Next are sesamoid bones, which are particularly unique because they lie within tendons. The largest sesamoid bone is the patella, or the kneecap, which is a triangular bone that covers the anterior side of the knee joint.
Lastly are irregular bones, which are bones that don’t fall into any of these categories, mainly because they have more complex shapes.
Examples of irregular bones are the vertebrae which form the spine. Ok, now the surface of many bones is rather rough, with many bumps and depressions, called bone markings.

Bone markings2:02–6:38

These bone markings allow bones to come in contact with other structures, like ligaments, tendons, fascia and blood vessels.
Now, there are three classes of bone markings: first, there are those that help in joining bones; second, there are extensions and projections; and third, depressions and holes.
Let’s start with bone markings that form bony depressions and holes, which include notches, fossae, and foramina. A notch describes a dip or indentation in the margins of a bone, like the mandibular notch of the lower jaw.
A fossa is a shallower depression, like the iliac fossa of the hip bone. Lastly is a foramen, which is an opening within a bone that gives passage to nerves and vessels.
These foramina may be small, like the infraorbital foramen of the orbit, or quite big, like the obturator foramen of the hip bone.
Next up are the bone markings that help in joining bones, which include the head, condyle, epicondyle, and facet. First is a head, which describes a large, round articular end of a bone that fits into a depression or fossa of another bone, forming a joint.
An example here is the head of the humerus, which fits into the glenoid fossa of the scapula, forming the shoulder joint.
Next is a condyle, which describes a rounded articular end, smaller than a head and usually occurring in pairs. Examples of condyles include the medial and lateral condyles of the femur, which articulate with the tibia, forming the knee joint.
While we’re at it, a small bump or eminence that lies on top of or beside a condyle is called an epicondyle. So, it makes perfect sense for the femur to have two epicondyles, one above each condyle.
These epicondyles don’t articulate with other bones, instead, they form attachment points for muscles. Finally is a facet, which is a smooth surface that is flat or slightly curved.
Typically, a facet articulates with another flat surface or facet, forming a joint. For example, the ribs have many facets that join other facets of the vertebrae.
Alright, now the next category on the list are bone extensions and projections, which include protuberances, spinous processes, spines, trochanters, tubercles, tuberosities, malleoli, lines, crests, and grooves.
Let’s start with a protuberance, which describes a protruding part of a bone. An example of a protuberance may be seen at the back of the skull, called the external occipital protuberance.
Next is a spinous process, which is a spike-like projection, as seen at the back of each vertebra of the vertebral column.
A spine however, describes a sharp, narrow process. An example would be the spine of the scapula seen on the posterior aspect of the scapula.
Next are trochanters, tubercles, tuberosities, and malleoli, which describe rounded, somewhat similar bumps. Trochanters are large, blunt elevations that lie on the sides of the upper end of the thigh bone, called the femur.
These trochanters are called the greater and lesser trochanters, and serve as attachment points for muscles and connective tissue of the lower limb.
The humerus has similar, but slightly smaller eminences, called the greater and lesser tubercles of the humerus. Next is a tuberosity, which is a large round elevation that also serves as an attachment point for muscles and connective tissue.
An example of a tuberosity lies near the middle of the shaft of the humerus, called the deltoid tuberosity where the deltoid muscle attaches.
Finally is a malleolus, which is a rounded process. The popular examples would be the medial and lateral malleoli on each side of the ankle, which are projections of the distal tibia and fibula.
Lastly, are lines, crests, and grooves, which are three elongated projections. Let’s start with a line, which is a linear process, like the one seen at the back of the femur, called the linea aspera.
A crest describes a raised or prominent ridge of bone. An example of a crest is the iliac crest, which lies at the top of the hip bone.
Lastly is a groove, which describes a narrow, linear depression. Typically, a groove houses and protects a nerve or a blood vessel.
For example, the radial groove of the humerus houses and protects the radial nerve and the deep artery of the arm. Ok, let’s take a quick break and try to identify these structures.

Quiz6:38–6:51

Great, now let’s get familiar with some terms related to long bones. First, the diaphysis describes the middle - or shaft - of a long bone, while the epiphysis describes its proximal and distal ends.

Long bones6:51–7:23

Near the epiphysis, the diaphysis becomes a bit wider, forming the metaphysis. Finally, between the metaphysis and the epiphysis is a layer of cartilage, called the epiphyseal plate, or growth plate, which allows these bones to keep growing during infancy, childhood and early adulthood.

Joint classification7:23–12:41

Okay, now let’s talk about joints! Joints are the junction points between two or more bones.
There are three categories of joints based on their structure: fibrous, cartilaginous, and synovial joints. Let’s start with fibrous joints.
These joints allow little, or no, movement because articulating bones are firmly connected by fibrous tissue. Now, based on the type and length of this fibrous tissue, fibrous joints may be further divided into three categories, called sutures, syndesmoses, and gomphoses: sutures are the zipper-like joints between the bones of the skull where short fibers connect the interlocking bones; syndesmoses connect bones through a fibrous membrane, like the interosseous membrane connecting the radius and ulna; and lastly, gomphoses, also called dentoalveolar syndesmoses, connect the roots of teeth to their bony sockets within the bones of the jaw.
Next are cartilaginous joints, which typically allow for slight movements, thanks to the cartilage tissue binding the bones together.
Just like fibrous joints, the cartilaginous joints are grouped into subcategories depending on the type of cartilage involved.
Primary cartilaginous joints, also known as synchondroses, are joined together by hyaline cartilage. These joints may be temporary, like the epiphyseal plate of the long bones or permanent like where the first rib is anchored to the manubrium by its costal cartilage.
Secondary cartilaginous joints - or symphyses - on the other hand, are united by fibrocartilage, which is stronger, but less flexible than hyaline cartilage.
An example are the intervertebral discs that connect adjacent vertebrae. Finally there are the synovial joints, which are the most common type of joint.
In synovial joints, the articular surfaces of the bones are covered with hyaline cartilage, but they are separated by a fluid-filled joint cavity that is surrounded by a joint capsule.
These features make synovial joints freely movable. There are six main types of synovial joints that are classified based on the structure of the articulating surfaces and the movement they permit.
The six types include plane, hinge, pivot, saddle, condyloid and ball-and-socket joints. More simply, we can divide them into uniaxial, biaxial and multiaxial joints.
Let’s start with the uniaxial synovial joints. These are joints that move around a single axis, and they include the plane, hinge and pivot joints.
A plane joint describes a small joint between two flat bony surfaces, allowing for slight gliding, or sliding movements.
Typically, this type of joint is found between the clavicle and the acromion of the scapula or between the wrist and ankle bones.
A hinge joint looks and acts similarly to a door hinge, only allowing flexion and extension, like at the elbow joint between the humerus and forearm bones.
Lastly, a pivot joint is made of a bony process that fits into another bone, allowing rotational movement around a single axis.
An example of this joint is the atlantoaxial joint in the neck, which connects the atlas, the first vertebra of the spine, with the axis, which is the second vertebra.
This joint allows for rotation of the head, like when you shake your head “no”. Up next are the two biaxial synovial joints, which allow movements to occur in two planes.
These include the saddle and condyloid joints. The saddle joint consists of one convex surface fitting into another concave surface, resembling a person sitting on a horse’s saddle.
To visualize the movements of this joint, think about a horseback rider leaning back and forth for flexion and extension in the sagittal plane, or moving side-to-side for abduction and adduction in the frontal plane.
Combining these movements also allows a certain level of circumduction. A good example is the joint of the thumb and the hand, called the carpometacarpal joint of the thumb.
Move your thumb around and see for yourself! Next, is the condyloid joint, which is similar to the saddle, but since its articulating surfaces are ellipsoid, its circumduction is much more limited and movement is usually greater in one axis than in the other.
For example, let’s take the metacarpophalangeal joints, which are the knuckle joints in the hand. These joints allow much more flexion and extension in the sagittal plane than abduction and adduction - or side-to-side movements - in the frontal plane.
Lastly, there are multiaxial synovial joints, namely ball-and-socket joints, which allow a much further range of motion, on multiple planes.
This articulation consists of a head of one bone that nicely fits into a concavity, or socket of another bone. This special articulation allows for the most movement out of all types of synovial joints.
These movements include flexion, extension, adduction and abduction, as well as medial and lateral rotation, which helps perform a wider range of circumduction.
The hip joint is one of the most obvious examples for a ball and socket joint where the round head of the femur articulates with the socket-like acetabulum of the hip bone.
Ok, pause the video for as long as you need, to see if you can classify each of these joints. Alright now, most joints are secured by strong fibrous bands of connective tissue, called accessory ligaments.

Quiz12:41–12:51

Ligaments12:51–14:00

These ligaments strengthen the joint, stabilize it, and allow movement in only certain directions, basically holding the bones together and preventing their dislocation.
Now, the ligaments associated with synovial joints can be classified based on their relationship to the fibrous joint capsule.
There are capsular ligaments – also called intrinsic ligaments - that are really just thickenings of the joint capsule. For example, there are three capsular ligaments of the hip joint, including the iliofemoral ligament.
Then there are extracapsular ligaments - also called extrinsic ligaments - which are separate from the joint capsule. For example, the coracoclavicular ligament anchors the clavicle to the scapula, but is separate from acromioclavicular joint, where the clavicle articulates with the scapula.
Finally, some joints have intracapsular ligaments which are found deep to the joint capsule. For example, the anterior and posterior cruciate ligaments - or the ACL and PCL for short – are found within the knee joint, deep to the joint capsule.

Blood supply and innervation14:00–14:42

Joints, like any other tissue, require blood supply and innervation. Their arterial supply comes from articular arteries, which are branches from neighboring arteries that run in the joint capsule and anastomose, or form network connections with each other, in order to reach all parts of the joint.
The waste is then removed from the joint by the articular veins, which accompany the arteries. Joint innervation is also important for providing pain sensation, as well proprioceptive signals that carry information about the movement and position of the body.
The Hilton law states that the nerves that innervate the muscles surrounding or crossing a particular joint, also innervate the joint itself, and the skin surrounding it.
Alright, as a quick recap. The human skeleton is made up of axial and appendicular bones.

Review14:42–15:09

These same bones can be further classified based on their shape into long, short, flat, sesamoid, and irregular bones. The surface of bones have different markings, where they come in contact with other bones, blood vessels, and muscles.
Joints are where bones articulate with each other mainly to allow for movement, and they’re classified into