Introduction to the skeletal system

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.

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.

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.

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. 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. 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.

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.