The Systems of Our Body: Skeletal
Understanding the bones in our bodies
Has someone ever told you to “break a leg” before going on stage?
Obviously, we know the meaning isn’t literal and it’s really a wish of good luck. However, what happens when we genuinely break some bone in our body? What are the chemical and anatomical processes that occur to correct the damage and make the injured bone “good as new?” I had the same questions about our body since it’s especially fascinating how we consider rebuilding bones a typical phenomenon and nothing special.
In reality, there’s a rather complex process the body undergoes to repair bones; interestingly enough, the rebuilt bones are much stronger than the initial damaged ones, which makes our bones seem all-the-more interesting. Our bones are nothing more than a small component of a larger network known as the skeletal system.
The Composition of a Bone
Bones are the key building block to the skeletal system, which is why it’s especially important to understand a bone’s components before understanding the skeletal system. A bone is a rigid structure that forms a skeletal structure in any vertebrae and consists of several critical layers.
Periosteum
The outermost layer of a bone is called the periosteum and is especially known for its rigidity, which prevents our bones from shattering like glass at small incidents. Compact tissue is comprised of osteons (Haversian canals), a small structure within compact tissue that maintains rigidity in the bone.
An osteon is a cylindrically-shaped vessel with an osteogenic canal in the center, which is surrounded by similarly-shaped “rings” of the matrix (a material consisting of mostly collagen) called lamellae. This “central canal” also contains blood vessels that transport blood and nerve fibers to receive electrical signals.
An interesting fact to note about osteons is their alignment; in compact tissue, osteons are directionally similar to the lines of stress on a bone, thereby preventing the bone from easily bending or fracturing. Depending on the position of a bone in the body, there will be a certain number of osteons present.
Cancellous Bone
The second layer in the bone is cancellous bone, otherwise known as “spongy bone” due to its structure. Unlike the periosteum (which contained osteons), cancellous bone consists of cells called osteocytes that are located in small “crooks” called lacunae.
Osteocytes are formed from osteoprogenitor cells, which are a type of stem cell that predominantly form structures in the bone, including osteoblasts (a type of cell contained in bone marrow). Essentially, osteocytes are a type of osteoblast that are superfluous with matrix and thus have a different function. Unlike most cells, osteocytes’ extendable cytoplasm allows them to exchange nutrients (the region where this occurs is called a gap junction). Using a mechanism called osteocytic osteolysis, osteocytes are able to terminate other bone cells, including themselves when necessary.
Blood vessels from compact tissue will travel to the inner cancellous bone and bone marrow, allowing for the creation of new red blood cells. Occasionally, osteocytes that are close to the blood vessels can utilize the nutrients being transferred through their extendable cytoplasm.
Cancellous bone also stores bone marrow (myeloid tissue), which is a component of spongy tissue that forms when blood vessels are tightly constricted and later condensed. Unlike compact tissue containing yellow bone marrow, spongy tissue’s red bone marrow is critical for processes like erythropoiesis (the production of red blood cells).
Divisions of the Skeletal System
There’s no denying that the skeletal system is huge. Although it’s common knowledge, the fact that an adult has 206 bones can be crazy to wrap your head around, especially considering how difficult it would be to know all the bones in our body. Luckily, the skeletal system is divided into two components: the axial and appendicular skeleton.
The axial skeleton contains parts of the skull, larynx, spinal column, and thoracic cage (ribcage). On the other hand, the appendicular skeleton accounts for the upper/lower limbs, shoulders, and pelvic region. The term “appendicular” refers to the bones involved “appending” to the axial skeleton.
The Axial Skeleton
As previously mentioned, there are four components to the axial system. Let’s take an in-depth look at each part and its functionality!
The Skull
In any vertebrae, the skull is responsible for protecting the brain from external damage and supporting facial structures. This critical part of the skeletal system contains two parts: the cranium and mandible. While the cranium comprises the “superior” part of the skull (the top region), the mandible refers to the lower jawbone.
The cranium can be further divided into the cranium roof and base; the “roof” (calvarium) contains frontal, occipital, and two parietal bones and the “base” contains six additional bones. The cranium base of the skull is also connected to facial structures, the mandible, and the region of the spinal column in the neck.
In addition to being the strongest bone on the face, the mandible also acts as a “container” for the lower teeth and connects with temporal bones in the cranium (this forms the temporomandibular joint). When combined with the maxilla (upper jawbone), the entire mouth structure is formed.
Though not commonly known, pieces of cartilage located in the ear also make up parts of the skull due to their proximity. Inside the temporal bone of the skull (close to the upper part of the mandible), three bones located in the ear are the malleus, incus, and stapes. When working collectively, these bones transfer vibrations to the inner ear.
The Larynx
The main purpose of the larynx is to control airflow through the trachea by opening and closing when necessary to prevent foreign substances (e.g. viruses, bacteria) from entering. However, the larynx can also develop phonetic sounds (allowing a variety of species to make vocal sounds) and allow coughing.
The larynx has a hyoid bone that secures it beneath the chin but also consists of several additional cartilage pieces. The three largest bones of the larynx are the cricoid, thyroid, and epiglottis; three additional pairs of cartilage in the larynx are the arytenoids, corniculate, and cuneiform. The larger parts of the larynx primarily control the body’s substance intake, whereas the smaller sections support other parts of the larynx and provide the larynx with a range of other functions (e.g. speaking).
Vertebral Column
Considered to be the longest part of the axial skeleton, the vertebral column consists of 24 vertebrae (building blocks of the vertebral column) as well as the coccyx and sacrum. Critical functions of the spinal column include protecting the delicate spinal cord (which is involved in the nervous system — more on that later 😉) and the guidance of several muscle groups through attachment.
The coccyx (more commonly called the tailbone) ensures stability between the hips when vertebrae are seated and allows for the connection of muscles, ligaments, and tendons. While also contributing to an equal distribution in weight, the sacrum also provides support and connection within the pelvic region.
Thoracic Cage
Also called the ribcage, the thoracic cage primarily protects the organs it houses, which include most of the respiratory system and parts of the upper limbs. The ribcage consists of ribs (quite obviously) and another structure called the sternum.
An individual rib contains a head, neck, and body. The “head” part of the rib is wedge-shaped, the “neck” part connects the head and body (of the rib), and the “body” part protects vessels and nerves from damage. This is the structure of a typical rib, which are ribs 3–9 (see diagram below).
The other five ribs (1–2, 10–12) are considered atypical ribs due to their peculiar structure. For example, Rib 1 is shorter and wider compared to other typical ribs, whereas Ribs 11–12 don’t have necks and thus “float” off of the ribcage (hence the name “floating ribs”). All ribs are connected to the spinal column, and can connect in two ways:
- Costovertebral Joint- This joint connects the head of ribs to vertebrae on the spinal column.
- Costotransverse Joint- This joint is formed between the tubercle (protective cap) of the rib with the vertebrae of the spinal column. These joints are primarily for gliding motions in the ribcage, and these joints are strongest with the “floating ribs.”
The sternum (also called the breastbone) is the central structure of the thoracic cage and resembles a “T-shape” that connects to the ribs through cartilage. This bone is designed to protect internal organs such as the heart, lungs, torso, and blood vessels in the chest. Another important thing to note about ribs is that Ribs 1–7 are true ribs, whereas Ribs 8–12 are false ribs. This is due to the true ribs directly attaching to the sternum, whereas the false ribs don’t have this quality.
The Appendicular Skeleton
In the appendicular skeleton, we look at bones that are attached to the axial skeleton. There are four critical sections of this skeleton, so let’s take a look at each!
Pectoral Girdles
The pectoral girdle(s), otherwise called the shoulder girdle, is responsible for connecting the upper extremities to the axial skeleton. In the human skeletal system, the two key components of the pectoral girdles are the clavicle and scapula. However, vertebrae have an additional bone in their shoulder girdle called the coracoid; instead, the coracoid in humans is fused with the scapula and forms a homologous bone. The joint that connects pectoral girdles to the axial skeleton is called the sternoclavicular joint, which contains more muscle and thus allows greater flexibility in our shoulder joints.
Inside the shoulder girdle, there are five joints that enable connectivity and flexibility in this bone section. The joints are the following:
- Glenohumeral joint
- Sternoclavicular joint
- Scapulothoracic joint
- Scapulothoracic joint
- Suprahumeral joint
As previously described, there are two key parts of the pectoral girdle, which are the clavicle and the scapula. The clavicle (collarbone) is a long, horizontally-positioned bone that restricts compression between the scapula and sternum, allowing free movement in the arms. The primary functions of the clavicle are to allow greater movement in the arms, protect nerves in the upper arms, and distribute external force to the axial skeleton.
The scapula (shoulder blade) is a triangular-shaped bone that connects the clavicle with the upper arm and is located in the back of the shoulder girdle and wall of the ribcage. In the scapula, there are three angles and another three borders that are used to determine the shape. The three angles of the shoulder blade are:
- Medial angle
- Inferior angle
- Glenoid angle
On the other hand, the three borders include:
- Superior border
- Axillary border
- Medical border
Critical functions of the scapula include enabling motions in the shoulder, including stabilizing nearby bones to ensure maximum movement in the shoulder region. Since there are several muscles connected to the pectoral girdle, ensuring movement in the shoulder region is a primary focus for the shoulder blade.
Pelvic Girdle
The pelvic girdle refers to a subdivision of the pelvic skeleton, including the sacrum, coccyx, and hip bones of the lower trunk. As a whole, the pelvic girdle connects the pelvic region to the lower limbs, and it connects the axial skeleton to those lower limbs. An important function of the pelvic girdle is to connect the trunk and legs, as well as protecting internal organs such as the intestines and bladder. Another important note about the pelvic girdle is its bowl-like shape, which increases rotation ability in the pelvic region.
The hip bones of the lower trunk consist of three parts: the ilium, ischium, and pubis. When working collectively, these bones form a cup-shaped socket called the acetabulum. A key function of this socket is to provide space for the thigh bone (femur).
Upper Extremities
The upper extremities allude to the bones in our arms, including the pectoral girdles on either side. There are six components to these upper extremities, all of which occur in pairs:
- Humerus: The humerus is the largest part of the upper extremities and connects the scrapula with bones in the lower half of the arm; its primary function is connecting muscles and ensuring mobility in the upper arm.
- Radius: The radius is the largest bone in the forearm and extends from the elbow to the left side of the wrist (the part containing our thumbs). Since this bone occurs within the humerus and ulna, its primary function is to ensure coordinated movement with these bones, allowing mobility in the arms.
- Ulna: The ulna is a thinner bone in the forearm that extends from the elbow to the side of the wrist opposite the thumbs. Along with the radius, the ulna allows for rotation in the arms and wrist.
- Carpals: The term “carpals” refers to eight bones that connect the forearm to the hand. These bones allow for maximum mobility in the wrist, including general movement and rotation.
- Metacarpals: The metacarpals are the five bones that occur between the phalanges and carpals. Most of the nerves and vessels in our fingers are connected to the metacarpals, so their function is to sustain this connectivity.
- Phalanges: Phalanges are bones found in both the upper region of the fingers and toes. Since the flesh near our fingertips is rather sensitive, phalanges sustain the non-skeletal parts of the fingertips and prevent damage to surrounding nerves.
Lower Extremities
While the upper extremities involved the arms, the lower extremities are a reference to leg bones. Similar to the bones in our arms, the bones of our legs occur in pairs and include:
- Femur: Also called the thigh bone, the femur extends from the hip bone of the pelvic region to the knee. This bone is the heaviest in the skeletal system since it provides stability for the body’s weight and ensures an equal distribution of weight.
- Tibia: The tibia (shinbone) is the larger of two bones located in the lower leg, extending from the knee to the ankle. Similar to the femur, the tibia accounts for weight distribution between the knee and bones in the feet.
- Fibula: The fibula is a thin bone in the lower leg and, similar to the tibia, extends from the knee to the ankle. While the fibula stabilizes the ankle, it’s also connected to muscles in the lower leg and supports them.
- Patella: The kneecap is a flat, triangular bone that connects the upper and lower regions of the leg. In addition to protecting the knee joint, the patella also connects certain ligaments (e.g. patellar ligament) to the femur.
- Tarsals: Similar to carpals located in the wrist, the tarsals are seven irregularly-shaped bones that are located in the ankle region. These bones are responsible for that “arch” in our foot that allows general stability and for the ankle to maintain weight.
- Metatarsals: These bones correspond to the metacarpals in the upper extremities, and connect the phalanges of the feet to the tarsals in the ankles. Metatarsals form arches along the feet, which allow for standard human functions (e.g. standing and walking).
Key Takeaways
- The key building block of the skeletal system is a bone, which contains two layers: the outer compact tissue and inner spongy tissue.
- The skeletal system is divided into two parts: the axial and appendicular skeleton. While the axial system focuses on the “vertical axis” of the body, the appendicular system contains the limbs, shoulders, and pelvic region.
- In the axial skeleton, the four key components are the skull, larynx, spinal column, and ribcage.
- In the appendicular skeleton, the bones involved are the limbs, shoulders, and pelvic region.
Glossary
- Appendicular Skeleton: The part of the skeleton containing the upper/lower limbs, shoulders, and pelvic region. This skeleton is called “appendicular” since it “appends” to the axial skeleton.
- Axial Skeleton: The part of the skeleton associated with the vertical axis of the body. Parts of the axial skeleton include the skull, larynx, spinal column, and ribcage.
- Bone: The building block of the skeletal system, which contains two layers: the compact bone (periosteum) and spongy tissue (cancellous bone).
- Bone Marrow: A component of spongy tissue that contains a majority of the body’s stem cells, which usually develop into red cells for the body. There is yellow bone marrow in the periosteum, but it mostly contains fat and doesn’t contribute much to erythropoiesis.
- Cancellous Bone: The innermost layer of bone, which contains bone marrow and cells called osteocytes.
- Gap Junction: The region where an osteocyte’s extended cytoplasm allows for the transfer of nutrients.
- Lamallae: Concentric rings made of the matrix (a substance made of primary collagen) that surround the osteogenic center.
- Larynx: The section of the axial skeleton that controls the airways and allows the creation of phonetic sounds.
- Lower Extremities: The section of the appendicular skeleton that involves the legs.
- Osteocytes: Structure derived from osteoprogenitor cells that maintain homeostasis in the cancellous bone layer.
- Osteon: A cylindrical structure containing an osteogenic center (which delivers nutrients to spongy tissue) and lamellae surrounding the central structure. Osteons are a key component of periosteum since they enable its functionalities.
- Pectoral Girdle: The section of the appendicular skeleton that enables movement in the shoulder region.
- Pelvic Girdle: The section of the appendicular skeleton that creates equal weight distribution in the trunk and connects the lower limbs to the axial skeleton.
- Periosteum: The outermost layer of the bone, comprised of cylindrical structures called osteons.
- Skeletal System: The system of vertebrae that’s responsible for supporting the body and preventing injury to fragile internal organs.
- Skull: The section of the axial skeleton that protects the brain from external damage and supports facial structures. The skull consists of two components: the cranium and mandible.
- Thoracic Cage: The section of the axial skeleton that protects critical internal organs (including the heart, lungs, and torso) and consists of subdivisions called ribs.
- Upper Extremities: The section of the appendicular skeleton that involves the arms.
- Vertebral Column: The section of the axial skeleton that protects the spinal cord and provides an essential structure for the human body.
