The skeletal system in an adult body is made up of 206 individual bones. These bones are arranged into two major divisions: the
axial skeleton and the
appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones in the following regions:
- Skull
- Hyoid
- Auditory ossicles
- Ribs
- Sternum
- Vertebral column
The appendicular skeleton is made up of 126 bones in the folowing regions:
- Upper limbs
- Lower limbs
- Pelvic girdle
- Pectoral (shoulder) girdle
SkullThe
skull
is composed of 22 bones that are fused together except for the
mandible. These 21 fused bones are separate in children to allow the
skull and brain to grow, but fuse to give added strength and protection
as an adult. The
mandible remains as a movable jaw bone and forms the only movable joint in the skull with the
temporal bone.
The bones of the superior portion of the skull are known as the
cranium and protect the brain from damage. The bones of the inferior and
anterior portion of the skull are known as facial bones and support the
eyes, nose, and mouth.
Hyoid and Auditory OssiclesThe
hyoid
is a small, U-shaped bone found just inferior to the mandible. The
hyoid is the only bone in the body that does not form a joint with any
other bone—it is a floating bone. The hyoid’s function is to help hold
the
trachea open and to form a bony connection for the
tongue muscles.
The malleus, incus, and stapes—known collectively as the
auditory ossicles—are
the smallest bones in the body. Found in a small cavity inside of the
temporal bone, they serve to transmit and amplify sound from the eardrum
to the inner ear.
VertebraeTwenty-six vertebrae form the
vertebral column of the human body. They are named by region:
With the exception of the singular sacrum and coccyx, each vertebra
is named for the first letter of its region and its position along the
superior-inferior axis. For example, the most superior thoracic vertebra
is called T1 and the most inferior is called T12.
Ribs and SternumThe sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the
thoracic region of the skeleton. The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.
There are 12 pairs of ribs that together with the sternum form the
ribcage of the thoracic region. The first seven ribs are known as “true
ribs” because they connect the thoracic vertebrae directly to the
sternum through their own band of costal cartilage. Ribs 8, 9, and 10
all connect to the sternum through cartilage that is connected to the
cartilage of the seventh rib, so we consider these to be “false ribs.”
Ribs 11 and 12 are also false ribs, but are also considered to be
“floating ribs” because they do not have any cartilage attachment to the
sternum at all.
Pectoral Girdle and Upper LimbThe pectoral girdle connects the
upper limb (arm) bones to the axial skeleton and consists of the left and right clavicles and left and right scapulae.
The humerus is the bone of the upper arm. It forms the ball and socket
joint of the shoulder with the scapula and forms the
elbow joint
with the lower arm bones. The radius and ulna are the two bones of the
forearm. The ulna is on the medial side of the forearm and forms a hinge
joint with the humerus at the elbow. The radius allows the forearm and
hand to turn over at the wrist joint.
The lower arm bones form the wrist joint with the carpals, a group of
eight small bones that give added flexibility to the wrist. The carpals
are connected to the five metacarpals that form the
bones of the hand
and connect to each of the fingers. Each finger has three bones known
as phalanges, except for the thumb, which only has two phalanges.
Pelvic Girdle and Lower LimbFormed by the left and right hip bones, the pelvic girdle connects the
lower limb (leg) bones to the axial skeleton.
The
femur is the largest bone in the body and the only bone of the thigh (femoral) region. The femur forms the ball and socket
hip joint with the hip bone and forms the
knee joint
with the tibia and patella. Commonly called the kneecap, the patella is
special because it is one of the few bones that are not present at
birth. The patella forms in early childhood to support the knee for
walking and crawling.
The tibia and fibula are the bones of the lower leg. The tibia is
much larger than the fibula and bears almost all of the body’s weight.
The fibula is mainly a muscle attachment point and is used to help
maintain balance. The tibia and fibula form the ankle joint with the
talus, one of the seven tarsal bones in the
foot.
The tarsals are a group of seven small bones that form the posterior
end of the foot and heel. The tarsals form joints with the five long
metatarsals of the foot. Then each of the metatarsals forms a joint with
one of the set of phalanges in the toes. Each toe has three phalanges,
except for the big toe, which only has two phalanges.
Microscopic Structure of BonesThe skeleton makes
up about 30-40% of an adult’s body mass. The skeleton’s mass is made up
of nonliving bone matrix and many tiny bone cells. Roughly half of the
bone matrix’s mass is water, while the other half is collagen protein
and solid crystals of calcium carbonate and calcium phosphate.
Living
bone cells are found on the edges of bones and in small cavities inside
of the bone matrix. Although these cells make up very little of the
total bone mass, they have several very important roles in the functions
of the skeletal system. The bone cells allow bones to:
- Grow and develop
- Be repaired following an injury or daily wear
- Be broken down to release their stored minerals
Types of BonesAll of the bones of the body can be broken down into five types: long, short, flat, irregular, and sesamoid.
- Long. Long bones are longer than they are wide and are the
major bones of the limbs. Long bones grow more than the other classes of
bone throughout childhood and so are responsible for the bulk of our
height as adults. A hollow medullary cavity is found in the center of
long bones and serves as a storage area for bone marrow. Examples of
long bones include the femur, tibia, fibula, metatarsals, and phalanges.
- Short. Short bones are about as long as they are wide and
are often cubed or round in shape. The carpal bones of the wrist and the
tarsal bones of the foot are examples of short bones.
- Flat. Flat bones vary greatly in size and shape, but have
the common feature of being very thin in one direction. Because they are
thin, flat bones do not have a medullary cavity like the long bones.
The frontal, parietal, and occipital bones of the cranium—along with the ribs and hip bones—are all examples of flat bones.
- Irregular. Irregular bones have a shape that does not fit
the pattern of the long, short, or flat bones. The vertebrae, sacrum,
and coccyx of the spine—as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all irregular bones.
- Sesamoid. The sesamoid bones are formed after birth inside
of tendons that run across joints. Sesamoid bones grow to protect the
tendon from stresses and strains at the joint and can help to give a
mechanical advantage to muscles pulling on the tendon. The patella and
the pisiform bone
of the carpals are the only sesamoid bones that are counted as part of
the 206 bones of the body. Other sesamoid bones can form in the joints
of the hands and feet, but are not present in all people.
Parts of BonesThe long bones of the body contain
many distinct regions due to the way in which they develop. At birth,
each long bone is made of three individual bones separated by hyaline
cartilage. Each end bone is called an
epiphysis
(epi = on; physis = to grow) while the middle bone is called a
diaphysis (dia = passing through). The epiphyses and diaphysis grow
towards one another and eventually fuse into one bone. The region of
growth and eventual fusion in between the epiphysis and diaphysis is
called the metaphysis (meta = after). Once the long bone parts have
fused together, the only hyaline cartilage left in the bone is found as
articular cartilage on the ends of the bone that form joints with other
bones. The
articular cartilage acts as a shock absorber and gliding surface between the bones to facilitate movement at the joint.
Looking at a bone in cross section, there are several distinct
layered regions that make up a bone. The outside of a bone is covered in
a thin layer of dense irregular connective tissue called the
periosteum. The periosteum contains many strong collagen fibers that are
used to firmly anchor tendons and muscles to the bone for movement.
Stem cells and osteoblast cells in the periosteum are involved in the
growth and repair of the outside of the bone due to stress and injury.
Blood vessels present in the periosteum provide energy to the cells on
the surface of the bone and penetrate into the bone itself to nourish
the cells inside of the bone. The periosteum also contains nervous
tissue and many nerve endings to give bone its sensitivity to pain when
injured.
Deep to the periosteum is the compact bone that makes up the hard,
mineralized portion of the bone. Compact bone is made of a matrix of
hard mineral salts reinforced with tough collagen fibers. Many tiny
cells called osteocytes live in small spaces in the matrix and help to
maintain the strength and integrity of the compact bone.
Deep to the compact bone layer is a region of spongy bone where the
bone tissue grows in thin columns called trabeculae with spaces for red
bone marrow in between. The trabeculae grow in a specific pattern to
resist outside stresses with the least amount of mass possible, keeping
bones light but strong. Long bones have a spongy bone on their ends but
have a hollow medullary cavity in the middle of the diaphysis. The
medullary cavity contains red bone marrow during childhood, eventually
turning into yellow bone marrow after puberty.
ArticulationsAn articulation, or joint, is a
point of contact between bones, between a bone and cartilage, or between
a bone and a tooth. Synovial joints are the most common type of
articulation and feature a small gap between the bones. This gap allows a
free range of motion and space for synovial fluid to lubricate the
joint. Fibrous joints exist where bones are very tightly joined and
offer little to no movement between the bones. Fibrous joints also hold
teeth
in their bony sockets. Finally, cartilaginous joints are formed where
bone meets cartilage or where there is a layer of cartilage between two
bones. These joints provide a small amount of flexibility in the joint
due to the gel-like consistency of cartilage.
Skeletal System Physiology
