The skeletal system includes all of the bones and joints in the body. Each bone is a complex living organ that is made up of many cells, protein fibers, and minerals. The skeleton acts as a scaffold by providing support and protection for the soft tissues that make up the rest of the body.
The skeletal system also provides attachment points for muscles to allow movements at the joints. New blood cells are produced by the red bone marrow inside of our bones. Bones act as the body's warehouse for calcium, iron, and energy in the form of fat.
Finally, the skeleton grows throughout childhood and provides a framework for the rest of the body to grow along with it.
Skeletal System Anatomy
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
Skull
The 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 Ossicles
The 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 boneNit 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 are known collectively as the auditory ossiclesNare 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.
Vertebrae
Twenty-six vertebrae form the vertebral column of the human body. They are named by region: - Cervical (neck) - 7 vertebrae
- Thoracic (chest) - 12 vertebrae
- Lumbar (lower back) - 5 vertebrae
- Sacrum - 1 vertebra
- Coccyx (tailbone) - 1 vertebra
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.
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 are known collectively as the auditory ossiclesNare 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.
Vertebrae
Twenty-six vertebrae form the vertebral column of the human body. They are named by region: - Cervical (neck) - 7 vertebrae
- Thoracic (chest) - 12 vertebrae
- Lumbar (lower back) - 5 vertebrae
- Sacrum - 1 vertebra
- Coccyx (tailbone) - 1 vertebra
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.
RIDS ana sternum
The sternum, or breastbone, is a thin, knifeshaped 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 Limb
> The 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 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 Bones
The 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:
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.
Articulations An 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.