Interesting and Surprising Facts About the Human Skeleton
Our Impressive Skeleton
The human skeleton is an interesting and complex structure. It's more than just a scaffolding for our body or a structure that enables us to move. The bones that make up the skeleton are made of living tissue that has vital functions.
In addition to supporting the body and allowing it to move, the skeleton protects organs, makes blood cells, and stores fat and minerals. Bones release minerals into the bloodstream and absorb them from the blood as needed. In addition, researchers are discovering that the skeleton makes chemicals that trigger effects not only in the bones but also in other parts of the body.
There are two divisions of the skeleton—the axial skeleton and the appendicular skeleton. The axial skeleton is located in the midline of the body and is composed of the skull, the vertebral column or backbone, the sternum or breast bone, and the ribs. It also includes smaller bones which aren't connected to the rest of the axial skeleton. These are the hyoid bone in the neck and the ossicles in the middle ears.
The appendicular skeleton is made of the limbs and their associated bones. It includes the bones of the hands, arms, feet, and legs as well as the pelvic bones, the scapula or shoulder blade, and the clavicle or collar bone.
- The skull is made of the cranium and the facial bones.
- The cranium is made of eight bones that fit tightly together.
- There are fourteen facial bones.
- Some of the facial bones contain a space called a sinus which is filled with air and has a lining that produces mucus.
- Sinuses are connected to the nose via tubes called ducts.
- When skulls of people who died a long time ago are discovered, only the bridge of the nose remains. Like the ear lobes, the rest of the nose is made of cartilage, not bone. Cartilage decays faster than bone after death.
A suture is a fibrous joint that occurs between the bones of the cranium. Sutural or Wormian bones are extra bones that develop within a suture. They are usually small and aren't present in everybody.
The Vertebral Column, Spine, or Backbone
- The vertebral column is made of seven cervical vertebrae in the neck, twelve thoracic ones in the upper back, five lumbar ones in the lower back, five fused vertebrae in the sacrum at the back of the pelvis, and three to five fused vertebrae in the coccyx or tail bone.
- The first vertebra in the neck is called the atlas because it holds the head up. It's named after Atlas, an Ancient Greek deity who supported the world on his shoulders.
- The second vertebra in the neck is called the axis. It acts as a pivot that allows the atlas to rotate.
The Sternum and Ribs
- Most people have twelve pairs of ribs.
- The first seven pairs are known as true ribs. They're connected to the vertebrae at the back of the body and are joined via a strip of cartilage to the sternum or breast bone at the front.
- The next three pairs are known as false ribs because they are connected to another rib at the front of the rib cage instead of directly to the sternum.
- The last two pairs are known as floating ribs because they aren't attached to any other bone at the front of the rib cage.
- Some people have an extra rib known as a cervical rib. This arises from the last cervical vertebra and can be present on either side of the body or on both sides. The rib may be only partially developed.
- Most cervical ribs cause no problems. Occasionally they may press on nerves or blood vessels and contribute to a condition known as thoracic outlet syndrome.
Although bones are often said to be connected to other bones, the bones are not connected directly. They are linked via a structure called a joint. Joints are movable, slightly movable, or immovable.
The Hyoid Bone
- The hyoid bone has a horseshoe shape. It's located in the neck between the lower jaw (mandible) and the larynx.
- Unlike nearly all other bones, the hyoid bone isn't connected to another bone. It's held in place by muscles.
- The larynx, or voice box, houses the vocal cords that produce sound. The tongue and the hyoid bone allow a wider variety of vocalizations to be produced than the vocal cords on their own.
The outer ear consists of the pinna or auricle, the ear canal, and the outer part of the eardrum or tympanic membrane. Only the pinna is visible from the outside of the body. The rest of the outer ear, the middle ear, and the inner ear are located inside bones of the skull.
- The three tiny bones in the middle ear are called ossicles.
- The first ossicle is called the malleus or hammer. It transmits vibrations from the eardrum.
- The vibrations from the malleus are sent to the second ossicle, which is called the incus or anvil.
- The incus sends vibrations to the third ossicle, or stapes. The stapes transmits the vibrations to the round window of the inner ear.
- The stapes is also known as the stirrup because it looks like the stirrup used by horse riders. It's the smallest bone in the body and is only around 2.8 mm in length.
- The ossicles vibrate as sound waves reach them from the outer ear. They transmit the vibrations to the fluid in the inner ear, which in turn stimulates hair cells. The hair cells then stimulate the auditory nerve, which sends nerve impulses to the brain. The brain creates the sensation of sound.
The Skeletal System: Crash Course Biology
Facts About the Appendicular Skeleton
- The part of the body with the most bones is the hand. Each hand contains 27 bones. There are eight carpal or wrist bones, five metacarpal bones in the palm, and fourteen phalanges in the digits (three in each finger and two in the thumb).
- The long bone in the upper leg is the femur. The femur is the biggest bone in the body and is also the strongest.
- The clavicle or collar bone is the only long bone in the body that normally lies in a horizontal position. It connects the sternum to the shoulder blade, or scapula.
- Each hip bone is composed of three fused bones—the ilium, the ischium, and the pubis.
- The two hip bones are joined to the sacrum at the back of the body and the pubic symphysis at the front. The resulting ring-like structure is called the pelvis.
- The space inside the ring is significantly larger in females than in males in order to accommodate childbirth.
- The pubic symphysis is a cartilaginous joint, not a bone.
Sesamoid bones are located in tendons, which are the fibrous structures that connect muscles to bones. At least one sesamoid bone is present in everyone's body. This bone is the patella or kneecap, which is located in the front of the knee. Other sesamoid bones vary in number and position and may not be present in all people.
Some common sites for the location of sesamoid bones in addition to the knee are the wrist, the hands, and the feet. With the exception of the patella, sesamoid bones are small in size. Despite this fact, the bones can break and may become inflamed, causing pain.
There are various theories for the function of sesamoid bones. One is that they improve the action of a tendon, acting as a fulcrum. Another is that they reduce friction in an area. Sesamoid bones on the bottom of the foot may assist with weight bearing.
The Funny Bone or Ulnar Nerve
The funny bone is actually the ulnar nerve. This is a very long nerve that travels from the neck down the arm to the hand. It's well protected over most of its route but is less protected at the elbow. If we hit our elbow in a certain place we may push the ulnar nerve against bone. This produces a strange sensation of numbness, tingling, and pain that travels down the forearm. People often say that they have hit their funny bone when they experience this event.
The term "funny bone" may have arisen due to the funny or strange sensation that is produced. Another possibility is that the term developed because the sensation happens at the bottom of the upper arm bone, whose technical name is the humerus. This name makes some people think of the word "humorous".
Structure of Bone
There are two types of bone tissue—compact bone and spongy bone. Spongy bone is also known as cancellous or trabecular bone. Compact bone is found in the outer part of bones and spongy bone is located in the inner part.
Compact bone is made of "building blocks" called osteons. Osteons are made of calcium, phosphate, and protein, a mixture that is known as bone matrix. Each osteon has a central canal, also called the Haversian canal, which contains blood vessels, a lymphatic vessel, and a nerve. The bone cells are located in small spaces in the osteon known as lacunae. The lacunae are arranged in concentric circles around the central canal. Tiny passageways called canaliculi connect the lacunae to each other.
Spongy bone consists of a mesh-like structure with spaces in between the bars and plates of the mesh. These spaces are often filled with bone marrow. The solid part of spongy bone contains bone matrix, lacunae, and canaliculi, but these aren't arranged in osteons.
Microscopic Structure of Bone
Osteoblasts and Osteoclasts
- The cells in bone are the osteocytes, which are mature bone cells, the osteoblasts, which build bone, and the osteoclasts, which break down bone.
- Bone is continually being remodelled by the osteoblasts and osteoclasts.
- When bone is broken down, minerals are released into the bloodstream. When bone is made, minerals are absorbed from the bloodstream. The chief minerals in bone are calcium and phosphorus.
- Unfortunately, as we age the osteoblasts become less active while the osteoclasts are relatively unaffected. This is especially true in women who are past menopause. Bone may be lost as a result. Exercise—especially weight-bearing exercise—can stimulate the activity of osteoblasts and restore some of the lost bone.
Blood Cell Production
- Red bone marrow contains stem cells that produce the red blood cells, white blood cells, and platelets.
- In a newborn baby, all of the bone marrow is red. As a person grows, some of their red bone marrow is gradually replaced by yellow marrow. This type of marrow stores fatty acids instead of making blood cells.
- In adults, red bone marrow is located in the spongy bone at the ends of the humerus and femur and in the skull, sternum, ribs, vertebrae, and hip bones.
- In cases of serious blood loss, the body can convert yellow bone marrow into the red type.
Red bone marrow makes about 2 million red blood cells every second. These are used to replace the old and damaged red blood cells destroyed by the liver.
Other Functions of Bones
- When muscles contract, they exert a pulling force on tendons. The tendons in turn pull on bones, enabling the body to move.
- The skeleton protects vital organs and tissues. For example, the cranium protects the brain, the vertebrae protect the spinal cord, and the rib cage protects the heart and the lungs.
- Osteocalcin is a protein hormone made by the osteoblasts in bone. It stimulates bone building but also has effects outside the bones. It seems to be involved in a feedback loop involving the beta cells in the pancreas, which make insulin, and the adipocytes, or fat cells.
The Three Types of Joints: Learning Through Song
- The bones of the skeleton are connected to other bones via joints.
- Joints are classified as movable, slightly movable, and immovable.
- Fibrous joints (synarthroses) are immovable. The bones are joined by fibrous connective tissue and there is no cavity between the bones. The joints between the skull bones are fibrous joints.
- Cartilaginous joints (amphiarthroses) are slightly movable. The bones are joined by cartilage and there is no cavity between the bones. The intervertebral disks located between the vertebrae are cartilaginous joints.
- Synovial joints (diarthroses) are movable and are the most common type of joint in the body. The bones are joined together via ligaments and there is a fluid-filled cavity between the bones. Some examples of this type of joint are those found in the shoulder and hip joints, the elbow and ankle joints, and the finger and toe joints.
- Synovial joints are classified into other categories based on their structure and type of movement.
Some Strange Facts About Skeletons
- A baby is born with about 300 "bones", although some of these bones are made of cartilage. As the baby grows, a lot of the cartilage ossifies, or turns into bone, and some of the bones fuse. As a result, an adult has only about 206 bones, even though their body is bigger than a baby's.
- Teeth are considered to be part of the skeletal system, although they are made of dentine and enamel instead of bone and have a different function from the rest of the skeletal system.
- According to Guinness World Records, Evel Knievel holds the record for the largest number of broken bones in a lifetime. Knievel was a stunt performer who was born in 1938. By the end of 1975 he had suffered from 433 broken bones. He retired from major competitions in 1976.
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© 2014 Linda Crampton