The Skeletal System: An Overview
Composed of bones, the skeletal system is responsible for the body’s shape, movement, and many other functions. This article will provide an overview of the skeletal system and its many functions.
The skeletal system of an adult is made up of 206 bones which work together as a skeleton to provide shape to the body, help the body move with assistance from the muscular system, protect softer internal organs, store minerals, and produce blood cells. At birth, newborns have 270 bones. Due to bones fusing over time, adults have only 206 bones. Bones can vary in shape and size, and are thus classified as long bones, short bones, flat bones, or irregular bones. Long bones have a greater length than width, and examples include the humerus, radius, and ulna. Short bones have similar lengths and widths, with examples being carpals and tarsals- the bones of the wrist and feet respectively. Flat bones are bones with two layers and space between them, such as the bones of the skull and the breast bone. Irregular bones are the remaining bones that do not fit the descriptions for the other 3 categories, such as the vertebrae and facial bones.
Bones are similar to organs- they take in nutrients and perform functions. Bones have different layers, each having a unique function. Long bones have many structures including a medullary cavity/canal, diaphysis, epiphyses, periosteum, red marrow, and cartilage. The medullary canal is an open space present at the center of the bone. It contains yellow marrow, which are fat cells, and endosteum, which covers the yellow marrow. The diaphysis is commonly known as the shaft of the bone and it spans the length between the two ends of the bone. The Epiphyses are the ends of the bone. The proximal epiphysis is the end of the bone closer to the torso of the body. The distal epiphysis is the end of the bone further away from the torso of the body. For example, the top of the humerus (upper arm bone) would be the proximal epiphysis, and the bottom would be the distal epiphysis since the top of the humerus connects to the torso. Periosteum is the outer layer of bone that contains blood, lymph vessels, and nerves. It is a white and fibrous layer and is the site of bone growth, repair, and nutrition. This is where tendons and ligaments attach. Red marrow is the bone tissue that produces red blood cells and white blood cells. More information on these types of cells can be found in the article about the cardiovascular system. Red bone marrow is found in long, flat, and irregular bones. Cartilage is the connective tissue that covers the ends of bones and helps cushion bones. Cartilage also forms the flexible part of the skeleton, such as ear lobes.
The skeletal system is split into two subgroups- the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones that make up the skull, inner ear, the spinal column, ribs, sternum, and the hyoid bone. The axial skeleton is made up of 60 bones: 29 in the head and 51 in the trunk. The cranium is a key part of the axial skeleton, and it has two parts: the skull and the facial bones. The skull consists of 8 bones: 2 temporal bones which shape the area around the ears; 1 occipital bone which shapes the base and back of the head; 2 parietal bones which shape the top and sides of the head; 1 sphenoid bone which forms a part of the cranial floor, supports the sides of the skull, and bridges the cranial and facial bones; and 1 ethmoid bone which makes the roof and sides of the nasal cavity and the medial surface of the orbit of eyes. The cranial bones are joined together via sutures, which connect around 2 years after birth. There are 14 facial bones: 5 nasal bones which shape the nose, 2 lacrimal bones which are in the inner corner of the eye, 2 maxillary bones which shape the upper part of the jaw, 2 zygomatic bones which shape the cheeks, 1 mandible which shapes the lower jaw, and 2 palatine bones which make up the roof of the mouth. There are many air cavities within the bones of the cranium, which are known as sinuses. This helps develop voice quality. There are also openings in the bones, known as foramina, which allow blood vessels and nerves to pass. There are 33 vertebrae in the spinal column which protect the spinal cord, support the head, and shape the back. Intervertebral disks made of cartilage support the vertebrae. The vertebrae are spread from top to bottom into 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused sacral vertebrae, and 4 fused coccygeal vertebrae. There are 12 ribs attached to the thoracic vertebrae. They give shape to the chest and help protect the delicate internal organs inside. The first 7 ribs are “true ribs” because they connect to the breast bone, the sternum. The next 5 are “false ribs” since they do not connect to the cartilage. The upper 3 “false ribs” connect to the cartilage attached to the sternum, and the bottom two do not connect to anything, so they are known as “floating ribs”
The appendicular skeleton consists of bones that make up the shoulders, arms, hands, legs, feet, and pelvis. It is made up of 126 bones: 64 in the upper extremities and 62 in the lower extremities. The upper extremities include the shoulder girdle, the arm, and the wrist and hand. The shoulder has two curved shoulder bones known as clavicles and two triangle shoulder blades known as scapulae. The long bone that makes up the upper arm is the humerus. It is connected to the scapula by ligaments and muscles. The forearm has two long bones: the radius, which runs on the side of the thumb, and the ulna, which runs on the side of the pinky. 8 carpals are present in the wrist oriented in 2 rows. The hands have 5 metacarpal bones in the palm and 14 phalanges which make up the fingers. Each finger has three phalanges with the thumb as an exception, which has two. The lower extremities include the pelvic girdle, the leg, and the ankle and foot. The pelvic girdle is the location of attachment for the leg and it protects the organs of the lower abdomen. During childhood, the girdle is made of 3 bones- the ilium, ischium, and pubis. These fuse to form the pelvic girdle near adulthood. The pelvic girdle fuses with the sacrum of the spine to form the symphysis pubis. The upper leg has a long bone known as the femur, which is the longest bone of the body. The femur attaches to the pelvic girdle at the acetabulum. The calf has two long bones: the fibula and tibia. The patella covers the front of the knee joint. There are 7 tarsals in the ankle which connect the foot and leg bones. The foot has 5 metatarsals which make the arch of the foot, as well as 5 toes with 14 phalanges.
The movable connection between two bones is a joint. The synovial membrane covers joints and produces synovial fluid, which lubricates joints and enables mobility.Ligaments are connective tissues that connect one bone to another. Joints like the elbow, knee, and shoulder also have bursa, which is a small fluid-filled sac that cushions and reduces friction in joints. Joints allow for various forms of movement, which can be seen in the following image.
As individual ages, their skeletal health starts to deteriorate. Changes include decreased height, bone mass, and flexibility. In addition, joints and cartilage erode, vertebrae thin, and bone minerals decrease over time. The skeletal system also has many diseases, including arthritis, back pain, carpal tunnel syndrome, scoliosis, lordosis, kyphosis, fractures, osteomyelitis, and osteoporosis. More information on each of these diseases will be posted in future articles, so be on the lookout for those!
To limit these effects and prevent these diseases, one should get exercise, avoid joint overextension, eat a diet with good amounts of calcium and vitamin C, stretch before physical activity, maintain good posture, lift objects properly, and use the correct protective equipment.
In conclusion, the skeletal system is composed of bones and is responsible for the body’s shape, movement, protection, mineral storage, and red blood cell production. Look out for my next article on the respiratory system which will be posted soon!
References
Mitchell, Dakota, and Lee Haroun. Introduction to health care. Clifton Park, NY: Delmar, Cengage Learning, 2012. Print.
Skeletal System. (2020). Retrieved from https://my.clevelandclinic.org /health/articles/21048-skeletal-system.
