How the Skeletal System Works

Although bones, muscles, joints, ligaments, and tendons all work together, they each have a special job. Bones provide the framework for your body but ligaments and tendons provide the attachments for muscles to contract and relax.

Bones store mineral salts, and the inner core of a bone is composed of hematopoietic (blood cell–forming) red bone marrow. Other areas of the bone are used as storage areas for minerals necessary for growth, such as calcium and phosphorus. And you thought bones just gave your body its shape. In fact, they operate as your body’s Fort Knox of vital minerals. Talk about double duty!

Red bone marrow is red because red blood cells form in it. In adults red marrow is eventually replaced by yellow marrow, which stores fat. Bones are complete organs, chiefly made up of connective tissue called osseous or bony tissue plus a rich supply of blood vessels and nerves.

Colles’ fracture was first described by Dr. Andrew Colles, an Irish surgeon in 1814. It is a fracture of the distal end of the radius (distal meaning the portion of a body part farthest from the point of origin). But you don’t have to be Irish to sustain one. In this case, the point of origin is the shoulder, looking at the limb separately, and not the body as a whole.

Bones and osteology

Now it’s time to get down with osteology. No, it’s not a slick new dance move. Osteology is the study of bones. Notice the root word osteo? You might know it as part of the word osteoporosis — a common condition typical in women involving the loss of bone density (in fact, four out of five Americans with osteoporosis are women). So osteo is the focus in this chapter. The first step into the world of osteology is looking at the actual makeup of our bones.

Bones of the hands and feet make up over half of our 206 total bones, while the skull is comprised of 22 bones.

Bones are classified by their shape — long, short, flat, irregular, and sesamoid. Like this:

• Long bones, found in arms (the humerus is the upper arm) and legs (the femur is your thigh) are strong, broad at the ends where they join with other bones, and have large surface areas for muscle attachment.
• Short bones are found in the wrists and ankles and have small, irregular shapes.
• Flat bones are found covering soft body parts. The shoulder blades, ribs, and pelvic bones are examples of flat bones.
• Vertebrae are examples of irregular bones.
• Sesamoid bones are small, rounded bones found near joints. The kneecap is an example of a sesamoid bone.

There’s more to bones than the hard, white substance you think of when you envision one. For starters, the shaft or middle region of a long bone is called the diaphysis. Each end of a long bone is called the epiphysis. Both are joined by the physis, also called the growth plate. The periosteum of the bone is a strong, fibrous membrane that covers the surface, except at the ends. Bones other than long bones are completely covered by the periosteum.

Fractures are often classified by a system called the Salter-Harris system, which identifies whether a fracture involves only the physis, or could involve the epiphysis and/or the diaphysis as well. Basically, it’s a fancy way of saying what area of bone is affected by a break and classifying the level of seriousness of that break.

Beneath the periosteum is a level of osteoblasts, which deposit calcium and phosphorus compounds in the bony tissue. Articular cartilage covers the ends of long bones. This cartilage layer cushions the bones where they meet with other bones, or at the joints. The compact bone is made of dense tissue lying under the periosteum in all bones. Within the compact bone is a system of small channels containing blood vessels that bring oxygen and nutrients to the bone and remove waste products such as carbon dioxide.

Bones are 31-percent water, and pound-for-pound, they are four times stronger than concrete. The hardest of these is your jaw bone.

Cancellous bone, sometimes called spongy bone, is more porous and less dense than compact bone. Spaces in cancellous bone contain red bone marrow, which is richly supplied with blood and consists of immature and mature blood cells in various stages of development. The ribs, the pelvic bones, the sternum or breastbone and vertebrae, as well as the epiphyses of long bones, contain red bone marrow within cancellous tissue. Figure 12-1 illustrates the skeleton.

Axial skeleton

Think of the word axis when you think about the axial skeleton. The bones that make up this particular part of the skeleton tend to encircle important organs or rotate in an axial motion. The axial skeleton includes the heavy hitters in this section.

Cranium

The bones of the cranium (skull) protect the brain. The bones of the skull include the frontal bone, which forms the forehead and bony sockets that contain the eyes. The parietal bone forms the roof and upper sides of the skull. Two temporal bones form the lower sides and base. The mastoid process is a small round part of the temporal bone behind the ear.

The occipital bone forms the back and base of the skull and joins the parietal and temporal bones, forming a suture (a joining line) of cranial bones. The occipital bone has an opening called the foramen magnum through which the spinal cord passes. The sphenoid bone extends behind the eyes and forms part of the base of the skull. It joins the frontal, occipital, and ethmoid bones and serves as an anchor to hold these bones together. The ethmoid bone is a thin delicate bone, supporting the nasal cavity and forms part of the orbits of the eyes.

Be careful when working with a newborn cranium, as the cranial bones of a newborn are not completely joined. There are gaps of unossified tissue (tissue that is still in the fibrous membrane stage) in the skull, called the soft spot or fontanelle. The lines where the bones of the skull join are called cranial sutures. The pulse of blood vessels can be felt under the skin in these areas.

Facial bones

All the facial bones except one are joined together. Only the mandible, or lower jaw bone, is capable of moving, which is necessary for chewing and speaking. Other facial bones include the nasal bones, and the maxillary bones. Two large bones compose the upper jaw. Both the mandible and maxilla contain sockets called alveoli, in which the teeth are embedded. The mandible joins the skull at the temporal bone, forming the lengthily named temporomandibular joint. The zygoma or zygomatic bones form the cheek. Together, these bones create a sort of mega-bone that makes up the upper portion of your face.

Vertebral column

The vertebral column, or spinal column, is composed of 26 bone segments called vertebrae (singular vertebra), which are arranged in five divisions: cervical, thoracic, lumbar, sacrum, and coccyx (tailbone).

The first seven vertebrae are called the cervical vertebrae (C1-C7). These vertebrae do not join with the ribs. The first cervical vertebra, C1 (also known as the atlas), articulates with the occipital bone of the skull at the back of neck. It supports the head and allows it to move forward and back. The second cervical vertebra, C2 (the axis), acts as a pivot, about which the atlas rotates, allowing head to turn from side to side, extend, and flex.

The second division consists of 12 thoracic vertebrae (T1-T12). These vertebrae join with the 12 pairs of ribs. The third division consists of five lumbar vertebrae (L1-L5). They are the strongest and largest of the back bones. The sacrum is a slightly curved triangular bone, composed of five separate segments, or sacral bones, that gradually become fused. The coccyx is the tailbone. It is also a fused bone, formed from four small coccygeal bones.

The Greek diskos means “flat plate.” An example is the lumbar disk. And coccyx comes from the Greek word for cuckoo; it resembles a cuckoo’s beak.

A vertebra is composed of a disk-shaped portion called the vertebral body, which is the solid anterior portion (closest to body front, farthest from the body back). A lamina is a part of the posterior (back) portion of a vertebra. Spinous processes, thoracic processes, and transverse processes are little wing-like projections that project or extend from each vertebra. The foramen is the opening in the middle of each vertebra that the spinal cord passes through.

Between the body of one vertebra and the bodies of vertebrae lying beneath are vertebral disks, which help to provide flexibility and cushion shock to the vertebral column.

Thorax

The thorax (not to be confused with a character invented by Dr. Seuss) starts with the clavicle, or the collarbone, connecting the sternum (breastbone) to each shoulder. The scapula is the shoulder blade, consisting of two flat triangular bones, one on each back side of the thorax. The scapulae extend to join with the clavicle at the acromion.

The shoulder blade connects to the body by 15 different muscles but not by a single bone connection.

The sternum is the breastbone, the flat bone extending down the midline of the chest. The uppermost part of the sternum joins to the sides of the clavicle and ribs, whereas the other, narrowed portion is attached to the diaphragm. The lower portion of the sternum is the xiphoid process, the small, mobile bone tag on the very end of the sternum. This is the thing you would feel for when placing your hands on a chest to perform CPR. The 12 pairs of ribs are close neighbors with the sternum. The first seven pairs join the sternum anteriorly (at the chest) by attachments of costal cartilage. Ribs 1–7 are called true ribs. Ribs 8–12 are called false ribs. The false ribs join with the vertebral column in the back, but join the 7th rib anteriorly and do not attach to the sternum. Ribs 11 and 12 are called floating ribs because they are completely free at their anterior end.

The ribs move five million times a year — every time you breathe.

Pelvis

The pelvic girdle or hip bone is a large bone that supports the trunk of the body and joins with the femur (thigh bone) and sacrum. The adult pelvic bone is composed of three pairs of fused bone: the ilium, the ischium, and the pubis.

The ilium is the uppermost and largest portion. The connection between the iliac bones and the sacrum is so firm that they are often referred to as one bone, the sacroiliac. The iliac crests are found on both the anterior and posterior portions of the pelvis. They are filled with red bone marrow and serve as an attachment for abdominal wall muscles.

The ischium is the posterior portion of the pelvis. The ischium and the muscles attached to it are what we sit on.

The pubis is the anterior portion containing two parts that are joined by way of a disk. This area of fusion is called the pubic symphysis. The region within the bone formed by the pelvic girdle is called the pelvic cavity. The rectum, sigmoid colon, bladder, and female reproductive organs are contained in this cavity.

Appendicular skeleton

Think of the word appendage when your thoughts turn to the appendicular skeleton. Your reachers, grabbers, and hoofers are all covered in this section. Appendages fall into two major categories of bones.

Upper extremities

Arms and hands are part of this category. The bones of the arm and hand include the humerus, the upper arm bone. The large head of the humerus is round and joins the scapula and clavicle. The ulna and radius are the bones of the lower arm or forearm. The bony prominence of the ulna at the elbow is called the olecranon. Carpals are wrist bones. Finally, there are two rows of four bones. The metacarpals are five bones radiating to the fingers. Phalanges (the singular is phalanx) are the finger bones.

Each finger has three phalanges: the proximal, middle, and distal. The proximal is the phalange closest to the point of origin, whereas the distal is farthest from the point of origin. So, the proximal would be the first after the knuckle, the middle would be in the middle, and the distal at the fingertip. The thumb has only two phalanges: medial and distal at tip of the thumb, which is why it is categorized differently than the rest of your fingers.

Diaphysis comes from Greek diaphusis, meaning “state of growing between.” Diaphysis is the shaft of long bones that grows as children grow.

Leg and foot

The femur is the thigh bone. At the top end of it, a rounded head fits into a socket in the hip bone called the acetabulum. The patella, or kneecap, is a small flat bone that lies in front of the joint between the femur and one of the lower leg bones called the tibia. The tibia is the larger of the two lower bones of the leg, often referred to as the shin bone. The fibula is the smaller of the two bones.

The tarsals, or ankle bones, are short bones that are much like the carpal bones of the wrist, but larger. The calcaneus, the largest of these bones, is also called the heel bone. Metatarsals compose the forefoot or bones leading to the phalanges in the toes. There are two phalanges in the big or great toe and three in each of the other four toes. Just like the fingers, all the bones in the toes are phalanges, from proximal to distal. In the big or great toe they are called the proximal and distal, differing slightly from the thumb.

The femur is the longest bone in the body and accounts for one quarter of your total height.

Joints

Now think about the “glue” that holds all these bones together. Okay, joints aren’t really made of glue, but they sure do a good job of keeping everything connected. Let us articulate that concept a bit better: Joints, also called articulations, are the coming together of two or more bones. Some are not movable, such as the suture joints between the cranial bones. Some joints only partially move, such as joints between the vertebrae.

Most joints do allow movement. These freely movable joints are called synovial joints. An example is the ball and socket type — the hip joint, for example, in which the head of the femur fits into the acetabulum. Another synovial joint is the hinged type as seen at the elbow, knee, or ankle joints.

The bones of a synovial joint are separated by a capsule, composed of fibrous cartilage. Ligaments of connective tissue hold the bones together around the capsule to strengthen it. The bone surfaces at a joint are covered with a smooth surface called the articular cartilage. The synovial membrane is the inner layer of the capsule, the layer beneath the capsular surface.

The synovial cavity is filled with a lubricating fluid produced by synovial membranes. This fluid contains water and nutrients that help to lubricate the joints so that friction on the articular cartilage is minimal.

Bursae (singular bursa) are closed sacs of synovial fluid lined with synovial membrane. They lie in the spaces between tendons, ligaments, and bones and lubricate areas where friction would normally develop close to the joint capsule. The olecranon bursa at the elbow joint and the patellar bursa at the knee are examples of bursae.