Posted on 10 Oct 2012 14:34
The following is part one of a list of of the 206 the bones of the human body, separated into the axial skeleton and its parts; and the appendicular skeleton and its parts.
This part one will cover the axial skeleton.
The appendicular skeleton comprises the bones of the shoulder, pelvis, and upper and lower extremities (arms and legs). The axial skeleton comprises the bones of the skull, vertebral column, chest and hyoid bone.
Although bones can be classified in different ways, they are normally classified according to their structure and shape.
A basic explanation of the different types (shapes) of bones of the skeleton is provided first. Copious reference images are provided, as well.
Obviously a full description of each and every bone, or bone system in the body cannot be provided in an article that covers every bone in the skeleton. But pains have been taken to provide a very thorough basic reference.
Table of Contents
A Note on the Number of Bones
The number of bones in the human body is generally reported as 206. However, the actual number of bones can vary somewhat from person to person. Some people may lack certain bones and others may have extra bones. One example of extra bones are wormian bones, which develop in the skull. Normally, the bones of the cranium fuse together along lines called suture lines. Sometimes extra bones called wormian or sutural bones develop in these sutures. Also, some people develop extra sesamoid bones, which are covered below. Extra bones are not covered in this overview.
Types of Bones in the Axial and Appendicular Skeleton
Long bones, such as those of the arms and legs, are basically tubular. They provide the body's framework and make movement possible. Although they are called long bones, this does not mean that all long bones are very long. The designation "long" simply means that they are longer than they are wide. Some long bones are actually very tiny and short. The bones of the hands, the phalanges, metacarpals, and metatarsals, are long bones even though they are small.
These bones have a long shaft, known as a diaphysis. At each end of the shaft is an enlarged prominence called an epiphysis (plural epiphyses). The very ends of the epiphyses are covered with a coating of cartilage, called an articular cartilage. This is where the bone articulates with another bone to form a joint, and the cartilage allows smooth movement of the joint, as well as protecting the end of the bone from wearing down. The femur, illustrated and labelled below, is a good example of a long bone.
In young children who are still growing, the diaphysis is separated from the epiphyses by a cartilage known as an epiphyseal plate. This is where the bone grows, so it is also known as a growth plate. Once the bone is done growing, the growth plate closes and the cartilaginous tissue is replaced with bone, so that the diaphysis and epiphyses are permanently joined. This normally happens at a specific age for each bone and is controlled by sex hormones. The growth plates close earlier in girls than in boys.
Femur, Long Bone of the Thigh
Original image by Frank Gaillard via wikimedia
Short bones, like carpal and tarsal of the wrist and ankles, have no diaphysis, or shaft. Their width and length are roughly equal and they tend to be somewhat cube-like (cuboidal). As an example, the cuboid bone of the foot is shown below. Notice that it basically looks like the ends of a long bone with no long shaft between them.
Cuboid foot bone, a short bone
image by Anatomist90 via wikimedia
Flat bones are thin, and obviously, flat. They usually play a protective role and tend to have curved surfaces. The bones of the skull, the ribs, the pelvic bones, the scapula, and the sternum are all flat bones. The image below shows a posterior view of the scapula, with all its parts labelled. The posterior of the scapula is the surface that faces outwards from your back, which you can see under the skin.
Posterior Surface of Scapula
image by Anatomist90 via wikimedia
Irregular bones are bones that are irregularly shaped and so do not fit into any of the other categories. You try naming bones that have an irregular shape by appearance and see if you can come up with a better name! There are no better examples of irregular bones than the vertebra of the spine. Below is a picture of a cervical vertebra with all its parts labelled. It's about as irregular as a bone can get.
image by Anatomist90 via wikimedia
Not all sources list these bones as a separate type of bone, as they are sometimes just considered another short bone, which they are. However, sesamoid bones differ by their location, which is unique enough to warrant describing them. A sesamoid bone is a bone that is embedded within a tendon. Remember, tendons usually attach to bones, so sesamoid bones are special and play a unique role. These bones are small and oval-shaped and they get their name because they reminded anatomists of sesame seeds. Their actual size and shape can vary between people but they are surprisingly smooth compared to most bones, which fits with their location, as a bumpy, knobby, or squarish bone inside a tendon would probably not be a good thing.
The most well-known, and largest sesamoid bone in the body is the knee cap, shown below. Other sesamoid bones are much smaller. The kneecap lies within the tendon of the quadriceps at the front of the knee — you can't miss it. It is also called the patella. The purpose of the patella is to keep the tendon further away from the joint, which allows the quadriceps to produce better torque to move the knee joint. Another example of a sesamoid bone is the pisiform bone of the wrist. It is a very small pea-shaped bone that is located in the carpi ulnaris wrist tendon, although it is not entirely embedded in the tendon, and unlike other sesamoids, it actually has an articular facet to articulate with the triquetral bone. Its purpose is to protect the tendon and it may have other functions, although people who have had it removed seem to get along well without it.
Front (Anterior) View of Human Patella (Kneecap)
This concludes our description of the types of bones in the body. What follows is a list of the 206 bones of the body, separated into the appendicular and axial skeleton and listed by body part.
The axial skeleton is made up of 80 bones which comprise the central axis of the human body. This includes the vertebral column (spine), thoracic cage, and the skull.
Skull, Having 22 Bones
The skull is actually made up of the cranium and the facial bones. The eight bones of the cranium are fused together at immoveable joints called sutures. These suture lines are jagged and look much like cracks in the skull.
The mandible, or lower jaw, is not really a part of the skull, but is a moveable bone joined to the cranium by ligaments. None of the bones of the skull are moveable. The mandible, however, is listed last, under the facial bones, and it is part of the facial skeleton. You may notice that the word cranium or skull is used to describe the locations of the cranial bones. For all intents and purposes, these are the same things. Cranium is useful for differentiating the cranial bones from other more anterior (frontal bones), that may be facial bones. When a bone is located in the back part of the head, it makes no difference whether you say it is part of the back of the "skull" or "cranium" as it is obviously not part of the face!
The skull of females is generally lighter and smaller than males, and its muscular attachments are less pronounced. The forehead is higher, the facial area rounder, the jaw smaller, and the mastoid process less prominent.
Side-view of Cranial Bones of Skull
Cranial Bones (8 total)
- Frontal bone (1): This bone forms the forehead and the roof of the nasal cavity, as well as the roots of the eye sockets.
- Parietal bone (2): Forms the side walls and the roof of the cranium.
- Occipital bone (1): Forms the back of the skull and the base of the cranium.
- Temporal (2): Forms the side walls and floor of the cranium.
- Sphenoid bone (1): Forms parts of the cranium base, sides of skull, and floors and sides of eye orbits
- Ethmoid (1): Forms parts of the roof and walls of the nasal cavity, the floor of the cranium, and the walls of the eye orbits.
Interior View of Skull
Facial Bones (13 total)
- Maxilla bone (2): Usually called the upper jaw, the maxillae form the upper jaw, comprising the anterior roof of the mouth (hard palate), the floors of the eye orbits, the sides and floor of the nasal cavity, and the sockets of the upper teeth. This bone, although part of the jaw, immoveable; only the lower mandible bone being able to move. All the other facial bones are connected to the maxillae.
- Palatine bone (2): Located behind the maxillae, these bones are L-shaped and form the back part of the hard palate and the floor of the nasal cavity. Some portions which are perpendicular help form the side walls of the nasal cavity.
- Zygomatic bone (2): Also called malar/ bones and commonly known as the cheekbones, they form the prominences of the cheeks below and to the side of the eyes. They help form the side walls and floor of the orbits. This bone joins with the temporal bone via the temporal process which joins to the zygomatic process of the temporal. This region of their joining is called the zygomatic arch.
Side-view of Facial Bones and Cranial Bones
- Lacrimal bone (2): Very thin and scale-like bones located in the inner (medial) border of each eye socket, between the ethmoid and maxilla. The front of the bone has a groove that forms a pathway for the tube that carries tears from the eye to the nasal cavity, called the nasolacrimal or tear duct.
- Nasal bone (2): Long, thin, and roughly rectangular that lie right next to each other and are fused at the midline of the face, forming the bridge of the nose. Most of the cartilage that forms the shape of the nose is attached to these bones.
- Vomer (1): A thin and flat bone located along the midline within the nasal cavity. It joins in the back with the perpendicular part of the ethmoid, forming the inferior and posterior part of the nasal septum.
- Inferior Nasal Concha (2): Thin fragile bones attached to the side of the nasal cavity, below the conchae of the ethmoid. Helps support the mucous membranes inside the nasal cavity, along with the superior and middle conchae.
Mandible, or Lower Jaw Bone (1)
The mandible bone is the lower jaw. It is a horse-shoe shaped bone which is situated horizontally, with a flat ramus projecting upwards at each of its ends. The anterior (toward the front) part of the ramus has a projection called the coronoid process, which serves as an attachment for chewing muscles (some large chewing muscles are inserted on the lateral surface of the rami, rather than the coronoid process). The posterior (toward the rear) part of the ramus has a knob-like projection called the mandibular condyle, which articulates with the temporal bones.
The upper part (superior) of the curved front of the bone, contains the sockets of the lower teeth and is called the alveolar arch. The lower front of the curved part of the bone forms the chin, which is also called the mental protuberance.
Manbible (Lower Jaw)
Middle Ear Bones (6 total)
The middle ear bones are also called auditory ossicles. These bones help transfer the vibratory movements of the tympanic membrane through to the oval window. The ossicles are a moveable chain of tiny bones that vibrate at the same frequency as the tympanic membrane, and this vibration in turn sets up wavelike movments in the inner ear fluid. The full complexity of the auditory system, however, is well beyond the scope of this description.
- Malleus (2): Also called the hammer. The first of the auditory ossicles, it is attached to the tympanic membrane.
- Incus (2): Also called the anvil. The second of the auditory ossicles, it lies between the malleus and the stapes and is attached to both of them by means of joints.
- Stapes (2): Also called the stirrup. The last of the auditory ossicles, it is attached to the oval window, which is the entrance into the fluid-filled cochlea.
Hyoid Bone (1)
The hyoid bone is located in the neck between the larynx and the lower jaw, at the level of the third cervical vertebra when at rest. It does not actually attach (articulate) to any other bones, but is fixed in place by muscles and ligaments. It helps support the tongue and serves as an attachment for muscles, such as the digastric muscle and the suprahyoid and infrahyoid muscles, that move the tongue and function in swallowing. It is shaped like horse-shoe and it moves when you swallow.
Hyoid Bone and Location
image courtesy of David Darling via EOS
Vertebral Column (26 bones)
The vertebral column, (spine or spinal column), extends from the base of the skull to the pelvis, forming the vertebral axis of the axial skeleton. It is made up of irregular bones called vertebra which are separated from one another by fibrocartilage called intervertebral discs. The spinal column is strong enough to support the head and trunk yet flexible enough to allow many movements, such as forward and lateral flexion (bending to the front or the side) and bending back (hyperextension)1 The spine also permits a fair degree of rotation, or twisting. Each section has a different degree of rotation range of motion. The spinal column also protects the spinal cord, which passes through the vertebral canal, formed by openings in the vertebrae.
Diagram of Spinal Column, Side and Back-View
The vertebral column is described as having three parts, the cervical, thoracic, and lumbar, extending from the skull to the pelvis. Each of the regions of the spine has its own curve, which helps give the spine its resiliency to properly support the body in a correct position. Also, at the lower end are two bones that form a pelvic curve, the sacrum and coccyx, which are not vertebra, although they are formed from nine separate bones in the infant, which fuse together.
The vertebra in the different regions of the spine have special characteristics. However, all vertebra have certain features in common. The body, which is called the centrum and is the anterior part of the vertebra, is the thickest part of the bone. These drum-shaped vertebral bodies, all in a row, are what provides the support for the body, with the intervertebral discs fastened in between them to provide shock-absorption and cushioning.
From the rear of each vertebra projects two pedicles which form the sides of the vertebral foramen. Two plates called laminae grow out of the pedicles and fuse together in the back of the vertebra to form a spinous process. The pedicles, laminae, and spinous process form a bony circle or vetebral arch. The middle of this circle is the vetebral foramen, through which the spinal cord passes. There are other projections from the vertebra, called processes. The transverse process serves as an attachment point for various ligaments and muscles. The superior and inferior articulating facets have cartilage coatings which join each vertebra to the one above and below, forming joints.
Cervical Vertebra (7)
There are seven cervical vertebra, numbered from the first superior or topmost vertebra to the most inferior, or bottom cervical vertebra: C1 though C7. The discs between the cervical vertebra are thicker in the front than in the back, helping to give the region its curve.
Sideview of Cervical Spine
Atlas Vertebra C1
image by Gliu via wikimedia
The first cervical vertebra is called the atlas and the second is called the axis. These two vertebra are shaped differently than the other five. The atlas does not have much of a body at all, but it is the vertebra that the base of the skull moves about on, via the occipital condoyle. It is little more than a ring with two transverse processes and two large articular facets that allow the articulation with the cranium. The second vertebra, the axis, has a tooth-like protuberance called the odontoid process or dens, which projects upwards into the ring of the atlas. As the head is turned, the atlas pivots around this process. The illustration below shows labelled views of the atlas and axis, side by side.
image by Anatomist90 via wikimedia
Atlas and axis vertebrae, the first two vertebrae of the cervical spine
The cervical vertebra have unique bifid or split spinous processes as well as a foramen (a opening) in their transverse processes to allow blood vessels to pass through the cervical spine. In addition, the seventh cervical vertebra is longer and protrudes out beyond the other cervical spines. This is called the vertebra prominens because it can be felt through the skin, providing a useful landmark. It is the very large bony lump you can feel at the base of the back of your neck. Many people, having never noticed this before, upon feeling it one day, think they have hurt themselves or developed a tumor! Run your finger down the back of your neck until you encounter this lump. You have reached the end of the cervical spine, at C7. Next stop, the thoracic spine. But first, the image below shows a "typical" cervical vertebra, which you now know cannot be the first, second or seventh vertebra, so it has to be C3, C4, or C5.
Typical Cervical Vertebra, C3, C4, or C5
Note the notch at the end of the
spinous process and transverse processes,
as well as the hole through the transverse processes,
called a foramen. Other vertebra do not
image by Anatomist90 via wikimedia
Thoracic Vertebra (12)
The thoracic vertebra, called T1 through T12, are larger than the cervical vertebra. In addition to the superior articulating facet as shown in the image of the cervical vertebra above, they also have facets on the sides of their bodies,called fovea or costal facets, which articulate with the heads of the 12 ribs. There are full facets on T1, and T10 through T12; but demi-facets on T2 through T9. This means that in this T2 through T9 region, the head of each rib is joined to two thoracic vertebra, instead of one. Each vertebra has half of the facet for the rib. One-half is on the superior (upper) vertebra and the other half is on the inferior.
An easy way to remember how many thoracic vertebra there are is to remember there are 12 ribs, one for each vertebra. The thoracic vertebra also have longer pointed spinous processes which slope downward.
As you move down the thoracic spine, the thoracic vertebra become progressively larger, their bodies becoming thicker, so that T12 is much larger than T1.
image by Anatomist90 via wikimedia
Lumbar Vertebra (5)
The lumbar have the largest and strongest bodies of all the vertebra, since they have to support more weight. Their bodies are larger side to side and they are thicker in the front than in the back. Their pedicles are short and their spinous processes are broad and blunt, projecting almost horizontally backwards. Their transverse processes are small and project towards the back, upward and laterally. The discs between them are thick, and like the discs in the cervical region, they are thicker anteriorly than posteriorly, which helps give the area its concavity.
image by Anatomist90 via wikimedia
The Sacrum (Sacral Vertebra) (1)
The sacrum is a triangular shaped structure at the base of the spine. As mentioned above, it starts out early in life as five separate vertebra but these gradually fuse (or partially fuse) together, through a complex process, throughout life, until around age 30. The coxal bones of the hip articulate with the sacrum at the sacroiliac joint, forming the pelvic girdle. The sacrum being sandwiched thus between the hip bones provides a firm base for the rest of the spinal column.
The female sacrum is wider than the male and the sacral proontory, which is the area labelled S1 below, does not project forward as much. In addition, the female sacrum is curved more sharply to the rear than in men.
image by Anatomist90 via wikimedia
The Coccyx (1)
The coccyx is commonly known as the tailbone. It it the absolute end of the lower spine, inferior to the sacrum, and starts out early in life as four vertebra that gradually fuse together until about the 25th year of life. It is possible for them to remain separate, however.
The shape of the coccyx and its features are similar to the sacrum, but on a smaller scale. The coccyx is attached by ligaments to the sacral hiatus, which, as you can see from the image above, is the inferior end of the sacrum bone. The attachment to the sacrum forms a joint that allows some movement, with the female coccyx being more moveable than the male coccyx.
Pressure is put on the coccyx when you sit down, and it moves forward to act something like a shock absorber. This bone, when fallen on, is often fractured or dislocated. Many refer to this as a "vestigial tail," since we evolved from ancestors whose spines projected well past their bodies, forming a tail. This gives rise to the oft-stated myth that the "coccyx is useless." Since the coccyx serves as an attachment point for various ligaments and muscles, and since it forms a weight-bearing tripod together with the ischium bones, the tailbone taking on more weight as you lean back, it is certainly not entirely useless. In fact, it would be quite difficult to remove without a lot of trouble!
Thorax (Rib Cage and Sternum, 25 Total)
The 24 ribs, the sternum; the costal cartilages that attach the ribs to the sternum; together with the thoracic vertebra in the back, make up the thorax or thoracic cage, which is what most of us call the chest area. The ribs and rib cage will be considered in a brief explanation, with some attention paid to the general characteristics of a rib. The sternum is described separately.
The thoracic cage encloses and protects the thoracic cavity, which is above the diaphragm. The thoracic cavity contains the lungs, divided into two pleural cavities by the mediastinum, and the heart, which is contained in a third division called the pericardial cavity. These organs are surrounded by the the bones of the thorax like a bony cage, which we call the rib cage. However, this cage is not rigid. If it were, you'd have a hard time breathing because your lungs would be prevented from fully expanding within the bars of the cage. The bones are actually able to move so that the thoracic cage can expand and contract, allowing the lungs to expand. This movement is created by muscles, and when it occurs, it helps change the pressure within the cavity, thus helping the lungs to take in or expel air. The joints of the thorax allow this to happen. These are the chondrosternal, costochondral, and interchondral joints, which allow external rotation and elevation of the rib. In essence, this allows the ribs to move slightly outwards and slightly upwards.
Although your liver, stomach, pancreas, spleen, gallbladder, and even the upper part of your kidneys have ribs on either side or them, this lower part, where the ribs do not directly attach to the sternum and so are open to the front is not considered part of the thoracic cavity, but of the abdominal cavity.
The Ribs and Rib Cage (24)
Let's start with the obvious. Regardless of sex, male or female, each person has the same number of ribs, normally. The myth that women had one more rib than men was circulated for years even though counting the ribs is a fairly elementary undertaking. This myth came from the story of Genesis in the Bible: the part where it says a rib was taken from Adam to give to Eve. This created the idea that men were missing a rib and women had an extra.
It is possible to have extra ribs, and these are usually associated with the cervical or lumbar vertebrae. We know from the above explanation that, normally, we only have ribs connected to our thoracic vertebra, but some people, now and again, have extra, shorter ribs connected to a cervical vertebra above or a lumbar vertebra below. These anomalies will not be considered in this article.
The first seven pairs of ribs are called the true ribs or vertebrosternal ribs. This means that they join to the sternum directly by their costal cartilages. The other five pairs are called false ribs. This is because their costal cartilages do not reach the sternum directly. The cartilages of the first three upper false ribs join the costal cartilages of the true ribs above. The last two lower ribs have no costal attachments at all, and for this reason they are sometimes called floating ribs. Remember that regardless of their attachment to the sternum, all the ribs are anchored firmly to the thoracic spine in the back. No rib actually "floats." So a better name for these last two ribs are vertebral ribs, to describe the fact that they only attach to the vertebra, as opposed to vertebrosternal ribs, which attach to both the vertebra and the sternum.
The following image shows the organs of the thoracic cavity and the corresponding numbered ribs and the levels of the thoracic vertebra.
The Thoracic Cavity
A typical rib has a long, slender, flattened shaft that curves around the chest and slopes downward from back to front. On the back end, which attaches to the thoracic vertebra, is the head, which articulates with a facet on the vertebra, and usually also to the next higher vertebra. Just behind this head, which is lateral in the body, is the neck, which allows for the attachment of various ligaments. A little further on, near the neck, is the tubercle, a projection that articulates with the transverse process of the vertebra.
The Rib Cage
To the other end of the rib, in the front of the body, is attached a costal cartilage made of hyaline cartilage. This continues in line with the rib to connect to the sternum, in the case of the first seven ribs. The costal cartilages of the next three ribs attach to the costal cartilage of the rib above, so their attachment to the sternum is indirect. The last two ribs, the so-called floating ribs, do not attach in the front at all.
A typical rib
Re-labelled drawing from Gray's
The Sternum or Breastbone (1)
The sternum is located at the mid-line of the body in the front of the thoracic cage. It is somewhat flat and elongated, but very strong, serving to protect the structures beneath it. It has three parts, the manubrium, body, and xiphoid process.
The manubrium is the upper quarter of the sternum. It has a depression called the suprasternal or jugular notch at its superior border (top), which is located just at the region of the sternoclavicular joint. This joint is formed by two notches on its side called the clavicular notches, where it articulates with the clavicles (collarbones). It also has two first costal notches, where it joins with the first two ribs.
Below the manubrium is the body of the sternum, which makes up more than half of the length of the sternum. The line where the manubrium and body meet is called the transverse junction and forms the angle of the sternum. The body also has notches for joining with the corresponding ribs. The second rib meets the sternum just at the transverse junction so half of the sternal facet is on the manubrium and the other half on the body beneath it. The third through sixth ribs articulate with the sternal body. The seventh rib falls on the transverse junction between the body and the xiphoid process. The manubrium and body usually remain as separate bones until mid-life, when they fuse into one.
The xiphoid process begins as a piece of cartilage which slowly ossifies (becoming bone) and fuses to the body by mid-life. The sternum provides attachment for muscles of the thorax, neck, and abdomen such as the sternocleidomastoid, sternalis, intercostals, etc.
This concludes part one of the bones of the adult skeleton, the axial skeleton. Part two continues with the upper appendicular skeleton, covering the bones of the pectoral (shoulder) girdle, upper arm, forearm, and hand.