which bone is part of the axial skeleton?

The axial skeleton forms the central axis of the body and includes the bones of the skull, ossicles of the middle ear, hyoid bone of the throat, vertebral column, and the thoracic cage (ribcage) (Figure 1). The function of the axial skeleton is to provide support and protection for the brain, the spinal cord, and the organs in the ventral body cavity. It provides a surface for the attachment of muscles that move the head, neck, and trunk, performs respiratory movements, and stabilizes parts of the appendicular skeleton.

The bones of the skull support the structures of the face and protect the brain. The skull consists of 22 bones, which are divided into two categories: cranial bones and facial bones. The cranial bones are eight bones that form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head and neck. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and the ethmoid bone. Although the bones developed separately in the embryo and fetus, in the adult, they are tightly fused with connective tissue and adjoining bones do not move (Figure 2).

The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The auditory ossicles consist of six bones: two malleus bones, two incus bones, and two stapes (one of each bone on each side). These are the smallest bones in the body and are unique to mammals.

Fourteen facial bones form the face, provide cavities for the sense organs (eyes, mouth, and nose), protect the entrances to the digestive and respiratory tracts, and serve as attachment points for facial muscles. The 14 facial bones are the nasal bones, the maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones, the inferior nasal conchae, and the mandible. All of these bones occur in pairs except for the mandible and the vomer (Figure 3).

Although it is not found in the skull, the hyoid bone is considered a component of the axial skeleton. The hyoid bone lies below the mandible in the front of the neck. It acts as a movable base for the tongue and is connected to muscles of the jaw, larynx, and tongue. The mandible articulates with the base of the skull. The mandible controls the opening to the airway and gut. In animals with teeth, the mandible brings the surfaces of the teeth in contact with the maxillary teeth.

The vertebral column, or spinal column, surrounds and protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column comprises 26 bones: the 24 vertebrae, the sacrum, and the coccyx bones. In the adult, the sacrum is typically composed of five vertebrae that fuse into one. The coccyx is typically 3–4 vertebrae that fuse into one. Around the age of 70, the sacrum and the coccyx may fuse together. We begin life with approximately 33 vertebrae, but as we grow, several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae (Figure 4).

Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the spinal cord. The vertebral column is approximately 71 cm (28 inches) in adult male humans and is curved, which can be seen from a side view. The names of the spinal curves correspond to the region of the spine in which they occur. The thoracic and sacral curves are concave (curve inwards relative to the front of the body) and the cervical and lumbar curves are convex (curve outwards relative to the front of the body). The arched curvature of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring (Figure 4).

Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine and acts as a cushion to absorb shocks from movements such as walking and running. Intervertebral discs also act as ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age and becomes less elastic. This loss of elasticity diminishes its ability to absorb shocks.

The thoracic cage, also known as the ribcage, is the skeleton of the chest, and consists of the ribs, sternum, thoracic vertebrae, and costal cartilages (Figure 5). The thoracic cage encloses and protects the organs of the thoracic cavity, including the heart and lungs. It also provides support for the shoulder girdles and upper limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders. Changes in the volume of the thorax enable breathing.

The sternum, or breastbone, is a long, flat bone located at the anterior of the chest. It is formed from three bones that fuse in the adult. The ribs are 12 pairs of long, curved bones that attach to the thoracic vertebrae and curve toward the front of the body, forming the ribcage. Costal cartilages connect the anterior ends of the ribs to the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs.

The bones of the human skeleton are divided into two groups. The appendicular skeleton includes all the bones that form the upper and lower limbs, and the shoulder and pelvic girdles. The axial skeleton includes all the bones along the body’s long axis. Let’s work our way down this axis to learn about these structures and the bones that form them.

The axial skeleton includes the bones that form the skull, laryngeal skeleton, vertebral column, and thoracic cage. The bones of the appendicular skeleton (the limbs and girdles) “append” to the axial skeleton.

The skull consists of the cranial bones and the facial skeleton. The cranial bones compose the top and back of the skull and enclose the brain. The facial skeleton, as its name suggests, makes up the face of the skull.

The 14 bones of the facial skeleton form the entrances to the respiratory and digestive tracts. The facial skeleton is formed by the mandible, maxillae (r,l), zygomatics (r,l), and the bones that give shape to the nasal cavity: lacrimals (r,l), nasals (r,l), vomer, palatines (r,l), and the nasal conchae (r,l).

The eight cranial bones support and protect the brain: occipital bone, parietal bone (r,l), temporal bone (r,l), frontal bone, sphenoid, and ethmoid.

In fetuses and newborn infants, cranial bones are connected by flexible fibrous sutures, including large regions of fibrous membranes called fontanelles. These regions allow the skull to enlarge to accommodate the growing brain. The sphenoidal, mastoid, and posterior fontanelles close after two months, while the anterior fontanelle may exist for up to two years. As fontanelles close, sutures develop. Skull sutures are immobile joints where cranial bones are connected with dense fibrous tissue.

The four major cranial sutures are:

Inside the petrous part of the temporal bone are the three smallest bones of the body: the malleus, incus, and stapes. These three bones articulate with each other and transfer vibrations from the tympanic membrane to the inner ear.

The laryngeal skeleton, also known as the larynx or voice box, is composed of nine cartilages. It is located between the trachea and the root of the tongue. The hyoid bone provides an anchor point. The movements of the laryngeal skeleton both open and close the glottis and regulate the degree of tension of the vocal folds, which–when air is forced through them–produce vocal sounds.

The vertebral column is a flexible column formed by a series of 24 vertebrae, plus the sacrum and coccyx. Commonly referred to as the spine, the vertebral column extends from the base of the skull to the pelvis. The spinal cord passes from the foramen magnum of the skull through the vertebral canal within the vertebral column. The vertebral column is grouped into five regions: the cervical spine (C01-C07), the thoracic spine (T01- T-12), the lumbar spine (L01-L05), the sacral spine, and the coccygeal spine.

The thoracic cage, formed by the ribs and sternum, protects internal organs and gives attachment to muscles involved in respiration and upper limb movement. The sternum consists of the manubrium, body of the sternum, and xiphoid process. Ribs 1-7 are called true ribs because they articulate directly to the sternum, and ribs 8-12 are known as false ribs.

The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate. In the human skeleton, it consists of 80 bones and is composed of six parts; the skull (22 bones), also the ossicles of the middle ear, the hyoid bone, the rib cage, sternum and the vertebral column. The axial skeleton together with the appendicular skeleton form the complete skeleton. Another definition of axial skeleton is the bones including the vertebrae, sacrum, coccyx, skull, ribs, and sternum.[1]

Flat bones house the brain and other vital organs. This article mainly deals with the axial skeletons of humans; however, it is important to understand the evolutionary lineage of the axial skeleton. The human axial skeleton consists of 81 different bones. It is the medial core of the body and connects the pelvis to the body, where the appendix skeleton attaches. As the skeleton grows older the bones get weaker with the exception of the skull. The skull remains strong to protect the brain from injury.

The human skull consists of the cranium and the facial bones. The cranium holds and protects the brain in a large space called the cranial vault. The cranium is formed from eight plate-shaped bones which fit together at meeting points (joints) called sutures. In addition there are 14 facial bones which form the lower front part of the skull. Together the 22 bones that compose the skull form additional, smaller spaces besides the cranial vault, such as the cavities for the eyes, the internal ear, the nose, and the mouth. The most important facial bones include the jaw or mandible, the upper jaw or maxilla, the zygomatic or cheek bone, and the nasal bone.[2]

Humans are born with separate plates which later fuse to allow flexibility as the skull passes through the pelvis and birth canal during birth. During development the eight separate plates of the immature bones fuse into one single structure known as the Skull. The only bone that remains separate from the rest of the skull is the mandible.[3]

The rib cages are composed of 12 pairs of ribs plus the sternum for a total of 25 separate bones. The rib cage functions as protection for the vital organs such as the heart and lungs. The ribs are shaped like crescents, with one end flattened and the other end rounded. The rounded ends are attached at joints to the thoracic vertebrae at the back and the flattened ends come together at the sternum, in the front.[4]

The upper seven pairs of ribs attach to the sternum with costal cartilage and are known as “true ribs.” The 8th through 10th ribs have non-costal cartilage which connects them to the ribs above, and for this they are known as "false ribs". The last two ribs are called “floating ribs” because they do not attach to the sternum or to other ribs and simply “hang free.” The length of each rib increases from number one to seven and then decreases until rib pair number 12. The first rib is the shortest, broadest, flattest, and most curved.[5]

At birth the majority of humans have 33 separate vertebrae. However, during normal development several vertebrae fuse, leaving a total of 24, in most cases. The confusion about whether or not there are 32-34 vertebrae stems from the fact that the two lowest vertebrae, the sacrum and the coccyx, are single bones made up of several smaller bones which have fused together. This is how the vertebrae are counted: 24 separate vertebrae and the sacrum, formed from 5 fused vertebrae, and the coccyx, formed from 3-5 fused vertebrae. If you count the coccyx and sacrum each as one vertebra, then there are 26 vertebrae. If the fused vertebrae are all counted separately, then the total number of vertebrae comes to between 32 and 34 (due to the number making up the coccyx varying between 3 and 5).

The vertebral column consists of 5 parts. The most cranial (uppermost) part is made up by the cervical vertebrae (7), followed by thoracic vertebrae (12), lumbar (5), sacral (5) and coccygeal vertebrae (3-5).

Cervical vertebrae make up the junction between the vertebral column and the cranium. Sacral and coccygeal vertebrae are fused and thus often called "sacral bone" or "coccygeal bone" as unit. The sacral bone makes up the junction between the vertebral column and the pelvic bones.

The word "axial" is taken from the word "axis" and refers to the fact that the bones are located close to or along the central "axis" of the body. The term axis means the central point around which the other structures are distributed.[6]

The axial skeleton consists of 80 bones:

The five parts of your axial skeleton include the bones in your skull, ossicles (small bones) of your middle ear, hyoid bone of your neck, vertebra (bones of your spine) and thoracic cage (ribcage).