Axial Skeleton

What is the axial skeleton?

The axial skeleton is one of the two main groups of bones in the human body. It is made up of the bones that make up the central axis of the body, that is, those that make up the skull, neck, rib cage and spine, and whose main function is to protect vital organs.

The human skeleton, as well as that of most vertebrate animals, is made up of two groups of bones commonly known as the axial skeleton and the appendicular skeleton.

More than 50% of the bones of the human body belong to the appendicular skeleton, however and despite their lower proportion, the bones of the axial skeleton fulfill extremely important protective and support functions, since they protect vital organs such as the brain, spine dorsal and viscera.

Thus, the bones of the axial skeleton are those that form the head, vertebrae and trunk, while those of the appendicular skeleton, as its name indicates, are those that form the appendages of the axial skeleton, that is, the upper extremities and lower, which function in movement and locomotion.

Functions of the axial skeleton

The axial skeleton is a fundamental part of the human skeleton since the protection and support of the different internal organ systems depend on it: the nervous system, the digestive system, the cardiovascular system, the respiratory system and part of the muscular system.

The central nervous system, which is made up of the brain and spinal cord, lies mainly within the structures of the axial skeleton that correspond to the skull and spinal column.

In the skull, in addition, not only is the brain housed, but there are also spaces corresponding to:

– the eye sockets (where the eyes are arranged)

– the nasal cavity (part of the respiratory system)

– the jaws and mouth (part of the digestive system)

– the tympanic cavity (where the 3 ossicles of the ears are)

The cardiovascular and respiratory systems are found inside what is known as the thorax or trunk, where the heart and lungs, the main organs of each respectively, are protected mainly by the rib cage formed by the ribs.

Although it provides a tough defense, the ribs are arranged in the rib cage in such a way as to allow expansion of the lungs during inspiration, as well as their contraction during expiration.

Axial skeletal bones

The axial skeleton, which constitutes the central portion of the body, is made up of 80 bones distributed in three regions: the head, the vertebral column and the thorax.

Head

The bony component of the head is made up of 22 separate bones such as the skull, the facial bones, the ossicles of the middle ear in the cavity of the eardrum, and the hyoid bone (under the jaw).

The cranium

Skull bones (Source: Original by LadyofHats. Translated by Ascánder. / Public domain, via Wikimedia Commons)

There are 8 cranial bones that form the cavity where the brain is housed and provide an attachment site for the muscles of the head and neck. These bones are:

– Frontal bone

– Parietal bones (2)

– Temporal bones (2)

– Occipital bone

– Sphenoid bone

– Ethmoid bone

The auditory ossicles

The tympanic cavity, corresponding to the middle ear, contains three small “chained” bones, in fact, they are the three smallest bones in the human body and that is why they are known as the ossicles. The three ossicles are:

– Hammer (2, one in each ear)

– Anvil (2, one in each ear)

– Stapes (2, one in each ear)

The main function of these bones is to transmit vibrational sound waves that collide with the tympanic membrane (which separates the outer ear from the middle ear) into the cochlea, a fluid-filled cavity in the inner ear.

Face

There are 14 facial bones and they stand out for their relationship with the sensory organs:

– Nasal bones (2)

– Maxillary bones (2)

– Zygomatic bones (2)

– Palatine bones (2)

– Vomer bone

– Lacrimal bones (2)

– Nasal turbinates (2)

– Mandibular bone

Facial bones (Source: -Cristobal carrasco (talk) 01:50, 24 February 2010 (UTC) / Public domain, via Wikimedia Commons)

The other bone of the axial skeleton that is in the cephalic region (by the head) is the hyoid bone , which is located below the mandible, in the front of the neck, where it is connected with muscles of the jaw, of the larynx and the tongue.

Spinal column

This portion of the axial skeleton supports the weight of the head, protects the spinal cord, and is where the ribs and muscles of the neck and back attach. It is made up of 26 bones, 24 of them corresponding to the vertebrae and the other two to the sacrum and the coccyx. In total it has an approximate length of 70-71 cm.

The order in which these bones are arranged in the spine is as follows:

– C1, is the first vertebra, also known as the Atlas bone, it is the site where the skull connects with the spine

– C2, the second vertebra, also known as the Axis bone (axis); it is right between the Atlas and the third vertebra

– C3-C7 (5), called cervical vertebrae

– Th1-Th12 (12), called thoracic vertebrae

– L1-L5 (5), called lumbar vertebrae

– Sacral bone

– coccyx

Vertebral segments of the spine (Source: DrJanaOfficial / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons)

The vertebrae are bones that are arranged forming a hollow cylindrical cavity inside, which contains the nerves that make up the spinal cord, which is part of the central nervous system. The vertebrae also have notches through which spinal nerves can exit.

The thorax

The chest of the human body is made up of the skeleton that forms the thoracic cavity. The sternum and ribs belong to this part of the axial skeleton, totaling 25 bones.

The bones of the thorax not only protect vital organs such as the heart, lungs and other viscera, but also support the shoulder girdles and upper limbs, serve as an attachment site for the diaphragm, for the muscles of the back, neck , shoulders and chest.

The ribs forming the rib cage (Source: Mikael Häggström / Public domain, via Wikimedia Commons)

The bones of the thorax are:

– The sternum: manubrium, body and xiphoid process (three flat and long bones fused into one in the anterior region of the chest)

– The ribs (12 pairs, attached to the thoracic vertebrae at the back of the body)

With the exception of the 11th and 12th pairs of ribs, all the ribs are attached to the sternum through what is called “costal cartilage.”

Joints

In the head

The 8 bones that make up the cranial cavity are closely connected to each other through a type of fibrous joints with very little movement known as sutures , which are of the synarthrosis type, that is, immobile joints.

There are four types of sutures in the skull:

– Lambdoid suture (occipital-parietal)

– Coronal suture (frontal-parietal)

– Sagittal suture (parietal)

– Squamous sutures (temporal-parietal)

In addition, the teeth are articulated with the maxillary and mandibular bones through a type of joint known as gonphosis, which are fibrous and immobile.

In the spine

The vertebrae that make up the spinal column are joined together thanks to joints known as intervertebral discs, which are fibrocartilaginous joints of the symphysis type, which allow some movements and which contribute to the cushioning of the spine during movement.

On the chest

The unions between the ribs and the sternum are mediated by what is known as “costal cartilages” which are a type of cartilaginous joint called synchondrosis, which allow some freedom of movement, very important for breathing.

In addition, the expansion of the thoracic cavity also occurs thanks to the joints between the thoracic vertebrae and the posterior ends of the ribs, since these are synovial joints, of the diarthrosis type, which are known as costovertebral joints and which are joined by ligaments.

References

  1. Gray, H. (2009). Gray’s anatomy. Arcturus Publishing.
  2. Marieb, EN, & Hoehn, K. (2007). Human anatomy & physiology. Pearson education.
  3. Netter, F. (2010). Atlas of Human Anatomy. Netter Basic Science.
  4. Saladin, KS, & McFarland, RK (2008). Human anatomy (Vol. 3). New York: McGraw-Hill.
  5. Warren, A. (2020). Encyclopaedia Britannica. Retrieved September 16, 2020, from britannica.com

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