Axial skeleton: what is it, parts and characteristics

The skeleton or skeletal system is a biological system that provides support, support and protection to soft tissues and muscles in living organisms.

Vertebrates have an internal skeletal system (endoskeleton) made up of bones, while arthropods have opted for the exoskeleton as an evolutionary adaptation, a complex layer composed mainly of chitin, which gives these animals protection, support and the possibility of to breathe.

The functions of the skeleton in the human being are multiple: mechanical support, production of movement, protection, metabolic storage of substances such as calcium and formation of blood cells. Hematopoietic stem cells are found in the bone marrow (which is located inside many bones), which when differentiated, give rise to all circulating blood bodies. This includes lymphocytes, platelets, and red blood cells, among others.

Thus, the functionality of the skeleton in vertebrates goes far beyond postural maintenance. Today we will tell you all about the axial skeleton

, the one that makes up the central axis of the human being's body and enables us to remain in three-dimensional space upright.

• Related article: "Flat bones: what are they, types and functions"

What is the axial skeleton?

The human skeleton is made up of a total of 206 bones, each and every one of them with morphology different, from the femur of the thigh (the longest in the body) to the stapes (the smallest), which forms the ear half. In an adult human being, the skeletal system accounts for 15% of body weight: if a man has a mass of 75 kilos, 9 of them correspond only to bone matter.

Bones are 98% made up of extracellular matter (mainly hydroxyapatite and collagen based) and only 2% of Its tissues are cells themselves, that is, osteoblasts, osteoclasts and osteocytes (responsible for renewing the matrix that is). Due to its percentage of eminently mineral matter, bone structures store 99% of the body's calcium.

On the other hand, there are 2 parts to the human skeleton:

• Axial skeleton: made up of the skull, spinal column, ribs, and sternum. It consists of 80 bones.
• Appendicular skeleton: it is formed by the bones of the upper and lower limbs, together with the shoulder and pelvic girdles. It includes 126 bones.

So, following this short list, we could define the axial skeleton as the set of bones that make up the central axis of the human body. The term "axial" comes from the Latin word axis, which refers to a central point from which the rest of the structures and formations are distributed in a stable way. Without this main axis, the limbs would not have points to anchor, so the name comes to this skeletal system like a glove.

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Parts of the axial skeleton

The axial skeleton is made up of the skull, spine, ribs, and sternum.. We show you the particularities of each of these subsystems in the following lines.

1. Skull

The bones of the skull make up a bony box that protects the brain and gives us the facial structure, which in turn makes it possible to see, breathe, swallow, make sounds and basically perform all the functions that define us as living beings. In zoology, we really speak of an osteocranium and not just a skull, since other animals have cranial structures similar to ours that are not completely ossified.

In turn, the cranial bones are divided into 2 large groups based on their functionality: the neurocranium and the splanchnocranium. The first is the one that contains the brain and the cranial meninges, and is composed of the occipital, sphenoid, scales, mastoid, parietal and frontal bones. Perhaps the frontal bone is the most famous of all, because, as its name indicates, it occupies our forehead and over it the occipitofrontal muscle is inserted, which allows the elevation of the eyebrows and the expression of many emotions.

On the other hand, the splanchnocranium refers to the part of the skull that contains the anterior part of the digestive and respiratory systems. Also known as viscerocranium, it has the following bones: ethmoid, lacrimal, vomer, maxillary, zygomatic, tympanic portion, temporal styloid and mandible.

2. rib cage

The rib cage is made up of a total of 24 ribs (12 on each side of the body plane) and the sternum, adding up to a total of 25 total bone bodies.. The costal cartilages are also included in this ultrastructure, although they are not materials made from bone.

The rib cage has an eminently protective function, since it safeguards the most important organs of the entire human body: the heart and lungs. It also gives an anchor point to the bones of the shoulders (and therefore to the extremities upper), serves as a point of attachment to the diaphragm and the muscles of the chest, back, neck and arms. Without a doubt, the physiology of the human being could not be conceived as it is today without the rib cage.

Also worthy of special mention is the sternum, a long, flat, pointed bone that serves as a insertion for almost all ribs (except the last 2 pairs, which are therefore known as floating). In humans, this bone is made up of 3 sections: the handle or manubrium, the body and the process or xiphoid process, which has a highly variable shape.

3. Spine

At the time of birth, most human beings come into the world with 33 vertebrae, but throughout development some of these merge, giving a total of 24 in almost all cases.

Broadly speaking, the spine can be differentiated into several sections: cervical (7 vertebrae that make up the neck), thoracic (12 vertebrae) and lumbar (5 vertebrae). Finally, the sacrum and coccyx are presented, the latter being a “vestigial” remnant of the tail of ancestral mammals.

Spinal column provides protection to the spinal cord and nerve roots, which allow communication of all parts of our body with the brain. In addition to this, it is also the support base for ligaments, tendons and muscles. On the other hand, the spine is a structural support, it allows flexibility and mobility to trunk level and is an excellent mineral storage and blood cell production center red.

The axial skeleton summarized in 6 points

If the information previously provided has seemed a bit confusing, do not worry: it is normal, since the skeletal apparatus stands out for its anatomical complexity. With you remembering these 6 essential points, you will know the basics of the axial skeleton:

• The skull is the first section of the axial skeleton. This is composed of 8 bones of the neurocranium and 14 of the splanchnocranium, giving a total of 22.
• 6 ossicles of the ear (3 on each side) are also involved in the axial skeleton. These are the hammer, the anvil, and the stirrup.
• 1 hyoid bone in the neck, which rises and falls in movements such as swallowing or breathing, is also part of the central axis.
• The vertebral column is the next ultrastructure of the axial skeleton, composed of 24 vertebrae, the sacrum and the coccyx (a total of 26 bones).
• In last place we have the rib cage, with a total of 24 ribs and the sternum (25 bones).

Thus, the axial skeleton adds up to a total of 80 bones, as we have indicated at the beginning. Although we have focused on the bony elements that make up the skull, spinal column and cage thoracic, you should not forget that the ossicles of the ear and the hyoid are also part of this axis central.

If anything can be made clear about the human skeleton, it is that this it is a true work of art of biomechanics. Each small bone structure does not fulfill only one, but several functions and, however small it may be, it is essential to understand human physiology in all its meanings.

The axial skeleton is made up of a total of 80 bones and 3 large sections, but You should also not forget that there is an appendicular skeleton, with a total of 126 bone structures that allow locomotion and making efforts, among many other things.

Bibliographic references:

• On the axis of the axial skeleton: Bones that form the axial skeleton, visiblebody. Picked up on February 16 in https://www.visiblebody.com/es/learn/skeleton/axial-skeleton#:~:text=El%20esqueleto%20axial%20incluye%20todos, vertebral% 20y% 20la% 20box% 20tor% C3% A1cica.
• Ibarra, J. M., Romero, F. T., María, V. A., Gómez, J. A., & Cabrera, M. P. (2015). Minimally invasive surgery in the axial skeleton. Colombian Journal of Orthopedics and Traumatology, 29 (1), 17-27.
• Le Vay, D. (2008). Human anatomy and physiology. Editorial Paidotribo.
• Martins, J. M. (2017). Osseous or skeletal system: bone terminology, axial skeleton, appendicular skeleton.
• NUMBER, G. Description of the axial skeleton of the spine and thorax (Doctoral dissertation, Pontifical University of Valparaíso).
• Olivares, R., & Rojas, M. (2013). Axial and appendicular skeleton of a vertebrate. International Journal of Morphology, 31 (2), 378-387.
• Olivares, R., & Rojas, M. (2013). Axial and appendicular skeleton of a vertebrate. International Journal of Morphology, 31 (2), 378-387.