Along the Axis of the Axial Skeleton: Bones That Form the Axial Skeleton

Overview of Axial skeleton: Skull, Vertebrae, larynx and thorax

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.

1. Skull Bones Protect the Brain and Form an Entrance to the Body.

Cranial bones and Facial 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.

Bones of the face: Lacrimal, zygomatic, maxilla, and mandible

Facial Skeleton. 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).

Bones of the CraniumCranial Bones

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

Sutures of the skullSkull Sutures

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:

  • lambdoid suture (between the occipital and parietal bones)
  • coronal suture (between the frontal and parietal bones)
  • sagittal suture (between the two parietal bones)
  • squamous sutures (between the temporal and parietal bones)

 

2. The Hyoid Bone, Laryngeal Skeleton, and Bones of the Inner Ear Are Commonly Categorized with Skull Bones

Inner ear bones, the Incus, Malleus, and StapesBones of the Inner Ear

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.

Hyoid bone and larynxLaryngeal Skeleton

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.

3. The Bones of the Vertebral Column: The Vertebrae, Sacrum, and Coccyx

Vertebrae, sacrum, and coccyx in vertebral column

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.

4. The Bones of the Thoracic Cage Protect Internal Organs

Manubrium, true and false ribs, sternum, and xiphoid process in thoracic cage

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.

External Resources

Ancient jaw may hold clues to origin of human genus from Science News.

Visible Body provides free 3D models of the human skeleton: PC, Mac, and iPhone/iPad.

Chapter 28, Skeletal Development from the book Principles of Developmental Genetics.
Moody, Sally A., ed. Principles of Developmental Genetics. Waltham: Elsevier Inc., 2015. Print.