The axial skeleton comprises the skull, vertebral column and ribs.
The skull is composed of a large number of bones, most of which join at fibrous joints, with little, if any movement between them. Most of the skull bones are flat bones. The major components are:
Mandible: the lower jaw bone which articulates with part of the temporal bone at the temporo-mandibular joint. The mandible is divided into the horizontal ramus (which contains teeth) and the vertical ramus, which is the site of insertion of the main muscles of mastication.
Maxilla, incisive and palatine bones: together these make up the upper jaw, the hard palate and the floor of the nasal cavity.
Nasal bone: this forms the roof and lateral walls of the nasal cavity.
Frontal bone: this makes up part of the rostral wall of the cranium.
Zygomatic bone forms the zygomatic arch which runs from the maxilla, rostrally, to the temporal bone, caudally. It is easily palpated (the ‘cheek-bones’) as it forms part of the ventral and lateral walls of the orbit. The vertical ramus of the mandible lies medial to the zygomatic arch.
Temporal bone: although not palpable in the normal man is recognized on radiographs by the characteristic bony enlargement, the tympanic bulla, which houses the middle ear.
Parietal bone: forms the lateral walls of the cranium.
Occipital bone: forms the caudal wall of the cranium; its dorsal part is the occipital protuberance and is normally palpable.
Hyoid bones: a collection of small bones, fashioned as a cradle, which run from the temporal bone to the larynx. They support the larynx, allowing it to move, primarily in the cranio-caudal direction, at the time of swallowing.
The vertebral column is composed vertebrae, arranged in five sections. The vertebrae protect the delicate nervous tissue of the spinal cord which runs in the vertebral canal. They also provide sites for the attachment of muscles. As upright primates we tend to forget that the vertebral column is important in the locomotion.
Vertebral body is a common feature to all vertebrae with an overlying vertebral arch which surrounds the vertebral canal. Arising from the dorsal aspect of the vertebral arch is the spinous process. The transverse processes, by contrast, arise from the two sides of the vertebral body. These provide sites for the insertion of muscles.
At the articulation between vertebrae there are synovial joints between the articular processes on the vertebral arch. Lying between the vertebral bodies are intervertebral discs. These are roughly circular in cross-section, and consist of two regions:
- An outer fibrous layer, called the annulus fibrosus.
- An inner gelatinous area, the nucleus pulposus.
The intervertebral disc serves as a shock absorber between the vertebrae. It also allows movement between the bones. The tissue of the nucleus pulposus undergoes certain chemical and physical changes with age, with replacement of the gelatinous material by fibrous tissue plus cartilage. In addition, calcium may be laid down in the cartilage, so allowing the discs to be seen on radiographs. Prolapse of an intervertebral disc (sometimes called a “slipped disc”) involves the protrusion of disc material into the overlying vertebral canal. This places pressure on the spinal cord and often interferes with neurological function leading to paralysis.
The vertebral column is divided into five regions in which the vertebrae have certain distinguishing features.
Cervical Vertebrae (7 vertebrae)
The first two cervical vertebrae are modified to allow movements of the head. The first cervical vertebra is called the atlas (because in humans it carries the skull as, in mythology, Atlas carried the world); it articulates with the base of the skull. Uniquely it has no vertebral body, but instead has two large lateral processes called wings. The axis is the second cervical vertebra; it lies behind the atlas and has a short bony protrusion from the cranial end of the vertebral body called the odontoid peg. The vertebral arch of the axis is very large. The articulation between skull, atlas and axis allow the enormous range of head movements in normal human.
The remaining five cervical vertebrae are all similar; they possess a strong vertebral arch, however, they have poorly developed spinous processes but stout transverse processes.
Thoracic Vertebrae (12 vertebrae)
The thoracic vertebrae have short vertebral bodies and prominent spinous processes; these may be felt when palpating the spine. Each thoracic vertebra articulates with a rib, by means of a synovial joint.
Lumbar Vertebrae (5 vertebrae)
Lumbar vertebrae have long vertebral bodies, compared with the thoracic vertebrae, but lack a distinct spinous process. There are large transverse processes which extend cranially and laterally, providing sites for the insertion of muscles, and also protecting the underlying kidneys. Sacral Vertebra (1 vertebra)
he three vertebrae are fused in the adult to form the sacrum. This articulates with the ilium (part of the hip bone or pelvis) by means of a cartilaginous joint, the sacroiliac joint. This articulation allows mechanical forces to be efficiently transferred from the lower limb to the vertebral column.
Coccygeal Vertebra: One fused with sacrum.
The ribs support the lateral wall of the chest. Each rib is composed of a dorsal bony section which joins with a ventral cartilaginous section at the costo-chondral junction. The ribs are arranged in pairs, and each articulates with a thoracic vertebra dorsally. The cranial eight ribs (sometimes called the true ribs) articulate with the sternum ventrally, whereas the caudal five ribs do not articulate directly with sternum, and are called the “floating ribs’. The ribs provide extensive protection for the thorax (especially the heart and lungs) as well as providing a mechanical framework for its expansion during breathing.