Inferior aspect of cranium

The inferior aspect of cranium, also known as the norma basalis, shows how the skull appears from underneath. It can be divided into three parts: anterior, middle, and posterior.

Anterior part:

The anterior part is formed by the hard palate and the arches of the alveolar bone. The bones that contribute to the hard palate are the maxilla and the palatine bones, which are connected by sutures. Notable foramina in the hard palate include the incisive foramen in the front, and several palatine foramina, with one greater and multiple lesser ones, located in each posterolateral corner of the hard palate.

Middle part:

The middle part extends from the hard palate to the front edge of the foramen magnum. It is primarily composed of the sphenoid and temporal bones. In the central area, we can observe the body of the sphenoid bone, which connects posteriorly to the basilar part of the occipital bone. The basilar part features a prominent pharyngeal tubercle where the pharyngeal raphe attaches, allowing for the insertion of pharynx muscles. Towards the front, there is a midline vomer bone attached to the sphenoid. This bone contributes to the nasal septum, splitting the nasal cavity into two posterior nasal apertures. Moving laterally from the body of the sphenoid bone, we encounter the bony pterygoid process, consisting of medial and lateral pterygoid plates. These structures extend downwards from the sphenoid bone and provide attachment points for the pterygoid muscles used in mastication. Further outwards, we reach the infratemporal fossa region. Here, the roof of the fossa is formed by the greater wing of the sphenoid bone on the inner side, and the squamous part of the temporal bone on the outer side.

On the inferior aspect of cranium, within the greater wing of the sphenoid bone, we can observe two notable foramina. The first is the foramen ovale, which allows passage for the mandibular nerve. The second is the foramen spinosum, serving as an opening for the middle meningeal artery. Laterally to the greater wing, the squamous part of the temporal bone displays a prominent concavity known as the mandibular fossa. This concavity articulates with the head of the mandible, forming the temporomandibular joint.

Moving backward, towards the posterior region, we find the wedge-shaped petrous part of the temporal bone situated just behind the greater wing of the sphenoid bone and outer to the basilar part of the occipital bone. The apex of this wedge-shaped bone is directed medially, and together with the basilar part of the occipital bone and the sphenoid body, it forms the edge of an irregular foramen called the foramen lacerum. The petrous bone contains the opening for the carotid canal, through which the internal carotid artery enters the cranial cavity. Posteriorly, the petrous bone also serves as the margin of the jugular foramen, along with the occipital bone. This opening allows for the passage of the internal jugular vein, as well as the glossopharyngeal, vagus, and accessory cranial nerves.

Posterior part:

The posterior part of the inferior aspect of cranium is composed of the occipital bone. This bone features a large central foramen called the foramen magnum, which allows for the passage of the spinal cord, brainstem, meninges, vertebral arteries, and spinal accessory nerves. The narrow section of the occipital bone in front of the foramen magnum is known as the basilar part, which connects to the body of the sphenoid bone as mentioned earlier. On the sides of the foramen magnum, the lateral parts of the occipital bone contain prominent occipital condyles that articulate with the atlas vertebra in the neck. The flattened posterior part of the occipital bone, known as the squamous part, displays bony prominences such as the external occipital protuberance and the nuchal lines. This region serves as attachment sites for important neck muscles.