The bones of the cranium

The cranium is a box-like cavity that contains and protects the brain. It has a dome-shaped roof called the calvaria, or skull cap, and its floor is known as the base of the skull. The cranium consists of 15 bones:

The frontal bone is a large flat bone comprising the forehead and most of the roof of the orbit. There are rounded prominences, called the frontal tuberosities, one on each side of the midline, which vary in size between individuals and which together form the forehead proper. The bone contains two irregular cavities called the frontal sinuses, which lie one over each orbit and which open into the nasal cavity. The sinuses contain air and are lined with mucous membrane, which is continuous with the mucous membrane lining the respiratory tract. They add resonance to the voice and they serve to lighten the skull, but the mucous membrane may become infected, causing a condition known as sinusitis.

The parietal bones form the sides and roof of the cranium; they articulate with the frontal bone, the occipital bone and with each other to form the sutures or joints of the cranium (see Chapter 11). On the internal surfaces are small grooves to carry the blood vessels supplying the brain, and the impression of the folds or convolutions of the surface of the brain can be seen. At birth there are membranous gaps in the skull at the angles of the parietal bone which are called fontanelles (see Chapter 11).

The occipital bone (Fig. 9.3) forms the back of the skull. It carries a marked prominence, the external occipital protuberance, which provides attachment for muscles. Below this there is a large oval opening, the foramen magnum, through which the cranial cavity communicates with the vertebral canal. On either side of the foramen are two smooth oval processes, the occipital condyles, for articulation with the first cervical vertebra. This joint allows the nodding movement of the head.

Fig. 9.3 The occipital bone (from below).

The temporal bones (Fig. 9.4) are situated at the sides and base of the skull. Each consists of four parts:

Fig. 9.4 The temporal bone.

The ethmoid bone (Fig. 9.5) is very light and irregular in shape and consists of three parts:

Fig. 9.5 Section of the ethmoid bone (shown in colour).

The sphenoid bone (Fig. 9.6) is situated at the base of the skull, in front of the temporal bones. It is shaped rather like a bat with outstretched wings. The body contains two large air sinuses, which communicate with the nasal cavity, and a deep depression, the hypophyseal fossa, which contains the hypophysis cerebri or pituitary gland. The greater and lesser wings are perforated by many openings for the passage of nerves and blood vessels.

Fig. 9.6 The sphenoid bone.