The interior of the skull

In order to study the interior of the skull it is necessary to make sections in three directions, sagittal, coronal and horizontal.

This enables the student to examine the various points with facihty and displays the great proportion the brain cavity bears to the rest of the skull. The sagittal section should be made slightly to one side of the median line in order to preserve the nasal septum. The black line (fig. 94) drawn from the basion (anterior margin of the foramen magnum) to the gonion (the anterior extremity of the sphenoid) represents the basi-cranial axis; whilst the line drawn from the gonion to the subnasal point lies in the basi-facial axis. These two axes form an angle termed the cranio-facial, which is useful in making comparative measurements of crania. A line prolonged vertically upwards from the basion will strike the bregma. This is the basi-bregmatic axis and gives the greatest height of the cranial cavity. A line drawn from the ophryon to the occipital point indicates the greatest length of the cranium.

Near its middle, the cranial cavity is encroached upon by the petrosal; the walls are channelled vertically by narrow grooves for the middle and small meningeal arteries and towards the base broader furrows are found for the venous sinuses.

The coronal section is most instructive when made in the basi-bregmatic axis. The section will pass through the petrosal in such a way as to traverse the two external auditory passages and expose the tympanum and vestibule and will also partially traverse the internal auditory meatuses. Such a section wdll divide the ' parietal bones slightly posterior to the parietal eminences and a line drawn transversely across the section at the mid-point will give the greatest transverse measurement of the cranial cavity. A skull divided in this way facilitates the examination of the parts about the posterior nares.

The horizontal section of the skull should be made through a line extending j from the ophryon to the occipital point, passing laterally a few millimetres above I the pterion on each side. It is of great advantage to study the various parts on the floor of the cranial cavity in a second skull having the dura mater and its various processes in situ.

The Skull in Sagittal Section.

The floor of the cranial cavity presents three irregular depressions termed the anterior, middle and posterior fossae.

The anterior fossa: The floor of this fossa is on a higher level than the rest of the cranial floor. It is formed by the horizontal plate of the frontal bone, the cribriform plate of the ethmoid and the lesser wings of the sphenoid, which meet each other and exclude the pre-sphenoid from the anterior fossa. The free margins of the lesser wings and the optic groove mark the limits of this fossa posteriorly. The central portion of the fossa is depressed on each side of the crista galli, the depressions forming a part of the roofs of the nasal sinuses; laterally, the floor of this fossa is convex where it corresponds to the roof of the orbits and is marked by irregular furrows. It supports the frontal lobes of the cerebrum. The sutures traversing the floor of the fossa are the fronto-ethmoidal, forming three sides of a rectangle, that portion of the transverse facial suture which traverses the roof of the orbit and the ethmoido-sphenoidal suture, the centre of which corresponds to the gonion.

The skull in horizontal section

The points of interest in the fossa are:

A groove for the superior longitudinal sinus.
The foramen caecum which transmits a small vein.
The crista galli.
The ethmoidal fissure for the nasal branch of the fifth nerve.
The cranial orifice of the anterior ethmoidal canal, transmitting the nasal branch of the fifth and a meningeal branch of the anterior ethmoidal artery.
Ethmoidal foramina for the olfactory filaments.
Cranial orifice of the posterior ethmoidal canal, transmitting a meningeal branch of the posterior ethmoidal artery.
The ethmoidal spine of the sphenoid. Furrows for meningeal arteries.
The middle cranial fossa presents a central isthmus and two lateral depressed portions. It is limited anteriorly by the posterior border of the lesser wings of the sphenoid and the anterior margin of the optic groove. The posterior limits are the | dorsum ephippii and the superior border of the petrosals. Laterally it is bounded j by the squamosals and the parietal bones. The floor is formed by the body i and greater wings of the sphenoid and the anterior surface of the petrosal. It | contains the following sutures: spheno-parietal, petro-sphenoid, squamo-sphenoidal, squamous and a portion of the transverse suture. The central portion or isthmus of the middle fossa possesses the following points from before backwards: -
The optic groove, which lodges the optic chiasma.
The optic foramina, transmitting the optic nerve and ophthalmic artery. The olivary process, indicating the line of ankylosis between pre- and post- sphenoid.
The anterior clinoid processes.
The pituitary fossa, with the middle clinoid processes and grooves for the internal carotid arteries. The dorsum ephippii, with the posterior clinoid processes and notches for the third pair of cranial nerves.
This central depression is in direct relation with the parts of the brain surrounding the circle of Willis.
The lateral depressions receive the temporo-sphenoidal lobes of the brain and are marked by numerous furrows roughly corresponding to the convolutions of the cerebrum. Numerous narrow diverging channels pass upwards from the fossa towards the vertex; these lodge the ramifications of the middle and small meningeal arteries.
The following openings occur on each side of this fossa: -
The sphenoidal fissure, leading into the orbit and transmitting the third, fourth, ophthalmic division of the fifth, the sixth nerve and ophthalmic vein.
In the greater wing of the sphenoid near its uilion with the frontal bone there are small openings allowing twigs of the middle meningeal artery to enter the orbit.
The foramen rotundum, which conducts the second division of the fifth nerve into the spheno-maxillary fossa.
The foramen ovale: this transmits the third division of the fifth with its motor root, the small meningeal artery and the small superficial petrosal nerve.
The foramen Vesalii (not always present) for a small vein.
The foramen spinosum, for the middle meningeal artery and its vense comites.
The sphenotic foramen (middle lacerated foramen), which transmits at its inner angle the internal carotid artery, with the carotid plexus of nerves.

On the posterior wall of this fossa the objects of interest are: -

A depression which lodges the Gasserian ganglion.
The hiatus Fallopii, for the great superficial petrosal nerve and a twig from the middle meningeal artery.
A foramen for the small superficial petrosal nerve. An eminence formed by the superior semicircular canal.
Anterior and slightly external to the ridge formed by the superior semicircular canal, the bone is exceedingly thin and translucent. This is the roof of the tympanum (tegmen tympani). When the dura mater is in situ, the depression lodging the Gasserian ganglion is converted into a foramen, traversed by the fifth nerve. The notch in the side of the dorsum ephippii for the third nerve is also a foramen when the dura mater is present. In many skulls the middle clinoid process is prolonged to meet the anterior clinoid process and thus forms a foramen for the internal carotid artery. The grooves for the middle meningeal arteries are sometimes canals or tunnels in a part of their course, especially in old skulls. The grooves radiate from the foramen spinosum and extend to the vault. The bones most deeply marked are the squamosal, the greater wing of the sphenoid and the parietal.
The posterior cranial fossa is the deepest portion of the cavity. It is bounded by the dorsum ephippii and the superior borders of the petrosals, the mastoid portion of the petrosals, the posterior inferior angle of the parietals and the squamo-occipital below the level of the crest (supra-occipital). The upper limits are indicated by the grooves for the lateral sinuses. It is marked by the following sutures: the petro-occipital, occipito-mastoid, parieto-mastoid, and, in young skulls, the basilar suture.
The ridges limiting this fossa give attachment to the tentorium cerebelli and the fossa lodges the cerebellum with the pons and medulla. It communicates with the general cranial cavity by means of the foramen of Pacchionius when the tentorium is in situ.

It has the following objects of interest:

The clivus, extending from the dorsum ephippii to the anterior margin of th'e foramen magnum. This is in relation with the basilar artery, the pons and medulla.
The notch for the sixth nerve on each side of the dorsum ephippii. This is sometimes a foramen, termed the petro-sphenoid.
The foramen magnum, presenting on each side a tubercle for the check ligaments; and the anterior condyloid foramen (sometimes subdivided by a spiculum of bone) for the hypoglossal nerve. Behind the foramen there is a vertical ridge of bone, the internal occipital crest, for the falx cerebelli. This sometimes presents a depression known as the vermiform fossa.

The anterior boundaries of the fossa present:

A notch for the passage of the fifth nerve. This is a foramen when the tentorium is present.
The internal auditory meatus, for the facial and auditory nerves and the auditory branch of the basilar artery.
The jugular foramen, which transmits the glosso-pharyngeal, vagus and spinal accessory nerves, the internal jugular vein and the meningeal branch of the ascending pharyngeal artery.
The termination of the groove for the lateral sinus, with the internal orifice of the mastoid foramen.

The cranium of an average European has a capacity of 1450 cc. The circumference, taken in a plane passing through the ophryon anteriorly, the occipital point posteriorly and the pterion laterally, is 52 cm. The length from the ophryon to the occipital point is 17 cm.; the width between the parietals at the level of the zygomata is 12,5 cm.; and the height from the basion to the bregma is nearly the same. The cranio-facial angle is about 96°.