INTERIOR OF THE SKULL

The cranial cavity lies within the skull and contains the brain and its membranes and their blood-vessels. Its walls are formed by the frontal, parietal, sphenoid, temporal and occipital bones and, to a very small extent, by the ethmoid bone. They are lined by a fibrous membrane, termed the endocranium, which is the fibrous outer layer of the dura mater. It passes through the various foramina which lead to the exterior, and becomes continuous with the periosteum on the outer surfaces of the bones of the skull, often termed the pericranium. Both these fibrous membranes are continuous with the sutural ligaments, which occupy the narrow interosseous intervals at the sutures.

The walls of the cranial cavity vary in thickness in different skulls and in different parts of the same skull ; but they tend to be thinner in situations where they are well covered with muscles externally, e.g. the temporal and posterior cranial fossae.

Most of the cranial bones consist of an outer and an inner table, formed of compact substance and separated from each other by the diploe, which consist of spongy substance containing red bone marrow in its interstices. Many of the bones are so thin that the two tables are continuous, e.g. the vomer, pterygoid plates, etc. The inner table is thinner and more brittle than the outer table, which is often surprisingly resilient.

Although the skull tends to be thicker in primitive than in higher races no relationship exists between thickness of skull and mental capacity, and in all races the bones of the skull are thinner in women and children than in men.

The interior of the skull is described in two sections :
(1) the internal surface of the skull-cap ; and
(2) the internal surface of the base of the skull.

The skull-cap comprises most of the frontal and parietal bones and the uppermost portion of the squamous part of the occipital bone. It is marked, therefore, by the coronal, sagittal and lambdoid sutures, but they may or may not be visible, depending on the age of the subject, for the cranial sutures tend to become obliterated in old age and the process commences on the cerebral surface. The skullcap is deeply concave in all directions and presents numerous vascular furrows and other markings.

Figure 326
The internal surface of the skull cap inferior view - Figure 326
Anteriorly, in the median plane, the upper end of the frontal crest projects backwards. It gives attachment to the falx cerebri and is grooved by the commencement of the sagittal sulcus. This sulcus lodges the superior sagittal sinus, and widens progressively as it runs backwards in the median plane along the sagittal suture to the occipital bone. On each side of the sagittal sulcus the bone presents a number of irregular depressions, termed granular pits. They are more numerous and more obvious in aged skulls and are formed by the arachnoid granulations, which are tuft-like protrusions of the arachnoid mater.

The anterior branch of the middle meningeal artery grooves the bone deeply about r cm. behind the coronal suture, and its line corresponds more or less accurately to the precentral sulcus of the cerebrum. Its rami and those of the posterior branch of the same artery course upwards and backwards from the cut edge of the skull-cap, grooving the inner surface of the parietal bone. Smaller grooves produced by meningeal vessels may be present on the inner surfaces of the frontal and occipital bones. When present, the parietal foramina open on this surface near the sagittal groove about 3.5 cm in front of the lambdoid suture. Each transmits an emissary vein from the superior sagittal sinus.

The impressions for the cerebral gyri are less distinct on the skull-cap than on the base of the skull and are seen best near the cut edge.

 


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