INTERNAL SURFACE OF THE BASE OF THE SKULL. [BASIS CRANII INTERNA] (figs. 327. 328)

The internal surface of the base of the skull shows a natural subdivision into anterior, middle and posterior cranial fossa:. It is very irregular owing, partly, to the impressions for the cerebral gyri, which are especially conspicuous in the anterior and middle fossa:, where they reflect accurately the pattern of the surface of the corresponding parts of the cerebrum. The dura mater is firmly adherent to the whole area, and through the numerous foramina and fissures the endocranium is continuous with the periosteum (pericranium) on the exterior of the skull.

ANTERIOR CRANIAL FOSSA (figs. 327, 328)

The anterior cranial fossa is limited in front and on each side by the frontal bone. Its floor is formed by the orbital plates of the frontal bone, the cribriform plate of the ethmoid and the lesser wings and anterior part of the body of the sphenoid.

The cribriform plate of the ethmoid, which occupies the median plane, lies between the two orbital plates of the frontal bone, and is depressed below the level of the rest of the floor. It separates the fossa from the nasal cavity, the roof of which it helps to form (fig. 333). Anteriorly it presents a median crest-like elevation, termed the crista galli, which projects upwards between the two cerebral hemispheres. A depression intervenes between the front of the crista galli and the crest of the frontal bone, the floor of which is crossed by the fronto-ethmoidal suture and is marked by the presence of the foramen caecum. On each side the crista galli is separated from the orbital plate of the frontal bone by a narrow interval. The numerous small foramina which perforate the cribriform plate transmit the minute olfactory nerves from the nasal mucosa to the olfactory bulb. Posteriorly the cribriform plate articulates with the anterior part of the body of the sphenoid at the spheno-ethmoidal suture.

The orbital plate of the frontal bone forms the greater part of the floor of the fossa on each side of the median plane and separates the orbit and its contents from the inferior surface of the frontal lobe of the brain. Its surface is convex upwards and is marked by impressions for the cerebral gyri and by one or two small grooves for meningeal vessels. In its anteromedial part it is split into two laminar to contain an air–space, termed the frontal sinus. The medial part of the orbital plate covers the upper aspect of the ethmoidal labyrinth and shuts it out from the floor of the anterior cranial fossa. Posteriorly it articulates with the anterior border of the lesser wing of the sphenoid bone. In the median plane the cerebral surface of the frontal bone is marked by the frontal crest, which projects backwards into the interval between the two cerebral hemispheres and extends upwards on to the interior of the skull-cap.

Behind the cribriform plate the floor of the anterior cranial fossa is formed by the anterior portion of the upper surface of the body of the sphenoid. This part of the bone is termed the jugum sphenoidale, and it separates the fossa from an air space contained in the body of the sphenoid and named the sphenoidal sinus (fig. 331). Anteriorly the jugum articulates with the posterior margin of the cribriform plate ; posteriorly it is limited by the anterior border of a groove, termed the optic groove (sulcus chiasmatis), which crosses the body of the sphenoid in the forepart of the middle cranial fossa and leads from one optic foramen to the other. Lateral to the jugum the floor of the fossa is formed by the lesser wing of the sphenoid, which articulates in front with the orbital plate of the frontal bone. The posterior margin of the lesser wing, which curves medially and backwards, is free and overhangs the anterior extremity of the middle cranial fossa. Laterally the lesser wing tapers to a point and meets the suture between the frontal bone and the greater wing at or near the lateral end of the superior orbital fissure. The medial extremity of its posterior border forms a protection, termed the anterior clinoid process. Medially the lesser wing is connected to the body of the sphenoid by two roots, separated from each other by the optic foramen. The anterior root, broad and flat, is continuous with the jugum sphenoidale; the posterior root, somewhat smaller and thicker, lies in the middle cranial fossa and is connected to the body of the sphenoid opposite the posterior border of the optic groove.

Figure 327
The internal surface of the left half of the base of the skull - Figure 327
Particular features.—The crista galli; and the frontal crest give attachment to the falx cerebri. The foramen cecum between them usually ends blindly, but on rare occasions it is patent and transmits a vein from the nasal mucosa to the superior sagittal sinus. The narrow groove on the lateral side of the crista galli is related to the gyros rectos, and the olfactory bulb lies on the medial edge of the orbital plate of the frontal bone. The anterior ethmoidal canal opens on the line of the suture between the orbital plate of the frontal hone and the cribriform plate of the ethmoid (fig. 317). It is placed behind the crista galli and is difficult to identify, for it is directed medially and is overlapped above by the medial edge of the orbital plate. It transmits the anterior ethmoidal nerve and vessels, which run forward under the dura mater and gain the nasal cavity by passing downwards through a slit-like foramen at the side of the crista galli. The posterior ethmoidal canal opens at the posterolateral corner of the cribriform plate and is overhung by the anterior border of the sphenoid. It transmits the posterior ethmoidal vessels, but the posterior ethmoidal nerve terminates by supplying the mucous lining of the sphenoidal and posterior ethmoidal sinuses.

The free, posterior border of the lesser wing of the sphenoid fits into the stem of the lateral cerebral sulcus and inay be grooved by the sphenoparietal sinus. Above, the lesser wing is related to the posterior part of the inferior surface of the frontal lobe and medially to the anterior perforated substance. Inferiorly it forms the upper boundary of the superior orbital fissure and helps to complete the roof of the orbit. The anterior clinoid process gives attachment to the free border of the tentorium cerebelli and is grooved on its medial aspect by the internal carotid artery as it pierces the roof of the cavernous sinus. Not infrequently the anterior clinoid process is connected to the middle clinoid process by a thin bar of bone, which completes a foramen around the internal carotid artery. The flat surface of the jugum sphenoidale supports the posterior ends of the gyri recti and the olfactory tracts.

 


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