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be seen by holding the skull up to the light; it is convex from side to side, and bears the impress of the gyri of the inferior surface of the frontal lobes of the cerebrum, which rest upon it. In front and at the side there are a number of vascular grooves for the branches of the anterior and middle meningeal arteries.
The middle fossa, which in form may be compared to the wings of a bird united by the body, is bounded in front by the curved thin posterior edge of the small wings of the sphenoid; posteriorly, by the line of attachment of the tentorium cerebelli, extending from the posterior clinoid process along the superior margin of the petrous portion of the temporal bone. The median part of the fossa, which is narrow, corresponds to the fossa hypophyseos and the tuberculum sellæ of the sphenoid. It is limited anteriorly by a line connecting the anterior margins of the two optic foramina, and is overhung behind by the dorsum sellæ. In this area are lodged the structures which lie within the interpeduncular fossa on the base of the brain. The floor of the lateral parts of the fossa on each side is formed by the great wing of the sphenoid in front, the squamous part of the temporal bone to the lateral side, and the anterior surface of the petrous part of the temporal behind. In the hollows so formed the temporal lobes of the cerebrum are lodged. On either side of the tuberculum sellæ are seen the optic foramina; these pass into the orbital cavities and transmit the optic nerves and ophthalmic arteries. Immediately behind these openings the anterior and middle clinoid processes are sometimes united, so as to enclose a foramen. Through this the internal carotid artery passes upwards. Leading backwards from this, along the side of the body of the sphenoid, is the carotid groove, which turns downwards near the apex of the petrous part of the temporal, to become continuous with the carotid canal, which here opens on the posterior wall of an irregular aperture, placed between the side of the body of the sphenoid and the summit of the petrous part of the temporal, called the foramen lacerum. Through the medial angle of this opening the carotid artery accompanied by its plexus of veins and sympathetic nerves passes upwards. Running through the fibrous tissue, which in life blocks up this opening, the greater superficial petrosal nerve, coming from the hiatus facialis, passes downwards and forwards to reach the posterior orifice of the canalis pterygoideus, which is placed on the anterior and inferior border of the foramen lacerum. A small meningeal branch of the ascending pharyngeal artery also passes upwards through this foramen. In front and to the lateral side of the foramen lacerum, and separated from it by a narrow bar of bone, is the foramen ovale; through this pass both roots of the mandibular nerve, the accessory meningeal artery, and some emissary veins. Somewhat lateral and posterior to this is the foramen spinosum for the transmission of the middle meningeal vessels, together with a recurrent branch (nervus spinosus) from the mandibular nerve. Leading from the lateral extremity of the foramen lacerum there is a groove which passes laterally, backwards, and slightly upwards on the superior surface of the petrous part of the temporal to end in the hiatus facialis (a cleft opening into the canalis facialis), which gives passage to the greater superficial petrosal branch derived from the ganglion geniculi on the facial nerve, together with the small petrosal branch of the middle meningeal artery. Just lateral to the hiatus facialis there is another small foramen for the transmission of the lesser superficial petrosal nerve. Overhung by the posterior border of the lesser wing of the sphenoid is the superior orbital fissure, the cleft which separates the small from the great wings of the sphenoid, and which opens anteriorly into the hollow of the orbit; through this pass the oculomotor, trochlear, ophthalmic division of the trigeminal, and abducent nerves, together with the ophthalmic veins as well as the sympathetic filament to the ciliary ganglion and the small orbital branch of the middle meningeal artery. Just below its medial extremity is the foramen rotundum for the passage of the maxillary nerve to the pterygo-palatine fossa, Behind this, and between it and the foramen ovale, the foramen Vesalii may occasionally be seen, through which a vein passes to reach the pterygoid plexus.
The lateral parts of the middle fossa are moulded in conformity with the gyri of the temporal lobes, but towards its medial part the splitting of the dura mater in the region of the cavernous sinus serves to separate the cranial base from the inferior surface of the cerebrum. As may be seen by transmitted light,
FIG. 172.-BASE OF THE SKULL SEEN FROM ABOVE.
The frontal and occipital bones are coloured red; the ethmoid and temporal bones, blue; the parietal, orange; and the sphenoid is left uncoloured.
1. Frontal bone.
24. Ridge for the attachment of the falx cerebelli. 25. Fossa for the lodgment of the left cerebellar hemisphere.
26. Foramen occipitale magnum.
27. Groove for the sigmoid sinus turning into the jugular foramen.
28. Groove for the inferior petrosal sinus running along the line of suture between the petrous part of the temporal and the basi-occipital.
29. Depression for the semilunar ganglion.
30. Middle cranial fossa for lodgment of the temporal lobes of the brain.
31. Foramen lacerum.
32. Carotid groove.
33. Dorsum sellæ of sphenoid.
34. Leads into foramen rotundum.
35. Fossa hypophyseos.
36. Tuberculum sellæ of the sphenoid.
37. Anterior cranial fossa for lodgment of frontal lobes of the brain.
38. Cribriform plate of ethmoid.
39. Crista galli of ethmoid.
40. Foramen cæcum.
41. Crest for attachment of falx cerebri,
the floor of the lateral parts of the fossa is thin as it overlies the temporal, infratemporal, and mandibular fossæ. The grooves for the lodgment of the branches of the middle meningeal artery leading from the foramen spinosum are readily seen; one, coursing backwards a little below the line of the squamoso-parietal suture, is specially well marked. Amongst other features may be noticed the depression for the lodgment of the semilunar ganglion overlying the apex of the petrous part of the temporal; behind and to the lateral side of the hiatus facialis, the arcuate eminence, indicating the position of the superior semicircular canal; and immediately anterior and slightly to the lateral side of this the tegmen tympani, which roofs in the cavity of the tympanum, the thinness of which can readily be demonstrated if light be allowed to fall through the external acoustic meatus.
The posterior fossa is larger and deeper than the others. In front it is limited by a line on either side leading backwards and laterally from each posterior clinoid process along the superior border of the petrous part of the temporal bone, where laterally and posteriorly it becomes confluent with the superior lip of the transverse groove for the transverse sinus, ending posteriorly in the middle line at the internal occipital protuberance. Along the line thus indicated the process of dura mater called the tentorium cerebelli, which roofs in the posterior fossa, is attached. The floor of the fossa, in which the cerebellar hemispheres, the pons, and medulla oblongata are lodged, is formed by the petrous and mastoid portions of the temporal bone, with part of the body of the sphenoid and the basilar portion of the occipital bone wedged in between them. Above the mastoid part of the temporal a small part of the mastoid angle of the parietal enters into the constitution of the side wall of the fossa. Behind and within these the lateral parts and inferior portions of the squamous part of the occipital complete the floor. In the median plane the floor of the fossa is pierced by the foramen magnum, in which lies the lower part of the medulla oblongata, together with its membranes, and through which pass upwards the vertebral arteries and the accessory nerves. On either side of the foramen magnum, and a little in front of a transverse line passing through its centre, is the opening of the canalis hypoglossi for the passage of the hypoglossal nerve, a small meningeal branch from the ascending pharyngeal artery and an emissary vein. Overhanging the opening of the canalis hypoglossi there is a thickened rounded bridge of bone, to the lateral side of which is placed the irregular opening of the jugular foramen. The size of this is apt to vary on the two sides, and the lumen is frequently subdivided by a spicule of bone which runs across it; the posterior and lateral rounded part of the foramen is occupied by the transverse sinus, which here joins the internal jugular vein. A meningeal branch from the ascending pharyngeal or occipital artery also enters the skull through this compartment. The anterior and medial part of the foramen is confluent with the groove for the inferior petrosal sinus, which turns downwards in front of the spicule above referred to. The interval between the portions of the foramen occupied by the two veins allows the transmission of the glosso-pharyngeal, vagus, and accessory nerves in this order from before backwards. About a quarter of an inch above and to the lateral side of the anterior part of the foramen jugulare the posterior surface of the petrous portion of the temporal bone is pierced by the internal acoustic meatus, through which the facial and acoustic nerves, together with the nervus intermedius, and the auditory branch of the basilar artery, leave the cranial cavity. Behind the jugular foramen and close to the margin of the foramen magnum the opening of the canalis condyloideus, when present, may be seen. This gives passage to a vein which joins the vertebral vein inferiorly. The inner aperture of the mastoid foramen is noticed opening into the groove for the transverse sinus, a little below the level of the superior border of the petrous part of the temporal. Through it passes an emissary vein which joins the occipital vein laterally; the mastoid branch of the occipital artery also enters the cranial cavity through this foramen.
The posterior fossa is divided into two halves posteriorly by the internal occipital crest, to which the falx cerebelli is attached, the floors of the hollows on either side of which are often exceedingly thin and are for the lodgment of the hemispheres of the cerebellum. The grooves for the following blood sinuses are usually distinctthe superior petrosal running along the superior border of the petrous part of the
temporal, the inferior petrosal lying along the line of suture between the petrous part of the temporal and the basilar part of the occipital bone; the occipital sinus grooving the internal occipital crest; and the transverse sinus curving forwards and laterally from the internal occipital protuberance, across the cerebral surface of the squamous part of the occipital, to reach the mastoid angle of the parietal bone, in front of which it turns downwards and medially to reach the jugular foramen, describing a sigmoid curve, and grooving deeply the inner surface of the mastoid and posterior aspect of the petrous portions of the temporal bone. Before it terminates at the jugular foramen it again reaches the occipital bone and channels the upper surface of the jugular process of that bone. Slight grooves for meningeal arteries are also seen-some pass upwards, whilst others turn downwards and are occupied by branches from the posterior offsets of the middle meningeal arteries.
Median Sagittal Section of the Skull.
Such a section should be made a little to one or other side of the median plane, so as to pass through the nasal cavity lateral to the septum; one-half will then display the nasal septum in position, whilst in the other the lateral wall of the nasal cavity of that side will be exposed.
The form of the cranial cavity is, of course, subject to many variations dependent on individual and racial peculiarities. The following details are, however, worthy of note. The posterior border of the foramen magnum (opisthion), and consequently the floor of the posterior cranial fossa, occupies the same horizontal plane as the hard palate. The anterior border of the foramen magnum (basion) lies a little higher, so that the plane of the foramen is, in the higher races at least, oblique, and is directed downwards and slightly forwards. From the basion a line passing upwards and forwards to reach the suture between the sphenoid and ethmoid passes through the basi-cranial axis formed by the basi-occipital, the basi-sphenoid, and the presphenoid. The basi-cranial axis is wedge-shaped on section posteriorly, whilst anteriorly it is of considerable width, and has within it the large sphenoidal air sinus. Its upper surface leads upwards and forwards with a varying degree of obliquity from the basion to the overhanging edge of the dorsum sellæ, in front of which the sella turcica, the floor of which is quite thin, is well seen in the section. From the tuberculum sellæ the floor of the anterior fossa follows a more or less horizontal direction, corresponding pretty closely to the level of the axis of the orbital cavity. The roof of the orbit is seen to bulge upwards to a considerable extent into the floor of the anterior fossa; whilst the floor of the middle fossa sinks to a level corresponding to that of the under surface of the basi-cranial axis, where it forms the roof of the choana. The maximum length of the skull is measured from the glabella (a point between the superciliary arches) to the occipital point posteriorly. It is noteworthy that the maximum occipital point does not necessarily correspond to the external occipital protuberance (inion). The greatest vertical height usually corresponds to the distance from the basion to the bregma (point of union of the sagittal with the coronal suture), though to this rule there are many exceptions. On looking into the posterior fossa the hypoglossal canals and jugular foramina and the internal acoustic meatus are seen in line, sloping from below upwards. The internal acoustic meatus lies in a vertical plane, passing through the basion. The grooves for the middle meningeal artery and its branches are very obvious. The anterior groove curves forwards and laterally, and reaching the cerebral surface of the pterion, passes towards the vertex at a variable distance behind and more or less parallel to the coronal suture. From this grooves pass forwards across the suture to reach the frontal bone. Another groove curves upwards and backwards a little below the line of the parieto-squamosal suture. From this an
upwardly directed branch radiates on the cerebral surface of the parietal bone, in the region of the parietal tuberosity, whilst a lower branch passes backwards some little distance above the lambdoid suture, and gives offsets which curve downwards and medially over the cerebral surface of the squama occipitalis of the occipital bone.
Cavum Nasi.In the section through the nasal cavity the structures which form its lateral wall can now be studied. These are the nasal bone; the frontal process of the maxilla; the lacrimal bone; the labyrinth of the ethmoid, comprising the superior and middle conchal bones; the perpendicular part of the palate bone; the inferior conchal bone; and the medial surface of the medial pterygoid lamina.
The roof as seen in the section is formed by the nasal and frontal bones, the cribriform plate of the ethmoid, the body of the sphenoid and the sphenoidal conchæ, the sphenoidal process of the palate and the ala of the vomer. The floor, which is
FIG. 173.-MEDIAL ASPECT OF THE LEFT HALF OF THE SKULL SAGITTALLY DIVIDED. The frontal, maxillary, and sphenoid bones are coloured red; the parietal, nasal and palate bones, blue; the basilar part of occipital, yellow, and squama occipitalis, purple. The ethmoid and inferior concha, together with the left ala of the vomer, are left uncoloured.
nearly horizontal from before backwards, is formed by the palatine processes of the maxillæ and palate bones. On sagittal section the nasal cavity appears somewhat triangular in shape with the angles cut off; the base corresponds to the floor; the apertura piriformis and choana to the truncated anterior and posterior