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meatus the posterior border of the parietal bone turns obliquely upwards and backwards, and forms with the squamous part of the occipital bone the strongly denticulated sutura lambdoidea (lambdoid suture). Inferiorly this suture is continued obliquely downwards between the occipital bone and the posterior border of the mastoid portion of the temporal, where it forms the sutura occipitomastoidea

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Fig. 169.-NORMA LATERALIS OF THE SKULL. The occipital, sphenoid, and lacrimal bones and the mandible are coloured blue. The parietal, zygomatic, and nasal bones are coloured red. The temporal, frontal, ethmoid, and maxillary bones are left uncoloured. 1 Mental foramen. 12. Lambdoid suture.

22. Stephanion. 2. Body of the mandible. 13. Occipital bone.

23. Frontal bone. 3. Maxilla. 14. Lambda.

24. Pterion. 4. Ramus of mandible.

15. Obelion placed betweeu the two 25. Temporal fossa. 5. Zygomatic arch.

parietal foramina.

26. Great wing of sphenoid. 6. Styloid process. 16. Parietal bone.

27. Zygomatic bone. 7. External acoustic meatus. 17. Lower temporal line.

28. Zygomatico-facial foramen. 8. Mastoid process. 18. Upper temporal line.

29. Lacrimal bone. 9. Asterion.

19. Squamous part of temporal 30. Nasal bone. 10. Superior nuchal line of occipital bone.

31. Infra-orbital foramen. bone. 20. Bregma.

32. Piriform aperture and anterior 11. External occipital protuberance. 21. Coronal suture.

nasal spine. (occipito-mastoid suture), much simpler and less serrated than the two previously mentioned. These three sutures just described meet in triradiate fashion at a point called the asterion.

Anteriorly the curve of the squamous suture is continued downward between the anterior, edge of the squamous part of the temporal and the posterior border of the great wing of the sphenoid ; inferiorly it lies in plane with the middle of the zygomatie arch

The sutures around the summit of the great wing of the sphenoid are arranged like the letter H placed obliquely, the cross-piece of the H corresponding to the spheno-parietal suture. When this is short, and becomes a mere point of contact, the arrangement then resembles the letter X. This region is named the pterion.

Curving over the lateral region of the calvaria in a longitudinal direction is the temporal line. This is often double. The lower line marks the limit of the attachment of the temporal muscle, whilst the upper ridge defines the attachment of the temporal fascia. Commencing in front at the zygomatic process of the frontal, the line sweeps upwards and backwards across the inferior part of that bone, and then crossing the coronal suture—at a point called the stephanion -it passes on to the parietal, over which it curves in the direction of its mastoid angle. Here it is continued on to the temporal bone, where it sweeps forwards to form the supra-mastoid crest, which serves to separate the squamous from the mastoid portion of the temporal bone laterally. Carried forwards, this ridge is seen to become continuous with the upper border of the zygomatic arch above the external acoustic meatus. In front, the temporal ridge separates the temporal fossa from the region of the forehead; above and behind, it bounds the temporal fossa which lies within its concavity, and serves to separate that hollow from the surface of the calvaria which is overlain by the scalp. Above the level of the temporal lines the surfaces of the frontal and parietal bones are smooth, the latter exhibiting an elevation of varying prominence and position, but usually situated about the centre of the bone, called the tuber parietale (parietal tuberosity). A slight hollowing of the surface of the parietal behind and parallel to the coronal suture is not uncommon, and is referred to as the post-coronal depression.

As seen in profile, the part of the calvaria behind and below the lambdoid suture is formed by the squamous part of the occipital bone. In line with the zygomatic arch this outline is interrupted by the external occipital protuberance or inion. The projection of this point is variable; but its position can usually be easily determined in the living. Passing forwards from it, and blending anteriorly with the posterior border of the mastoid process of the temporal bone, is a rough crest, the linea nuchæ superior (superior nuchal or curved line), a little above which there is often a much fainter line, the linea nuchæ suprema (highest curved line); this affords attachment to the galea aponeurotica. These two lines serve to separate the part of the cranium above, which is covered by scalp, from that below, which serves for the attachment of the fleshy muscles of the back of the neck, the latter surface (planum nuchale) being rough and irregular as contrasted with the smooth superior part (planum occipitale). The fulness of these two parts of the occipital bone varies much. There is frequently a pronounced bulging of the planum occipitale, and the position of the lambda can often be easily determined in the living; similarly the planum nuchale may be either comparatively flat or else full and rounded. These differences are of course associated with corresponding differences in the development of the cerebral and cerebellar lobes, which are lodged in relation to the cerebral aspect of these parts of the bone. The further description of the planum nuchale is best deferred till the external aspect of the base of the skull is studied.

Fossa Temporalis.—Within the limits of the temporal lines the side of the cranium slopes forwards, medially, and downwards, thus leaving a considerable interval between its lower part and the zygomatic arch. This space or hollow is called the temporal fossa ; bounded above and behind by the temporal lines, its inferior limit is defined by the level of the zygomatic arch. Deepest opposite the angle formed by the frontal and temporal processes of the zygomatic bone, the fossa becomes shallow towards its circumference.

Its floor or medial wall, which is slightly concavo-convex from before backwards about mid-level, is formed above by the temporal surface of the frontal, behind by the sphenoidal angle of the parietal, as well as the lower portion of that bone, below the temporal line; below and in front by the temporal surface of the great wing of the sphenoid, and behind and below by the squamous portion of the temporal bone. Inferiorly, the floor is limited in front by the free inferior border of the great wing of the sphenoid, which forms the upper boundary of the pterygopalatine fossa ; behind that, by a rough ridge, the infra - temporal crest, which crosses the lateral surface of the great wing of the sphenoid, to become continuous posteriorly with a ridge on the lower surface of the squamous part of the temporal from which the anterior root of the zygoinatic process springs. Anteriorly the temporal fossa is separated from the orbit by the zygomatic process of the frontal above, and by the orbital process of the zygomatic and its junction with the lateral border of the great wing of the sphenoid between its orbital and temporal surfaces.

Laterally and in front, the fossa is overhung by the backward projection of the fronto-sphenoidal process of the zygomatic bone, and it is under cover of this, and within the angle formed by the frontal and orbital processes of the zygomatic bone, that we see the opening of the zygomatico-temporal foramen, which pierces the orbital plate of the zygomatic bone and transmits the zygomatico-temporal branch of the zygomatic nerve—a filament of the maxillary division of the trigeminal nerve. The anterior part of the inferior orbital fissure opens into the lower part of the temporal fossa, and thus establishes a communication between it and the orbit. If the floor of the fossa is carefully examined, some more or less distinct vascular grooves may be seen. One passing upwards over the posterior part of the squamous temporal, immediately in front of and above the external acoustic meatus, is for the middle 'temporal artery; two others, usually less distinct, pass up, one over the temporal surface of the great wing of the sphenoid, the other over the anterior part of the squamous part of the temporal; these are for the anterior and posterior deep temporal branches of the internal maxillary artery. Inferiorly the temporal fossa communicates with the infratemporal fossa, beneath the zygomatic arch, the two being separated by an imaginary horizontal plane passing medially at the level of that bony bridge. The fossa contains the temporal muscle with its vessels and nerves, together with the zygomatico-temporal branch of the zygomatic nerve and some fat; all of which are enclosed by the fascia which stretches over the space from the upper temporal line above to the superior border of the zygomatic arch below. The extent and depth of the fossa depends on the size of the temporal muscle, the development of which is correlated with the size and weight of the mandible.

Springing from the front and lower part of the squamous part of the temporal is the zygomatic process of that bone; it has two roots, an anterior and a posterior, between and below which are placed the mandibular fossa in front, and the opening of the external acoustic meatus behind. Of compressed triangular form, the process at first has its surfaces directed upwards and downwards, but curving laterally and forwards, it twists on itself, so that its narrowed surfaces are now turned laterally and medially, and its edges upwards and downwards ; passing forwards, it expands somewhat, and ends in an oblique serrated surface, which unites with the temporal process of the zygomatic bone completing the zygomatic arch. It is the superior edge of this bridge of bone which forms the posterior root. The inferior border, turning medially, forms the anterior root, and serves to separate the temporal from the infra-temporal surface of the squamous part of the temporal, blending in front with the infra-temporal crest on the lateral surface of the great wing of the sphenoid. The inferior surface of this root is convex from before backwards, and is thrown into relief by the mandibular fossa, which passes up behind it. In this way a downward projection, which is called the tuberculum articulare (O.T. eminentia articularis), is formed.

The spina angularis of the sphenoid (angular spine) lies immediately to the medial side of the articular part of the mandibular fossa. Its size and projection vary. It is well to remember its relation to the condyloid process of the mandible when that bone is in position ; lying, as it does, to the medial side and a little in front of that process, it affords attachment to the spheno-mandibular ligamnent. As will be seen hereafter, the anterior extremity of the osseous part of the auditory tube lies just to its medial side.

A noteworthy feature about the articular part of the mandibular fossa is the thinness of the bony plate which serves to separate it from the middle cranial fossa above. The vaginal process is a crest of bone which runs obliquely forwards from the front and medial side of the mastoid process, just below the external acoustic meatus, to the angular spine of the sphenoid. Passing downwards and slightly forwards from the centre of this, and ensheathed by it in front and at the sides, is the pointed styloid process, the length of which is extremely variable.

In the recess between the posterior root of the zygoma and the upper curved edge of the meatus there is usually a depression, though in some instances this may be replaced by a slight bulging of the bone. If from the posterior root of the zygoma a vertical line be let fall, tangential to the posterior edge of the meatus, a small triangular area is mapped off which has been named by Macewen the suprameatal triangle. Surgically this is of importance, as it is the spot selected in which to trephine the bone to reach the tympanic antrum.

In the suture between the posterior border of the mastoid part of the temporal and the squamous part of the occipital, there is usually a foramen (mastoid) for the transmission of an emissary vein from the transverse sinus within the cranium to the cutaneous occipital vein of the scalp; this opening, which may be double, varies greatly in size, and is usually placed on a level with the external acoustic meatus.

Fossa Infratemporalis.—The side of the cranium in front of the anterior root of the zygomatic process of the temporal bone is deeply hollowed, forming the infra-temporal fossa. The student must bear in mind that, in examining this space, the ramus and coronoid process of the mandible form its lateral wall; but this bone for the present being withdrawn, enables us to get a better view of the boundaries of the space

In front its anterior wall is formed by the convex posterior or infra-temporal surface of the maxilla, which rises behind the socket for the last molar tooth to form the tuber maxillare (maxillary tuberosity). Anteriorly, the infra-temporal surface of the maxilla is separated from its anterior aspect by the rounded inferior margin of the zygomatic process which supports the zygomatic bone. This latter curves laterally and backwards, forming part of the upper and anterior wall of the fossa. On the medial surface of this wall will be seen the suture uniting the zygomatic and maxillary bones (sutura zygomaticomaxillaris), which runs obliquely upwards and medially to reach the lateral extremity of the inferior orbital fissure, the inferior border of which forms the superior boundary of the infra-temporal surface of the maxilla. On this aspect of the bone are to be seen the openings of the foramina alveolaria, two or more in number, which transmit the nerves and vessels to the upper molar teeth. The medial wall of the infra-temporal fossa is formed by the lateral surface of the lateral pterygoid lamina, the width and shape of which varies greatly ; its posterior border is thin and sharp, and often furnished with spiny points, to one of which the pterygo-spinous ligament, which stretches from this border to the angular spine of the sphenoid, is attached. It occasionally happens that this ligament becomes ossified. Anteriorly the lateral pterygoid lamina is separated from the maxilla above by an interval called the pterygo-maxillary fissure. Below this the bones are apparently fused, but a careful inspection of the skull, together with an examination of the disarticulated bones, will enable the student to realise that, wedged in between the two bones at this point, is a part of one of the smaller bones of the face, the pyramidal process of the palate bone (O.T. tuberosity of palate bone).

The inferior border of the lateral pterygoid lamina is usually curved and slightly everted. Superiorly, where the lateral pterygoid lamina is generally narrower, it sweeps upwards to become continuous with the broad inferior surface of the great wing of the sphenoid ; this, which overhangs in part the infra-temporal fossa superiorly, is limited laterally by the infra-temporal crest, which separates its infra-temporal from its temporal surface. The infra-temporal surface of the great wing of the sphenoid is limited in front and below by the edge which forms the superior boundary of the inferior orbital fissure, whilst behind it reaches as far back as the medial extremity of the petro-tympanic fissure, where it terminates in the angular spine. It is from this point that the suture (sutura sphenosquamosa) curves forwards and upwards to reach the region of the pterion. The infra-temporal surface of the great wing of the sphenoid, and the lateral surface of the lateral pterygoid plate, alike afford extensive attachments for the external pterygoid muscle, whilst the former is pierced by minute canals for the transmission of emissary veins. Occasionally a larger vascular foramen is present (foramen Vesalii),

through which a vein runs from the cavernous sinus within the cranium to the pterygoid venous plexus situated in the infra- temporal fossa. Immediately behind the root of the external pterygoid plate there is a large oval hole, the foramen ovale, and behind that, and in line with the angular spine, is the smaller foramen spinosum. These two foramina cannot usually be seen in a side view of the skull, and are better studied when the base is examined; they are mentioned, however, because they transmit structures which here pass to and from the cranium, viz., the mandibular division of the trigeminal nerve, together with its motor root, and the accessory meningeal artery through the foramen ovale, and the middle meningeal artery and its companion vein through the foramen spinosum. A part of the squamous part of the temporal also forms a small portion of the roof of this fossa ; it consists of a triangular area immediately in front of the tuberculum articulare, and between it and the anterior root of the zygomatic process of the temporal, which is here curving medially and forwards, to become continuous with the infra-temporal crest. Medially this surface is continuous with the infra-temporal surface of the great wing of the sphenoid, separated from it, however, by the posterior part of the spheno-squamosal suture.

When the mandible is in position, the infra-temporal fossa is concealed by the ramus of the mandible, the medial surface of which, in its upper half, forms the lateral wall of that space. Viewed from the lateral side, the ramus of the mandible displays considerable differences in different skulls. These are mainly due to variations in its width and in the nature of the angle which it forms at its fusion with the body of the bone. A considerable interval separates the posterior border of the ramus from the front of the mastoid process. Within this space may be seen the free inferior edge of the tympanic plate (vaginal process), from which, just below the external acoustic meatus, the styloid process of the temporal bone is observed passing downwards and slightly forwards. The width and height of the coronoid process vary much, oftentimes reaching the level of the top of the condyle. Its extremity, when the lower jaw is closed, lies just within the anterior part of the zygomatic arch; at other times it rises to a much higher level, so that its point may be seen above the level of the upper border of the zygomatic arch. The posterior edge of the coronoid process forms the anterior border of the mandibular noteh, and limits in front the interval left between the lower border of the posterior half of the zygomatic arch and the upper hollowed edge of the ramus. On looking into this interval, the floor of the infra-temporal fossa may be seen, formed anteriorly by the lateral pterygoid lamina; whilst posteriorly it is possible to pass a probe right across the base of the skull from one mandibular notch to the other, the shaft of the probe lying immediately behind the pterygoid processes of the sphenoid, and crossing the foramina ovalia, through which the mandibular divisions of the trigeminal nerves pass.

The ramus and coronoid process are so placed as to occupy a position intermediate between the zygomatic arch laterally and the lateral pterygoid lamina medially; their medial surface, therefore, forms the lateral wall of the infra-temporal fossa. On a level with the surface of the crowns of the teeth of the mandible, and situated about the middle of this aspect of the ramus, is the mandibular foramen, the superior opening of the canalis mandibulæ (mandibular canal), which traverses the body of the bone. Through this foramen there pass the inferior alveolar branch of the mandibular division of the trigeminal nerve, together with the inferior alveolar artery and its veins. As will now be seen, when the mandible is in position, the infra-temporal fossa is closed in laterally, by the ramus of the mandible. In front there is an interval between the anterior border of the ramus and the infra-temporal surface of the maxilla, through which pass the buccinator branch of the trigeminal nerve and the communicating vein between the pterygoid plexus and the anterior facial vein. Above, in the interval between the mandibular notch and the inferior border of the zygomatic arch, there pass from the fossa the vessels and nerves which supply the masseter muscle. Between the posterior border of the ramus and the styloid process there enter and leave the large vessels which are found within the space. Superiorly under cover of the zygomatic arch, the infra-temporal fossa communicates with the

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