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The Skull from the Front (Norma Frontalis).
In front, the smooth convexity of the frontal bone limits this region above, whilst inferiorly, when the lower jaw is disarticulated, the teeth of the maxillæ form its lower boundary. The large openings of the orbits are seen on either side; whilst placed centrally, and at a somewhat lower level, is the apertura piriformis (anterior nasal aperture) leading into the nasal cavity.
The frontal region, convex from above downwards and from side to side, is limited laterally by two ridges, which are the anterior extremities of the temporal lines. Superiorly the fulness of the bone blends with the convexity of the vertex. Inferiorly the frontal bone forms on each side the arched superior border of the orbit (margo supraorbitalis). The space between these borders corresponds to the root of the nose, and here are seen the sutures which unite the frontal with the nasal bones medially, and with the frontal process of the maxilla on each side, called the naso-frontal and fronto-maxillary sutures, respectively. The supra-orbital margin is thin and sharp laterally, but becomes thick and more rounded towards its medial end, where it forms the medial angular process and unites with the frontal process of the maxilla and the lacrimal bone in the medial wall of the orbit. This arched border is interrupted towards the medial side by a notch (incisura supraorbitalis), sometimes converted into a foramen, for the transmission of the supra-orbital nerve and artery. In the median plane, just above the naso-frontal suture, there is often the remains of a median suture (sutura frontalis), which marks the fusion of the two halves from which the bone is primarily ossified. Here also a prominence, of variable extent-the glabella-is met with; from this there passes out on each side above and over the orbital margin a projection called the superciliary arch.
The orbital fossæ, of more or less conical form, display a tendency to assume the shape of four-sided pyramids by the flattening of the superior, inferior, and lateral walls. The base, which is directed forwards and a little laterally, corresponds to the orbital aperture. The shape of this is liable to individual and racial variations, being nearly circular in the Mongoloid type, whilst it displays a more or less quadrangular form in Australoid skulls. The superior margin, as has been already stated, is formed by the frontal bone between the zygomatic and medial angular processes. The lateral and about half the inferior margins are formed by the sharp curved edge between the facial and orbital surfaces of the zygomatic bone. The medial border and the remainder of the inferior margin are determined by the lateral surface of the frontal process of the maxilla, and the sharp edge separating the facial from the orbital surface of the same bone. Three sutures interrupt the continuity of the orbital margin-zygomatico-frontal laterally, the fronto-maxillary medially, both lying about the same level, and the zygomatico-maxillary inferiorly. The apex of the space is directed backwards and medially, so that the medial walls of the two orbits lie nearly parallel to each other, whilst the lateral walls are so disposed as to form almost a right angle with each other. The depth of the orbit measures, on an average, about two inches (5 cm.). At the apex there are two openings; the larger, known as the superior orbital fissure (O.T. sphenoidal), passes from the apex of the space laterally and a little upwards for the distance of three-quarters of an inch or so, between the roof and lateral wall of the orbit. The medial third of this fissure is broad and of circular form. Laterally it is considerably reduced in width. Through this the oculomotor, trochlear, ophthalmic division of the trigeminal, and the abducent nerves enter the orbit, whilst the ophthalmic veins pass backwards through it. Above and medial to the medial end of the sphenoidal fissure there is a smaller circular opening, the optic foramen, for the transmission of the optic nerve and ophthalmic artery.
The roof of the orbit, which is very thin and brittle towards its centre, is formed in front by the orbital part of the frontal bone, and behind by a small triangular piece of the small wing of the sphenoid, which surrounds the optic foramen and forms the upper border of the superior orbital fissure. Laterally
this surface is separated from the lateral wall by the superior orbital fissure
25. Squamous part of the temporal.
31. Mastoid process.
32. Incisor fossa.
33. Angle of jaw.
34. Mental foramen.
26. Left nasal bone.
27. Zygomatic bone.
28. Inferior orbital fissure.
FIG. 168.-THE FRONT OF THE SKULL.
The nasal bones, lamina papyracea of the ethmoid, vomer, inferior conchæ, zygomatic, and parietal bones are coloured red. The sphenoid, lacrimal, perpendicular part and middle concha of the ethmoid, and mandible are coloured blue. The maxille are coloured yellow. The frontal and temporal bones are left uncoloured.
29. Zygomatic arch.
30. Apertura piriformis, displaying nasal septum and inferior and middle conchæ.
posteriorly, anteriorly by an irregular suture between the orbital part of the
frontal and the upper margin of the orbital surface of the great wing of the sphenoid, lateral to which the zygomatic process of the frontal articulates with the zygomatic bone, often forming a ridge which limits the fossa for the lodgment of the lacrimal gland inferiorly (Whitnall). Medially the roof is marked off from the medial wall by a suture, more or less horizontal in direction, between the orbital plate of the frontal and the following bones, in order from before backwards, viz., the frontal process of the maxilla, the lacrimal bone, and the lamina papyracea of the ethmoid. In the suture between the last-mentioned bone and the frontal there are two foramina, the anterior and posterior ethmoidal foramina; both transmit ethmoidal vessels and the ethmoidal branches of the naso-ciliary nerve as well. The roof is concave from side to side, and to some extent also from before backwards. About midway between the fronto-maxillary suture and the supraorbital notch or foramen, but within the margin of the orbit, there is a small depression, occasionally associated with a spine (fovea vel spina trochlearis), for the attachment of the cartilaginous pulley of the superior oblique muscle of the eyeball. Under cover of the zygomatic process the roof is more deeply excavated, forming a shallow fossa for the lodgment of the lacrimal gland (fossa glandulæ lacrimalis). In front, the roof separates the orbit from the frontal sinus, and along its medial border it is in relation with the ethmoidal air-cells. The relation to these air spaces is variable, depending on the development and size of the sinuses. The rest of the roof, which is very thin, forms by its upper surface part of the floor of the anterior cranial fossa, in which are lodged the frontal lobes of the cerebrum.
The floor of the orbit is formed by the orbital surface of the maxilla, together with part of the orbital surface of the zygomatic bone, and a small triangular piece of bone, the orbital process of the palate, which is wedged in posteriorly. Laterally, for three-quarters of its length posteriorly, it is separated from the lateral wall, which is here formed by the great wing of the sphenoid, by a cleft called the inferior orbital fissure. Through this there pass the maxillary division of the trigeminal nerve on its way to the infra-orbital canal, the zygomatic branch of the maxillary nerve, the infra-orbital vessels, a branch connecting the inferior ophthalmic vein with the pterygoid plexus, and some twigs from the sphenopalatine ganglion. By means of this fissure the orbit communicates with the pterygo-palatine fossa behind, and the infra-temporal fossa to the lateral side, though in the recent condition the fissure is bridged over by the involuntary orbitalis muscle of Müller. Medially the floor is limited from behind forwards by the suture between the following bones, viz., the orbital process of the palate below with the body of the sphenoid above and behind, and the lamina papyracea of the ethmoid above and in front-anterior to which the orbital surface of the maxilla below articulates with the lamina papyracea of the ethmoid and the lacrimal above and in front. At the anterior extremity of this line of sutures the medial edge of the orbital plate of the maxilla is notched and free between the point where it articulates with the lacrimal posteriorly and the part from which its frontal process arises. Here it forms the lateral edge of a canal, down which the membranous naso-lacrimal duct passes to the nose. The floor of the orbit is thin behind and at the sides, but thicker in front, where it blends with the orbital margin. Passing in a sagittal direction through its substance is the infra-orbital cal, the roof of which is usually deficient behind, where it becomes continuous with a broad, shallow groove, which leads forwards from the anterior margin of the inferior orbital fissure. This canal (canalis infraorbitalis) opens on the anterior surface of the maxilla immediately below the orbital margin (foramen infraorbitale) and transmits the maxillary division of the trigeminal nerve, together with the infra-orbital vessels. The floor forms a thin partition which separates the orbit from the maxillary sinus, which lies beneath it. Medially it completes the lower ethmoidal air-cells, and separates the orbit from the middle meatus of the nasal cavity.
The lateral wall of the orbit, which is the strongest, is formed by the orbital surface of the great wing of the sphenoid and the superior part of the orbital surface of the zygomatic bone. Above it, behind, is the superior orbital fissure, whilst below,
and extending much farther forward, is the inferior orbital fissure. The posterior portion of this wall, formed by the great wing of the sphenoid, serves as a partition between the orbit and the anterior extremity of the middle cranial fossa, in which is lodged the pole of the temporal lobe of the cerebrum. In front of this, and behind the line of the spheno-zygomatic suture, this wall is strengthened on its outer aspect by its confluence with the cranial wall. Still more anteriorly, the lateral wall separates the orbit from the temporal fossa. The anterior margin of the lateral wall is stout and formed by the zygomatic bone, behind which, formed in part by the orbital process of the zygomatic bone and the zygomatic edge of the great wing of the sphenoid, it forms a fairly thick partition between the orbit in front and the temporal fossa behind. Crossing this surface from above downwards, close to the anterior extremity of the inferior orbital fissure, is the suture between the zygomatic bone and the great wing of the sphenoid (sutura sphenozygomatica). This wall is pierced in front by one or two small canals (foramina zygomatico-orbitalia), which traverse the zygomatic bone and allow the transmission of the zygomatico-temporal and zygomatico-facial branches of the zygomatic portion of the maxillary division of the trigeminal nerve. small tubercle, which can be more readily felt than seen, is situated just within the orbital margin near the middle of the anterior part of this wall, and indicates the site of attachment of the lateral palpebral raphe (Whitnall).
The medial wall of the orbit is formed from before backwards by a small part of the frontal process of the maxilla, by the lacrimal, and by the lamina papyracea of the ethmoid, posterior to which is a small part of the lateral aspect of the body of the sphenoid in front of the optic foramen. Above, the orbital part of the frontal bone forms a continuous suture from before backwards with the bones just enumerated; whilst below, the lacrimal and the lamina papyracea of the ethmoid articulate with the orbital plate of the maxilla; posteriorly the posterior extremity of the lamina papyracea and the anterior part of the body of the sphenoid articulate with the orbital process of the palate. The orbital surface of the lacrimal bone is divided into two by a vertical ridge-the lacrimal crest (crista lacrimalis posterior)- which forms in front the posterior half of a hollow, the fossa sacci lacrimalis, the anterior part of which is completed by the channelled posterior border of the frontal process of the maxilla. In the fossa is lodged the lacrimal sac, whilst passing from it and occupying the canal, of which the upper opening is at present seen, is the membranous naso-lacrimal duct. The lower part of the fossa separates the orbit from the anterior part of the middle meatus of the nasal cavity. To the medial side of the upper part of the fossa for the lacrimal sac lie the anterior ethmoidal cells, the passage leading from the nose to the frontal sinus (infundibulum ethmoidale), and the part of the bone behind the lacrimal crest forms the thin partition between the orbit and the ethmoidal cells. Behind, where the body of the sphenoid forms part of the medial wall of the orbit, the sphenoidal air sinus is in relation to the apex of that space, though here the partition wall between the two cavities is much thicker.
The skeleton of the face on its anterior surface is formed by the two maxilla, the frontal processes of which have been already seen to pass up to articulate with the medial angular processes of the frontal bone, thus forming the lower halves of the medial margins of the orbits. Joined to the maxillæ laterally are the zygomatic bones, which are supported by their union with the temporal bones posteriorly through the medium of the zygomatic arches. The suture which separates the zygomatic from the maxilla (sutura zygomaticomaxillaris) commences above about the centre of the inferior orbital margin and passes obliquely downward and laterally, its inferior end lying in vertical line with the lateral orbital margin. The two maxilla are separated by the nasal cavities, which here open anteriorly. Above, the two nasal bones are wedged in between the frontal processes of the maxilla; whilst below the apertura piriformis, the maxillae themselves are united in the middle line by the intermaxillary suture (sutura intermaxillaris).
The apertura piriformis (piriform aperture) (O.T. nasal aperture or anterior nares), which lies below and in part between the orbits, is of variable
shape and size-usually piriform, it tends to be long and narrow in Europeans, as contrasted with the shorter and wider form met with in the negroid races. Its edges are formed below and on either side by the free curved margin of the body and the frontal process of the maxilla; and above, and partly at the sides, by the free border of the nasal bones. In the median plane, inferiorly, corresponding to the upper end of the intermaxillary suture there is an outstanding process— the anterior nasal spine, formed by the coalescence of spicules from both maxillæ ; arising from this, and passing backwards and upwards, is a thin bony partition
-the osseous septum of the nose. Often deflected to one or other side, it divides the cavity of the nose (cavum nasi) into a right and a left half. Projecting into these chambers from their lateral walls can be seen the medial surfaces and free borders of the middle and inferior conchæ, the spaces below and between which form the inferior and middle meatuses of the nose, respectively.
Below the orbit, and to the lateral side of the piriform aperture, the anterior or facial surface of the body of the maxilla is seen; this is continuous inferiorly with the lateral surface of the alveolar process, in which are embedded the roots of the upper teeth.
A horizontal line drawn round the maxillæ on the level of a point midway between the lower border of the piriform aperture and the alveolar edge corresponds to the plane of the hard palate. Below that the alveolar process separates the cavity of the mouth from the front of the face; whilst above, the large air space, the maxillary sinus, lies within the body of the maxilla.
The zygomatic bone forms the lower half of the lateral and lateral half of the lower border of the orbit. Its lateral aspect corresponds to the point of greatest width of the face, the modelling of which depends on the flatness or projection of this bone.
When the mandible or lower jaw is in position, and the teeth in both jaws are complete, the lower dental arch will be seen to be smaller in all its diameters than the upper, so that when the jaws are closed the upper teeth slightly overlap the lower both in front and at the sides. Exceptionally, a departure from this arrangement is met with.
Lateral Aspect of the Skull (Norma Lateralis).
Viewing the lateral aspect of the skull, in the first instance without the mandible, it is seen to be formed in part by the bones of the cranium, and in part by the bones of the face. A line drawn from the fronto-nasal suture to the tip of the mastoid process serves to define roughly the boundary between these portions of the skull. Of ovoid shape, the cranium is formed above by the frontal, parietal, and occipital bones from before backwards; whilst below, included within these are the sphenoid and temporal bones. The sutures between these several bones are arranged as follows: Commencing at the zygomatic process of the frontal, the suture between that bone and the zygomatic bone is first seen; tracing this backwards and a little upwards, the lower edge of the frontal next articulates with the upper margin of the great wing of the sphenoid for a distance varying from three-quarters of an inch to one inch. Here the posterior border of the frontal turns upwards and slightly backwards, forming with the parietal the sutura coronalis (coronal suture). The lower border of the parietal bone, which is placed immediately behind the frontal, articulates anteriorly with the posterior part of the superior border of the great wing of the sphenoid. The extent of this suture (sutura sphenoparietalis) is liable to very great individual variation at times being broad, in other instances being pointed and narrow, whilst occasionally the parietal does not articulate with the sphenoid at all. Behind the spheno-parietal suture the parietal articulates with the squamous part of the temporal (sutura squamosa), the posterior extremity of which is about one inch behind the external acoustic meatus. Here the suture alters its character and direction, and in place of being scaly, becomes toothed and irregular, uniting, for the space of an inch or so, the mastoid angle of the parietal with the mastoid process of the temporal bone. This suture (sutura parietomastoidea) is more or less horizontal in direction, and lies in line and on a level with the superior border of the zygomatic arch. At a point about two inches behind the external acoustic