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temporal fossa, whilst inferiorly it is continuous with the infra-maxillary region. Medially, on the floor of the fossa there is an r-shaped fissure, the horizontal limb of which corresponds to the inferior orbital fissure, forming a channel of communication between the fossa and the orbit, through which passes the zygomatic branch of the maxillary division of the trigeminal nerve; whilst the vertical cleft is the pterygo-maxillary fissure, which leads into a small fossa placed between the front. of the root of the pterygoid process of the sphenoid and the back of the maxilla, called the pterygo-palatine fossa.
The following foramina open into the infra-temporal fossa-the foramen ovale, foramen spinosum, foramina alveolaria, mandibular foramen, minute foramina for the transmission of emissary veins; of these one of large size is occasionally present, the foramen of Vesalius.
Fossa Pterygopalatina. This space, which corresponds to the angular
FIG. 170.-FRONTAL SECTION THROUGH THE PTERYGO-PALATINE FOSSA OF THE RIGHT SIDE.
interval between the pterygo-maxillary and inferior orbital fissures, and which lies between the maxilla in front and the root of the pterygoid process behind, is bounded medially by the perpendicular part of the palate bone, which separates it from the nasal cavity, with which, however, it communicates by means of the spheno-palatine foramen, which lies between the orbital and sphenoidal processes of the palate bone and the inferior surface of the body of the sphenoid. Opening into this fossa, above and behind, are the foramen rotundum, the pterygoid canal and the pharyngeal canal, in that order from lateral to medial side, whilst below is the superior orifice of the pterygo-palatine canal, together with openings of the lesser palatine canals. Its roof is formed by the inferior surface of the body of the sphenoid and the orbital process of the palate bone. Anteriorly it lies in relation to the apex of the orbit, with which it communicates by means of the inferior orbital fissure; whilst laterally, as already stated, it communicates with the infra-temporal fossa through the pterygo-maxillary fissure.
Posterior Aspect of the Skull (Norma Occipitalis). The view of the cranium as seen from behind includes the posterior halves of the two parietal bones above, the squamous part of the occipital bone below, and the mastoid portions of the temporal bones on either side, inferiorly. The shape of this aspect of the skull varies much, but ordinarily the greatest width corresponds to the level of the parietal tuberosities. The sutures on this view of the calvaria display a triradiate arrangement, one limb of which is vertical, and corresponds to the posterior part of the interparietal or sagittal suture. The other two limbs pass laterally and downwards in the direction of the mastoid processes, uniting the two parietal bones in front with the occipital bone behind; these constitute the A-shaped lambdoid suture. The point of confluence of the sagittal and lambdoid sutures is called the lambda. This can generally be felt in the living, owing to the tendency of the squamous part of the occipital to project slightly, immediately below this spot. About one inch and a quarter above the lambda the two small parietal foramina are seen, through which pass the small emissary veins of Santorini, which connect the intra-cranial venous system with the superficial veins of the scalp. These small holes lie about of an inch apart on either side of the sagittal suture, which here, for the space of about an inch, displays a simplicity of outline in striking contrast with its serrated arrangement elsewhere. The term obelion is applied to a point on the sagittal suture in line with the two parietal foramina. The lambdoid suture is characterised by great irregularity of outline, and not infrequently chains of separated ossicles are met with in it, the ossa suturarum (sutural bones). The squamous part of the occipital bone is divided into two parts by the superior nuchal or curved line, the central part of which forms the external occipital protuberance or inion. The part above, called the planum occipitale or occipital surface comes within cur present consideration; the part below, called the planum nuchale or the nuchal surface, though seen in perspective, had best be considered when the base is examined. A little above the level of the superior curved line the occipital surface is crossed on either side by a faint lunated line, the linea nucha suprema (highest nuchal or curved line), to which are attached the occipitales muscles and the galea aponeurotica. The projection of the occipital surface varies much in individual skulls; most frequently it overhangs the external occipital protuberance, forming a distinct boss; exceptionally, however, the latter may be the most projecting part of the bone. The extremity of the superior nuchal line on either side corresponds to the position of the asterion (p. 285). Lateral to these points the outline of the skull is determined by the downward projection of the mastoid processes, the medial surfaces of which are deeply grooved by the mastoid notches for the attachment of the posterior bellies of the digastric muscles, thus causing these processes to appear more pointed when viewed from this aspect.
Upper Aspect of Skull (Norma Verticalis).
This is the view of the calvaria as seen from above. It is liable to great diversities of form. Thus, its shape may vary from an elongated oval to an outline more nearly circular. These differences have been classified, and form important distinctions from a craniometrical standpoint, the rounder varieties being termed the brachycephalic, whilst the elongated belong to the dolichocephalic group. Another noteworthy point in this view is the fact that in some instances the zygomatic arches are seen, whilst in others they are concealed by the overhang and bulge of the sides of the anterior part of the cranium. The former condition is described as phænozygous, the latter as cryptozygous, and each is more or less closely associated with the long or round varieties of head-form respectively.
The sutures displayed have a T-shaped arrangement. Placed medially between the two parietal bones is the sagittal suture. This is finely denticulated, except in the region of the obelion, though, of course, this will not be apparent if obliteration of the suture has taken place through fusion of the two parietal bones. Posteriorly the sagittal suture unites with the lambdoid suture at the lambda, which marks in the adult the position of the posterior fontanelle of the foetus. Anteriorly it terminates by joining the transverse suture which separates the frontal bone
anteriorly from the parietals posteriorly; this latter is called the coronal suture, and the point of junction between the sagittal and coronal sutures is known as the bregma this corresponds in position to the anterior fontanelle of the foetus. The summit, of the vault of the calvaria corresponds to a variable point in the line of the sagitta. suture, and is named the vertex. The coronal suture is less denticulated centrally than laterally. Occasionally there is a persistence of the suture (metopic) which unites the two halves of the frontal bone; under these conditions the line of the sagittal suture is carried forward to the fronto-nasal suture, and a skull displaying this peculiarity is described as metopic. Behind the coronal suture may occasionally be seen the post-coronal depression, and in some instances the vault of the calvaria forms a broad, slightly elevated crest along the line of the sagittal suture. On either side the temporal ridges can be seen curving over the lateral and superior aspects of the parietal bones. As the lower of these crosses the coronal suture in front it marks a spot known as the stephanion, useful as affording a fixed point from which to estimate the bi-stephanic diameter. The interval between the temporal ridges on either side will vary according to the form of the skull and the development of the temporal muscle. In this view of the calvaria a small part of the lambdoid suture on either side of the lambda is visible posteriorly.
Basis Cranii Externa (Norma Basalis).
The external or inferior aspect of the base of the cranium-i.e. the skull without the mandible--includes a description of the under surfaces of the skeleton of the face (cranium viscerale) and the cranium (cranium cerebrale). The former includes the hard palate formed by the maxillæ and palate bones, the superior alveolar arch, and the bodies of the maxillæ as seen from below; whilst laterally, and united with the bodies of the maxillæ, the zygomatic bones are displayed, curving backwards to form the anterior halves of the zygomatic arches. In the median plane, passing from the upper surface of the hard palate, is the osseous septum of the nose, here formed by the vomer, which is united above to the under surface of the body of the sphenoid.
The under surface of the cranium is pierced by the foramen occipitale magnum for the transmission of the spinal medulla and its membranes. In front of this a stout bar of bone extends forwards in the median plane, formed by the union of the body of the sphenoid in front with the basilar part of the occipital bone behind. In adult skulls all trace of the fusion of these two bones has disappeared; when union is incomplete, it indicates that the skull is that of a person below the age of twenty-five. The sphenoid comprises that part of the calvaria which forms the roof and sides of the apertures which lie on either side of the nasal septum above the hard palate the choana. Laterally the inferior surfaces of the great wings of the sphenoid extend as far forward as the posterior border of the inferior orbital fissure; whilst posteriorly they reach as far as the angular spine, lateral to which the spheno-squamosal suture, separating the great wing of the sphenoid from the squamous portion of the temporal, curves forwards and upwards, medial to the tuberculum articulare, to reach the floor of the temporal fossa, along which its course has been already traced (p. 168). On a level with the front of the foramen magnum the jugular process of the occipital bone forms an irregular curved border, which sweeps laterally to terminate at a point just medial to the root of the styloid process. Here, in line with the spheno-squamosal suture, from which, however, it is separated by a considerable interval, its extremity turns backwards, and may be traced at first medial to, and then turning upwards, behind the mastoid process of the temporal bone, separated from this latter by the occipito-mastoid suture. The bone behind the foramen magnum, which is included between the two occipito-mastoid sutures, comprises the nuchal surface of the squamous portion of the occipital bone, an area which is limited behind by the superior nuchal line, which separates it from the occipital surface of the same bone. The remaining portions of the base of the calvaria, as at present exposed, are formed by the squamous and tympanic portions of the temporal bone, together with the petro-mastoid part of the same bone, the latter of which is wedged in between the great wing of
FIG. 171.-INFERIOR SURFACE OF BASE OF SKULL.
The occipital, vomer, maxillary, and zygomatic bones are coloured red. The temporal and palate bones, blue.
The sphenoid and parietal bones, and the teeth, are left uncoloured.
1. External occipital crest.
2. Superior nuchal line of
the occipital bone.
3. Foramen magnum.
4. Occipital condyle.
5. Mastoid notch.
6. Mastoid process.
3. Styloid process.
9. Mandibular fossa.
13. Lateral pterygoid lamina.
14. Hamulus of medial
15. Nasal septum.
16. Posterior nasal spine.
17. Horizontal part of palate bone.
meatus. 19. Incisive foramen.
18. Palatine process of maxilla.
10. Foramen spinosum.
11. Angular spine of the
sphenoid. 12. Foramen ovale.
21. Greater palatine foramen.
22. Zygomatic process of maxilla.
24. Infra-temporal fossa.
25. Zygomatic arch.
the sphenoid in front and the occipital bone behind. Stretching forwards from the squamous part of the temporal in front is seen the zygomatic process which, by its union with the zygomatic bone, completes the formation of the zygomatic arch.
Palatum Durum.-Studying next the various parts in detail, the hard palate may be first examined. Of horse-shoe shape as a rule, it presents many varieties of outline and size. Formed by the palatine processes of the maxillæ in front and the horizontal parts of the palate bones behind, its circumference in front and at the sides corresponds to the superior alveolar arch, in which are embedded the sixteen teeth of the two maxilla; posteriorly the edge of the hard palate is thin, presenting in the median plane a pointed process, the posterior nasal spine, on either side of which the posterior free border is sharp and lunated. The vault of the palate, which is concave from side to side, and from before backwards, varies in depth according to the projection and development of the alveolar processes. When the teeth are shed and the alveoli are absorbed, the palate becomes shallow and flat. Running throughout its entire length in the median plane is the median palatine suture, which separates the palatine processes of the maxillæ in front and the horizontal parts of the palate bones behind. A little behind the central incisor teeth, and in the line of this suture, is a little pit, the foramen incisivum. At the bottom of this may be seen the openings of some small canals, varying in number from one to four; these are usually described as arranged in two pairs, the one pair placed side by side, the other lying in the median plane in front and behind. The former are called the foramina of Stenson, and transmit the terminal twigs of the greater palatine arteries which ascend to reach the nasal cavities. The latter, called the foramina of Scarpa, open, the anterior into the left, the posterior into the right nasal cavity, and afford passage for the fine filaments of the left and right naso-palatine nerves, respectively. About half an inch (12 mm.) in front of the posterior nasal spine the median palatine suture is crossed at right angles by the transverse palatine suture. This, which indicates the line of union of the palatine processes of the maxilla with the horizontal parts of the palate bones, passes transversely laterally on either side until it reaches the medial aspect of the base of the alveolar process, along which it turns backwards, to disappear within the foramen palatinum majus (greater palatine foramen), the aperture of which lies just medial to the root of the dens serotinus (wisdom molar). Through this there pass the greater palatine artery and the large anterior palatine nerve. Leading from this foramen is a groove which curves forwards immediately to the medial side of the alveolar arch; not infrequently the medial edge of this groove forms a thin and sharp ridge on the surface of the palate. In this groove are lodged the afore-mentioned vessels and nerves. The surface of the palate in front of the transverse suture is rough, pitted for the palatine glands, and pierced by numerous small vascular foramina; the part of the palate behind the suture, formed by the under surface of the horizontal part of the palate bone, is much smoother. From this there rises, just posterior to the greater palatine foramen, a thin sharp crest, which curves medially immediately in front of the posterior free edge; to this are attached some of the tendinous fibres of the tensor veli palatini muscle.
Pterygoid Processes.-Buttressed against the posterior extremities of the alveolar arch are the pterygoid processes of the sphenoid. If carefully examined, these will be seen not to lie in actual contact with the maxillæ, but to be separated from them by the triangular wedge-shaped pyramidal processes of the palate bones. It is these latter which are pierced by the foramina palatina minora (lesser palatine canals), which lie just behind the greater palatine foramen, and through which pass the lesser palatine nerves. As here displayed, the pterygoid processes of the sphenoid lie on either side of the opening of the choana (O.T. posterior nares); each consists of two laminæ, a medial and a lateral; the latter is the broader, and is directed backwards and slightly laterally. Its lateral surface has been already studied in connexion with the infra-temporal fossa (p. 168). Medially it is separated from the medial pterygoid lamina by the pterygoid fossa, wherein is lodged a considerable part of the internal pterygoid muscle. The floor of the fossa is formed. in greater part by the coalescence of the two pterygoid lamine; but at the level of