The student should now examine the spheno-maxillary fossa with great care, so that he may gain a clear comprehension of the way in which it is built up and of its relations and connections, for the key to an understanding of a great deal of the deep anatomy of the face is to be found in this region.
We have seen that the palate bone is applied to the inner side of the maxilla, but that it projects back further than this bone to articulate with the internal pterygoid plate, so holding the jaw away from the pterygoid region by the amount of this projection, and thus leading to the existence of an interval between them that is the spheno-maxillary fossa. Evidently the vertical plate of the palate must form the inner wall of this fossa and must separate it from the nose, for this plate is part of the outer wall of the nasal fossa. Because the vertical plate articulates behind with the internal pterygoid plate along the whole length of its front border it is equally plain that the posterior wall of the fossa must be formed by the outer plate (Fig. 183) except in its extreme upper and inner part, where the inner plate is slightly wider than its front edge that carries the palate bone : in other words, the front edge of the internal plate turns in under the body of the sphenoid and thus leaves a small piece of the plate exposed to form part of the back wall of the fossa just-below and outside the body of the bone. This is the level of the Vidian opening, which is seen immediately outside this little piece of inner plate. Outside this the external pterygoid plate makes the chief part of the posterior wall : the plate broadens rapidly as it passes up and becomes continuous with the area of bone immediately below the orbital surface of the great wing.
In Fig. 183 the back wall of the fossa is shown, and the outer plate is seen to widen very much in its upper part and to open out into the spheno-maxillary surface of the great wing. The position of the vertical plate of the palate is also shown, and the small part of the inner plate that enters into the formation of the back wall. At the outer edge of the broad front surface of the outer plate the fossa becomes continuous with the zygomatic fossa, so it is evident that the breadth of the fossa depends on the width of the surface of the plate ; this varies in different bones, leading to a shallow or deep fossa, as the case may be. The opening, bounded by a margin of the plate, through which the fossa becomes continuous with the zygomatic fossa may be termed the pterygo-maxillary fissure, to distinguish it from the spheno-maxillary fissure, which opens into the orbit from the fossa. Observe that the anterior opening of the foramen rotundum is on the spheno-maxillary surface of the great wing, so that the nerve enters the fossa on leaving the foramen ; also notice that the Vidian opening is further in, and on a lower level, because it runs between the two plates and not through the great wing. A ridge of bone separates the two foramina and can be traced up to the side of the body of the bone. The inner wall of the fossa is made by the vertical plate of the palate. Articulate this bone with the maxilla and observe how the front portion of the vertical plate lies against the nasal surface of the maxilla, so that its orbital process can be seen from behind the jaw, at the top back corner of the bone, as it turns out to reach the orbit; in this way the orbital process comes to possess a zygomatic surface. The spheno-palatine foramen separates this process from the sphenoidal process, which is directed in as it follows the internal plate under the body of the sphenoid.
It is clear, therefore, that the inner wall of the fossa is made by that part of the vertical plate that lies below the foramen and which forms the sphenoidal process behind this. The foramen is closed above by the body and turbinate of the sphenoid and thus opens into the nasal fossa immediately below the roof, which is formed here by the under surface of the sphenoid : in this way it becomes possible for nerves and vessels passing through the foramen to turn down on the outer wall or to cross below the roof and reach the septum, and thus to supply both sides of the nasal cavity.
The extreme inner part only of the back of the maxilla forms the front wall of the fossa ; the remainder makes the front wall of the zygomatic fossa, and the continuity of the two fossae is very clear on this bone. The upper margin of the bone is the lower edge of the spheno-maxillary fissure, which opens from the orbit into the two fossae : the extent to which the fissure opens into the spheno-maxillary fossa is really determined by the size of the spheno-maxillary area on the great wing, as can be seen on comparison of different skulls, so varies with the depth of the fossa. The fissure is directed downwards and outwards, and thus the maxillary nerve reaches its level by running outwards as well as forwards, without an upward direction : when the fossa is shallow the nerve may pass from the spheno-maxillary into the zygomatic fossa before reaching the level of the fissure.
The direction and construction of the posterior palatine canal can be understood by articulating the palate and maxilla, and the" situation and relations of the tuberosity of the palate should be thoroughly investigated at the same time.
To sum up :-The fossa is connected with the cranial cavity by the foramen rotundum, with the region of the naso-pharynx and Eustachian tube (see p. 225) by the Vidian and pterygo-palatine canals, with the nasal fossa by the spheno-palatine foramen, with the mouth cavity by the posterior palatine canal (from which minute foramina also reach the outer wall of the nose), with the orbit by the spheno-maxillary fissure, and with the zygomatic fossa through the pterygo-maxillary fissure.
The structures passing through the spheno-maxillary fissure are :- Infraorbital nerve and vessels ; Temporo-malar nerve ;
Branches fiom Meckel's ganglion ascending to the orbit ; The inferior ophthalmic vein. Passing through the pterygo-maxillary fissure are :- Internal maxillary vessels ; Posterior dental nerves and vessels ; Infraorbital nerve (sometimes).
The spheno-maxillary fissure is closed externally by the malar, connecting the maxilla and the sphenoid : rarely the maxilla may meet the sphenoid directly here, behind the malar, and this is the only place where these two bones may come into contact.