The sphenoidal sinuses occupy the anterior portion of the body of the sphenoid, and are separated from one another by an osseous septum. Occasionally one or both may be absent, and they vary considerably in size. When large, the olfactory peduncle, optic commissure, pituitary body, and pons lie on the thin roof, while the optic nerve and ophthalmic artery lie on the superior external aspect, and the internal carotid artery, cavernous sinus, and structures occupying the sphenoidal fissure are in relation to the external lateral wall. The floor is sometimes thin, and under it lies the Vidian nerve. The sinus is sometimes invaded by tumours of the pituitary, and, on the other hand, septic conditions of the sinus may lead to involvement of the optic nerves, cavernous sinus, etc. The ostium is situated on the anterior wall, near the roof, and opens into the spheno-ethmoid recess on the internal surface of the superior turbinate. It does not, therefore, facilitate drainage ; it is sometimes so small that it will not admit a probe, and it is generally necessary to remove a portion of the middle turbinate before an instrument can reach it from the nose. It lies fully 3 inches from the nasal vestibule, at an angle of nearly 45 degrees upwards and backwards from the nasal floor.

The ethmoidal cells, situated in the lateral mass of the ethmoid, vary in number and size, and are divided into posterior and anterior sets, which, as a rule, do not communicate, by an oblique lamina of bone. Externally they are bounded by the os planum of the ethmoid, internally by the superior and middle turbinates, above by the orbital plate of the frontal, and below by the orbital plate of the superior maxilla. Anteriorly they are closed in by the nasal process of the superior maxilla and the lachrymal bone, and posteriorly by the sphenoidal spongy bones.

The posterior ethmoidal cells rarelv communicate with the sphenoidal sinus, but sometimes project into the body of the sphenoid, giving rise, when infected, to symptoms similar to those of sphenoidal sinus infection.

These cells open into the superior meatus by one or more ostia and are very inaccessible. A purulent discharge may find its way posteriorly into the naso-pharynx or anteriorly into the nose.

The anterior ethmoidal cells are closely related to the floor of the frontal sinus, in which they not infrequently form a projection called the bulla frontalis. They may also project upwards along the roof of the orbit, and downwards into the roof of the antrum of Highmore, and may thus, when infected, cause symptoms simulating disease of these sinuses, which may actually become involved by destruction of the thin bony septa.

The ethmoidal bulla, already spoken of, situated at the antero-inferior border of the lateral mass, also contains one or more cells, and further cells may be found in the middle and inferior turbinate bones. These latter (owing to blockage of the duct in nasal catarrh) sometimes become transformed into mucoceles, which may be so large as to obstruct the nasal passage.

Several ostia open into the infundibulum on its outer and posterior aspect, and not infrequently the infundibulum itself originates in the cells forming the bulla frontalis. Other ostia open directly into the middle meatus. Inflammatory conditions of the cells are frequently associated with the presence of soft polypi in the nose, and with dacryocystitis (inflammation of the lachrymal sac), while extension to the orbit or cranial cavity is to be feared.

The frontal sinuses are situated immediately above the root of the nose, between the tables of the frontal bone. They are separated by a septum and vary greatly in size, to which the prominence of the superciliary ridge is no guide. Sometimes one or both are absent. One sinus is often much larger than the other ; the cavities are frequently subdivided by incomplete septa, and, while the main septum is generally mesial, it is sometimes deflected. If a sinus be present it can generally be reached by making a horizontal skin incision at the level of the eyebrow, and going down immediately above the root of the nose in the angle between the middle line and the inner third of the supra-orbital margin. Care should be taken to clear out all the subdivisions of the sinus. The posterior wall of the sinus is in relation to the frontal lobe of the brain, and the floor forms part of the roof of the orbit.

Inflammatory conditions of the sinuses may involve the brain, and, where the bone is affected, perforation often takes place at the inner angle, the eyeball being pushed down and outwards, and diplopia resulting. Not infrequently the orbital cavity is involved in disease, tubercular or otherwise, originating in the sinus, the pus generally pointing at the inner side of the upper eyelid. The sinuses may be fractured without the cranial cavity being involved, and emphysema of the surrounding tissues may arise therefrom when the mucous membrane is torn.

The fronto-nasal duct, which varies much in size, begins at the lowest part, close to the septum, thus favouring drainage. It may open into the infundibulum or into the middle meatus direct, the ostium varying from a slit to an aperture ¼ inch in diameter. The introduction of a probe is not always an easy matter, it being frequently necessary to remove the middle turbinate, while the probe is also liable to enter the ostia of the anterior ethmoidal cells.