The width of the aperture varies considerably in different races ; compared with the height it is least in Europeans and greatest in certain Negro races. It is proportionately broader in the infant than in the adult, owing to the shallowness of the maxilla at birth.
Sphenoidal Sinus-Variable in extent in different individuals and usually on different sides in any one individual. The variations, and the situations of the different incomplete bony septa found within the cavities, may be largely explained by reference to developmental points. There are paired centres for the presphenoid, situated in front of paired centres for the basisphenoid : the centres for the lingula? lie outside these last and separate them from the inner parts of the great wings, which, however, come into relation, in front of the lingular, with the presphenoidal centres. Each sinus, growing back from the spheno-ethmoidal region, comes first into relation with the presphenoidal part and hollows this out, the condensed bone between the two ossific areas remaining as a median vertical septum. If now each cavity extends further back, equally on the two sides, deeper sinuses result : these involve the postsphenoid centres, and, being equal growths, still maintain a median antero-posterior septum in their deeper parts, formed by the condensation between the basal ossifications. This equal extension is uncommon, however, for usually the further extension is unequal : one sinus may. for example, involve the postsphenoid of its own side and then extend to the postsphenoid of the other side, so that the other sinus remains small, confined to the presphenoidal region, and the septum between them, median in front, swings to one side further back; where it corresponds with the condensation between the pre- and post-sphenoid. One or both of the " presphenoidal " sinuses may extend laterally (instead of, or in addition to, the backward growth) and so involve the neighbouring part of the great wing : in this case the backward and the lateral extensions are separated by a partial septum corresponding with the condensation of the lingula, and this septum, directed obliquely forwards and inwards, may be very prominent if the region of the lingula has not been invaded, or more or less destroyed if such an invasion has taken place. A septum of this sort lies under the carotid groove, as does also the outer part of a transversely directed septum associated with the condensation between pre- and post-sphenoids, and these might be called carotid buttresses within the cavity : they are usually better marked above than below and are situated of course in the deeper parts of the cavities at their sides. Various combinations of these extensions may be seen in different individuals, but in general, it may be recognised that there is a septum, median in front but as a rule directed to one or other side behind, with lateral and posterior loculi more or less separated by a carotid buttress : the deep and anterior parts of the cavities may show a partial separation by an incomplete transverse septum marking the plane of condensation between pre- and post-sphenoids, and this, like the deflected inter-sinus septum, may run into a carotid buttress. Exceptionally the sinus may extend (a) into the great wing, reaching as far, it may be, as the groove for the maxillary nerve, and even invading the base of the outer pterygoid plate, passing over the Vidian nerve, (b) into the roots of the small wing, the optic nerve and ophthalmic artery lying in a prominent bony canal projecting into the cavity, or (c) into the basi-occiput, from which it is nearly always separated by the condensation between this bone and the basisphenoid.
The sinus begins its growth about the third or fourth month of intra-uterine life, is at birth a definite separate small cavity, in relation with the front aspect of the sphenoidal ossification, and enclosed by its own turbinate, grows fairly rapidly in the next few years, so that a noticeable cavity, invading the presphenoid, is present at five years of age, and takes on a more rapid extension at or before the age of ten, and again at puberty.
The frontal sinus develops as an upgrowth from one of the groups of cells (anterior ethmoidal) which form under cover of the overhanging anterior and upper end of the middle turbinal. The enlarging cell extends slowly upwards, the extension beginning about the middle of intra-uterine life. The growing cavity does not, as a rule, reach the frontal bone before birth, but invades that bone within the first year, and grows steadily up to the ninth or tenth year : after this it seems to take on a more rapid growth. The extensions are usually unequal, so that the septum between the cavities is deviated in its upper part, though generally more or less median below. In metopic skulls (p. 200) a sinus never transgresses the middle line. In other cases one sinus may completely overshadow the other, so that it seems at first as if no septum were present. The opening of the sinus depends on the site of its origin : if it is an enlarged member of the lateral group of anterior cells, it opens into the top of the hiatus semilunaris, from which these cells take origin as out-pouchings, but, if from more medially situated cells, as is perhaps more frequently the case, the sinus does not open directly into the upper end of the hiatus.
The adult sinus is extremely variable in form and extent. In a very general way it might be described as roughly pyramidal, the apex being directed upwards, but it is often rounded or ovoid, and may present a partial subdivision : this subdivision is in some cases really due to the simultaneous upgrowth of two cells which have partly coalesced. The front wall of the sinus is the thickest wall, and contains diploic tissue, the posterior wall being made of compact bone. The floor shows a general slope downwards and inwards towards the opening in its hinder and inner part, but there is often a depressed fossa in front of this, and the floor is generally uneven. It lies over the inner part of the orbital roof, not often going further than, or even so far as, the line of the supra-orbital nerve, and covers the anterior ethmoidal cells internally. The amount of its backward extension, greatest internally, is very variable, but it seldom reaches the depth of an inch : an average measurement in this direction would be in the neighbourhood of J inch.
The maxillary sinus (antrum of Highmore, maxillary antrum) is a cavity situated in the maxilla, of an irregular pyramidal shape, with its base inwards. Its front wall is fairly thick, and is made by the facial surface of the maxilla, its floor is formed by the thick alveolar portion of the bone, and its roof and inner back wall are thin-as is its inner wall, which is made of the inner lamina of the maxilla, overlaid by the lower end of the lachrymal, the uncinate process of the ethmoid, the downturned maxillary process of the inferior turbinate, and the vertical plate of the palate bone (see Fig. 194). Its opening, narrowed by the approximation of these bones, is a small slit between the uncinate process, palate, and turbinate : occasionally there is a double opening, the accessory ostium being usually placed below and behind the ordinary opening. The sinus begins to grow as an out-pouching of the side wall of the nose during the third month, and is present at birth as a small cavity (circ. 5 x 5 X 12 mm.) in the inner part of the upper jaw. This extends out as far as the line of the infra-orbital nerve during the first year, so that the situation of this nerve is indicated by a bony ridge in the roof of the cavity during and after the second year. The sinus grows with the bone, fairly quickly, up to the 8th or 9th year, and after this rather more slowly, corresponding with the slower eruption of teeth now occurring : after the later years of puberty, its form is only altered by the addition of a postero-inferior angle associated with the last molar development. These three indefinite stages of its growth seem to be indicated in its floor, where there are two ridges, as a rule, visible : one of these is in the premolar region, and the other in the molar region. Other ridges seen in the cavity include the infra-orbital ridge and the crest running downwards and forwards from this, containing the anterior and middle dental nerves. At birth the sinus is, of course, some distance above the level of the nasal floor, but its floor reaches this level about the eighth year, and after this usually lies below the nasal level to a small extent. As the teeth fall out with age, the floor rises and comes a little above the level of the nasal floor. The floor is closely related to the roots of the molars, and does not often reach teeth in front of the second premolar : the canine may, however, be in the front wall of large sinuses, and abnormal cavities may occasionally extend in front of this. A large posterior ethmoidal cell may project into the back and upper part of the cavity, even giving in some cases the appearance of the existence of a double sinus.