THE ACCESSORY AIR-SPACES. 



meatus. The ostium, which is usually in the lateral wall of the infundibulum, 

 about one centimeter from the upper end of the hiatus, is an oval or elliptical cleft 

 of variable size, with extremes of length from 3-19 mm. (Zuckerkandl), and from 

 25 mm. in width. An additional communication (ostium accessorium), present 

 in about 10 per cent., likewise opens into the infundibulum, lying behind the chief 

 aperture. It is ordinarily small, its diameter being only a few millimeters. The 

 mucous membrane lining the maxillary sinus is directly continuous with that 

 covering the lateral wall of the nasal fossa. With the exception of being thinner, it 

 corresponds in structure with the mucous membrane of the respiratory region, being 

 invested with ciliated columnar epithelium and possessing numerous, although small 

 and scattered, tubo-alveolar glands. 



Variations. The investigations of Zuckerkandl (Kallius) have shown that enlargement of 

 the maxillary sinus may be produced by : (i) hollowing out of the alveolar process (alveolar 

 recess) ; (2) excavation of the floor of the nasal fossa by extension of the alveolar recess 

 between the plates of the hard palate (palatal recess); (3) encroachment of the sinus into the 

 frontal process of the maxilla ; (4) hollowing out of the zygomatic process of the malar bone 

 (malar recess) ; (5) extension to and appropriation of an air-cell within the orbital process of 

 the palate bone (palatal recess). Contraction of the maxillary sinus, on the other hand, may 

 follow : ( i ) imperfect absorption of the cancellated bone on the floor of the sinus, or secondary 

 thickening of its walls ; (2 ) encroachment due to approximation of the facial and nasal walls, 

 unusual depression of the canine fossa, excessive bulging of the lateral nasal wall, or imperfectly 

 erupted teeth. 



The crescentic projections which quite commonly are seen protruding from the walls into 

 the interior, occasionally are replaced by septa that completely divide the sinus into two cavities, 

 each having its independent opening into the nasal fossa, but not being in communication with each 

 other. These partitions vary in position and direction, sometimes subdividing the antrum into an 

 anterior and a posterior compartment, and at others, into an upper and a lower chamber. In 

 the last case the lower space may communicate with the inferior meatus (Zuckerkandl, Briihl). 



FIG. 1185. 



Right frontal sinus 



- Left frontal sinus 



Passage leading into 

 infundibulum and 

 middle meatus 



Nasal septum 



Portion of frontal section exposing frontal sinuses which are asymmetrical. 



The Frontal Sinus. The air-spaces between the outer and inner tables of 

 the frontal bones (sinus frontales) are very variable in extent and form. The relative 

 development and general position of these cavities are usually indicated by the 

 degree of prominence of the superciliary ridges, but by no means invariably, since 

 numerous exceptions to this correspondence occur. The sinuses are frequently 

 quite asymmetrical (Fig. 1185), one cavity being enlarged, sometimes at the expense 

 of the other, with accompanying displacement of the intervening septum. The 

 latter, usually approximately median in position, is often very thin, but only rarely 



