THE SINUS MAXILLARIS 111 



cation of the ostium is, we believe, due to a developmental 

 process similar to that producing the ostia maxillaria acces- 

 soria, which will be considered below, differing only in that 

 accessory ostia are located posterior to the infundibulum 

 and thus communicate directly with the meatus medius. 



Schaeffer found the primitive maxillary pouch duplicated 

 in some of his fetal specimens, and suggested that this may 

 explain some of the duplications of the ostium maxillare of 

 the adult sinus, the two pouches fusing distally, forming a 

 single cavity with duplicated ostia. To us, however, this 

 does not seem probable, since we find that accessory sinuses 

 which have developed from separate ostia do not directly 

 communicate, except in instances where the intervening wall 

 has been destroyed by a suppurative process. In the de- 

 velopment of accessory sinuses the pouching mucosa rests 

 upon a layer of compact bone as the process of pneumatiza- 

 tion extends into the body of the bone whether it be the 

 maxillary, the frontal, or the sphenoid bone. The layer 

 of compact bone advances as the process of resorption goes 

 on in the underlying cancellous bone. When two advancing 

 layers of compact bone are brought into contact by the re- 

 sorption of the intervening cancellous bone, resorption ceases 

 and a permanent layer of compact bone remains between 

 the layers of mucoperiosteum lining the sinus cavities. Ir- 

 regularities in the rate of resorption of the cancellous bone 

 surrounding any individual sinus may be of a character in 

 which the advancing layers of compact bone surrounding 

 different recesses of that sinus are brought into contact, 

 in which case a ridge or an incomplete septum persists. 

 The sinus maxillaris in Fig. 35, and the superior portion of 

 the left sinus frontalis in Fig. 52, show incomplete septa 



