158 THE NASAL ACCESSORY SINUSES IN MAN 



The anterior wall of the fully developed sinus is formed by 

 the anterior plate of the vertical portion of the frontal bone. 

 This is the thickest of the sinus walls, different specimens 

 varying from 1 to 8 mm., but averaging in adult cases 2.5 

 mm. in thickness. The thicker walls contain considerable 

 diploe. All observers agree that the prominence of the 

 superciliary ridges bears no constant relationship to the size 

 of the sinuses found beneath them. 



The posterior wall of the fully developed sinus is formed 

 by a part of the posterior plate of the vertical portion of the 

 frontal bone and also, in some cases, by a part of the hori- 

 zontal portion. This wall is composed almost entirely of 

 compact bone and averages 1.2 mm. in thickness. 



The medial wall is formed by the septum front ale, which 

 frequently shows deviations toward one side or the other. 

 The percentage of cases showing deviations toward the right 

 and the percentage toward the left are approximately equal. 

 In no instance was the septum frontale found to be incom- 

 plete. In average adult cases its thickness varies from 0.5 

 to 1 mm. 



The floor of the fully developed sinus is usually quite 

 uneven. Laterally it is formed by the orbital roof, while 

 medially it overlies the anterior ethmoidal cells. In the 

 posteromedial portion of the floor is the ostium frontale, 

 which is usually situated in the most dependent portion of 

 the sinus (Figs. 28, 33, 45, and 52), but is not invariably 

 so an antero-inferior recess sometimes being on a lower 

 plane (Figs. 35 and 49). The average diameters and levels 

 of the ostia, as found in the various ages, are given in the 

 table on pp. 134-5. Irregularities in the floor of the sinus, 

 due to underlying anterior ethmoidal cells, were considered 

 on pp. 54 and 63. 



