THE SINUS FRONTALIS 157 



2. By the extension of two cells lateral to the processus 

 uncinatus (cellulse infundibulares) in two instances. 



3. By the extension of an infundibular cell and the direct 

 extension of the infundibulum ethmoidale in one instance. 



4. By the extension of one of the cellulse frontales and the 

 direct extension of the infundibulum ethmoidale in one 

 instance (Fig. 52). 



THE FORM AND BOUNDARIES OF THE SINUS FRONTALIS 

 The shapes of the frontal sinuses, as well as the extent 

 of their pneumatization, show a wide variation in the differ- 

 ent specimens of approximately the same age. In average 

 cases, however, sinuses which have not extended into the 

 vertical portion of the frontal bone (infantile types of sinuses) 

 are rather ovoid in outline (Figs. 21, 24, 27, 33, and 36). 

 When pneumatization has extended into the vertical portion 

 of the bone (Figs. 49, 51, 53, 55, and 56), the outline of the 

 average sinus resembles somewhat a three-sided pyramid- 

 as described by Boege and by Miloslawski in their studies 

 of adult sinuses. Boege found, in his study of the frontal 

 sinuses in 203 adult macerated skulls, that in 4.9 per cent, of 

 cases the sinuses on both sides failed in their superior de- 

 velopment to reach the level of the supra-orbital margins; 

 while in 4.4 per cent, of cases the right sinus alone, and in 2.5 

 per cent, of cases the left sinus alone, failed to reach a height 

 equal to that of the supra-orbital margin. The average 

 diameters of the frontal sinuses as found in Boege's 203 

 cases are 23.6 mm. laterally, 20.8 mm. supero-inferiorly, and 

 16.1 mm. anteroposteriorly. The average diameters of de- 

 veloping sinuses as found in the various ages in this series 

 are given in the table on pp. 134 and 135. 



