THE SINUS FRONTALIS 133 



velop during the latter months of fetal life, yet, in the average 

 case, one cannot say definitely which of the extending proc- 

 esses of pneumatization represents the primitive sinus 

 frontalis until after the sixth month of postnatal life. In 

 some specimens the probable route of development can be 

 determined with a fair degree of certainty at the time of 

 birth or shortly thereafter (Figs. 17 and 20), while in other 

 cases the distinct beginning of a sinus frontalis is not de- 

 monstrable until near the end of the first year. 



From whichever of these sources a frontal sinus may have 

 its origin, the process of pneumatization gradually extends 

 from that portion of the anterior ethmoidal area toward and 

 into the inferior portion of the frontal bone. The sinus, 

 surrounded as it is by a thin lamina of compact bone, then 

 advances toward the ascending portion of the frontal bone, 

 advancing as the cancellous bone is resorbed. This rate of 

 resorption, in an average case, is such that the sinus begins 

 its ascent into the vertical portion of the bone during the 

 second year, and in the third year is 3.8 mm. above the level 

 of the nasion. 



As the sinus frontalis advances into the vertical portion of 

 the bone, its posterior wall is always thin and is composed 

 almost entirely of compact bone, while the anterior wall, as 

 found in the different specimens, varies greatly in its thick- 

 ness and usually contains a considerable amount of diploe. 



In the following table we have given the average diameters 

 and the level at which the ostia frontalia were found, the 

 origin of the sinuses, the thickness of the walls, the level 

 reached by the superior border of the sinus, and the vertical, 

 lateral, and anteroposterior diameters of the average sinus 

 as found in the given number of cases at the various ages. 



