THE SINUS FRONTALIS 



The areas from which the sinus frontalis may have its 

 origin were considered above in discussing the development 

 of the cellulse ethmoidales anterior. (See pp. 45-6, 53-4.) 

 We found that during the fourth month of fetal life there are 

 two expansions usually demonstrable in the anterosuperior 

 portion of the meatus medius. The expanding areas are sep- 

 arated by the anterior attachment of the processus uncinatus. 

 The recess beneath the anterosuperior attachment of the 

 concha media and medial to the processus uncinatus is 

 termed the recessus conchalis. The recess lateral to the 

 anterior portion of the processus uncinatus is termed the 

 recessus infundibularis, being the anterosuperior termination 

 of the infundibulum ethmoidale. From the recessus con- 

 chalis the cellule front ales develop between the conchse 

 frontales and between the conchae and the walls of the recess, 

 while from the recessus infundibularis the cellulse infundib- 

 ulares have their origin. A sinus frontalis may develop 

 as an anterosuperior extension of a cell originating from 

 either of these two recesses, as a direct extension of the in- 

 fundibulum ethmoidale, as a direct extension of a recessus 

 conchalis in which no frontal cells have developed, or, in 

 rare instances, as an extension from a cell originating in the 

 suprabullar furrow. 



Although the primitive recessus conchalis and the recessus 

 infundibularis are usually demonstrable during the fourth 

 fetal month, and distinct cellulse ethmoidales anterior de- 



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