128 J. PARSONS SCHAEFFER 



It is a well established fact that the sinus frontalis deA^elops 

 variously by a direct extension of the whole recessus frontahs; 

 from one or other of the anterior group of cellulae ethmoidales 

 which have their point of origin in frontal furrows; and occa- 

 sionally from the ventral extremity of the infundibulum eth- 

 moidale, either by direct extension or from one of its cellular 

 outgrowths. Indeed, the sinus frontalis may be unilaterally or 

 bilaterally present in duplicate or triplicate, indicating a genesis 

 from more than one of the aforementioned areas. The sinus 

 frontalis is in many instances, embryologically speaking, a 



^ F/v?/tefl /hrroivs 



/■'rot/t(fI folds _. .' -g's^ - "' ,B-oc. imd7ff/^s 



Co?icI'a 7/ns. 7/ud. - - 

 Bulla etJ/ 



Fig. 2 From a term fetus. Recessus frontalis exposed. Note frontal fur- 

 rows and folds. Especially note the continuity of the suprabullar furrow and the 

 fourth (most dorsal) frontal furrow (see reference in text). The infundibulum 

 ethmoidale is in line with the first frontal furrow, hut not directly continuous 

 with it. After Schaeffer. 



cellula ethmoidalis anterior which has grown sufficient!}^ far 

 into the frontal region to be topographically a sinus frontalis. 



The first evidence of the sinus frontalis must not be sought 

 in the frontal bone, but in the recessus frontalis of the meatus 

 nasi medius. Lack of observance of this rule has led to such 

 statements as: "in the newborn infant no trace of a frontal sinus 

 is visible," ''the earliest sign of a frontal sinus is seen about the 

 end of the first year in the form of a shallow depression," "the 

 frontal sinus is completely absent in the newborn infant." Poi- 

 rier states that the frontal sinus is first seen about the end of the 

 second year. Tillaux puts it as late as the twelfth year. Onodi, 

 Schaeffer, Davis and others recognize the sinus frontalis as such 



