680 Jacob Parsons Schaeffer. 



region, and in many cases might be classed as frontal sinuses. 

 In some adult specimens in which two large sinuses are present 

 on the same side of the frontal region, it is difficult to say whether 

 they are both frontal sinuses or extensively developed anterior 

 ethmoidal cells. Developmentally, in many cases, they are eth- 

 moid cells, and topographically they should be classed as frontal 

 sinuses. Occasionally the posterior ethmoid cell which develops 

 from the meatus nasi suprema I extends into the sphenoidal sinus. 

 The superior and the supreme conchae usually become m^ore or 

 less shell-like in the adult due to the developing posterior ethmoid 



cells. 



In the formation of the ethmoid labyrinth there is no uniform- 

 ity. In a general way the anterior group of ethmoidal cells is 

 ventral to the posterior group. There is, however, at times, con- 

 siderable overlapping of the two groups. Each cell as it grows 

 from a preformed furrow or recess tends more or less towards the 

 cribriform plate of the ethmoid bone. Even though a certain 

 cell has its anlage-point farther inferior than another cell, it may 

 outgrow its neighbor and force the latter to progress in a direction 

 other than to which it was primarily directed. Later the cells in 

 the homeycombing of the lateral ethmoidal masses grow in almost 

 any direction. Sometimes the intervening walls break down 

 and larger single cavities are thus formed. Seydel very aptly 

 says: " . . . . die Zellen der verschiedenen Reihen stehen 

 gewissermassen im Wettkampf mit einander, bald iiberwiegen die 

 der einen, bald die der anderen Reihe. Daher verlaufen die 

 Trennungslinien zwischen den Reihen keineswegs immer genau 

 wie die Muschelurspriinge." 



The sinus sphenoidalis 



Of the sinus sphenoidahs little need be said in this paper. It is 

 primitively nothing but a constriction from the dorsal and superior 

 portion of the nasal fossa, a fact already pointed out by Dursy. 

 This constriction of the nasal fossa in the formation of the anlage 

 of the sphenoidal sinus is evident fairly early, and at birth the 

 sinus is comparatively well advanced in most instances. 



