THE SINUS SPHENOIDALIS 



The sinus sphenoidalis differs in its early development 

 from the other accessory sinuses in that the primitive sinus 

 does not have its origin from one of the furrows situated on 

 the lateral nasal wall, but develops as an in vagina tion ex- 

 tending from the posterosuperior portion of the recessus 

 sphenoethmoidalis. 



In the narrowed posterosuperior portion of the recessus 

 sphenoethmoidalis there is demonstrable in sixty-five-day 

 embryos an invagination of the mucosa extending into the 

 posterior portion of the nasal capsule (Fig. 8). The site 

 of this primitive invagination persists as the ostium sphenoid- 

 ale. Embryos of eighty-five to one hundred days show the 

 development into the nasal capsule more distinctly (Fig. 11), 

 the advancement of the pouching process being most fre- 

 quent in a postero-inferior and slightly lateral direction. 

 The primitive sphenoidal sinuses thus come to lie posterior 

 to the nasal capsule and anterolateral to the body of the 

 sphenoid bone (Figs. 15 and 16). In that portion of the 

 nasal capsule which forms the antero-inferior wall of the 

 primitive sinus there develops an ossification center for the 

 concha sphenoidalis or ossiculum Bertini. Ossification of 

 this concha sphenoidalis is in many instances well advanced 

 in term fetuses and infants (Fig. 18), but it is not until the 

 second or third year that this bone becomes firmly attached 

 to and continuous with the body of the sphenoid. 



The extent of development of the sinus sphenoidales 



160 



