THE ACCESSORY AIR-SPACES. 



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usually by a crescentric cleft upon or above the ethmoidal bulla, but sometimes into 

 the infundibulum. The posterior cells communicate with the superior meatus by one 

 or more openings overhung by the upper concha. Very exceptionally the ethmoidal 

 cells may communicate with the sphenoidal or the maxillary sinuses, or may extend 

 into the substance of the middle turbinate bone. The mucous membrane clothing 

 the ethmoidal cells is exceedingly thin, but corresponds in its general structure, 

 even in possessing glands, with that lining the respiratory region of the adjacent 

 nasal fossae. 



The Sphenoidal Sinus. — The paired air-spaces (sinus sphenoidales) produced 

 by the absorption of the cancellated tissue within the body of the sphenoid bone are 

 separated by an osseous partition and seldom communicate. They are very variable 

 in size and often asymmetrical, with corresponding displacement of the septum. A 

 length of 22 mm., a width of 15 mm., and a height of 12 mm., are the approximate 

 dimensions of the average sinus. The capacity of the latter, as determined by Briihl, 

 is from 1-4 cc. When large, the spaces may appropriate not only a large part of 

 the sphenoid, extending into both wings, the pterygoid processes and the rostrum, 

 but also include the basilar process of the occipital bone. Not infrequently one or 



Fig. 11S7. 



Anterior ethmoidal cells 



Probe passes to middle meatus 



Posterior 



ethmoidal 



cells 



Sphenoidal sinuses 



Pituitary body 



Openings of sphenoidal sinus 

 and posterior ethmoidal cells 



Internal carotid artery 



^ Portion of section of frozen formalin-hardened head, exposing ethmoidal and sphenoidal air-spaces; 



viewed from above. 



more of the posterior ethmoidal air-cells projects or opens into the sphenoidal sinuses. 

 Very exceptionally these spaces may come into close relations with or even open into 

 the maxillary antrum (Zuckerkandl) — a condition normally found in some apes. 

 The sphenoidal sinus of each side communicates with the nasal fossa by means of 

 the spheno-ethmoidal recess, above the superior turbinate and close to the roof of 

 the fossa, by an aperture that pierces the upper part of the anterior wall of the sinus. 

 Through this opening, reduced in the recent condition, the respiratory mucous 

 membrane is prolonged into the sinus which it lines. 



The palatal sinus, the small air-space within the orbital process of the palate 

 bone, communicates indirectly with the nasal fossa by either the posterior ethmoidal 

 cells or the sphenoidal sinus into which it opens. 



Vessels. — Of the arteries supplying the nasal fossa the spheno-palatine branch 

 of the internal maxillary is the largest and most important. Entering the nose 

 through the spheno-palatine foramen, it divides into external (posterior nasal) and 

 internal (naso-palatine) branches, which supply an extended tract reaching from the 

 posterior to the anterior nares. The external branches are distributed to the turbinate 



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