1424 HUMAN ANATOMY. 



incomplete, so that the spaces very seldom communicate. Numerous instances have 

 been observed in which one sinus was entirely wanting. The average dimensions of 

 the frontal sinus, as given by A. L. Turner, include a height of 31 mm. (i^ in.), a 

 width of 30 mm., and a depth of 17 mm. The capacity varies from 3-8 cc. (Briihl). 

 These spaces are not recognizable in the new-born child, first appearing about the 

 seventh year, after the absorption of the cancellated bone. It is not until after 

 puberty, however, that they attain their full size. They are usually larger in the 

 male than in the female. 



The typical pyramidal form of the space is often modified by the enlargement 

 of the sinus beyond its usual limits, since when exceptionally developed it may 

 extend into the orbital plate of the frontal bone, at times reaching as far as the 

 lesser wing of the sphenoid, or into the median orbital wall, or laterally into the 

 external angular process, or, exceptionally, into the nasal spine beneath the root of 

 the nose. On the other hand, the frontal sinus may be encroached upon by 

 projecting ethmoidal cells. 



The frontal sinus communicates with the middle nasal meatus through either the 

 infundibulum, or a passage between the anterior attachment of the middle turbinate 

 and the uncinate process, or both. Its aperture (ostiura frontalis) lies from 2-10 

 mm. from the upper end of the hiatus semilunaris. The frontal sinus is lined by a 

 prolongation of the respiratory nasal mucous membrane, diminished in thickness but 

 otherwise of its usual structure. 



FIG. i i 86. 



Sphenoidal sinus 



-Frontal sinus 



^ jp^^s^H am 



Superior meatus- __^^^ ^^^^^^^ 



.Infundibulum 



MMH^^^H" '^K" _J '^. \ 



Middle meatus __ 



Entrance to 

 'middle meatus 



Naso-pharynx 



Inferior meatus 



, L \ Vestibule and 



Choana (posterior naris) 1TJ~ ,, a sal aperture 



Maxillary sinus 

 Cast of nasal fossae and accessory air-spaces, viewed from right side ; natural size. (Kallius.) 



The Ethmoidal Air-Ceils. These spaces (cellnlae ethmoidales) include a 

 series -of pneumatic cavities, very variable in number and size, that from birth lie 

 between the upper part of the nasal fossae and the orbits, from which they are separated 

 by osseous plates of papery thinness. They are all lined with mucous membrane 

 which covers the thin bony partitions that separate the spaces from one another. 

 When these partitions are deficient, as they often are in old subjects, the intervening 

 septa are entirely membranous. The ethmoidal air-spaces, completed by the articu- 

 lation of the ethmoid with the frontal, maxillary, lachrymal, sphenoid and palate 

 bones, usually form three groups, tin- anterior, the middle and the posterior cells. 

 Every space communicates with the nasal fossa, either directly by means of an 

 independent aperture, or indirectly through one or more cells of the same group. 

 Sometimes the cells are so fused that two general cavities, an anterior and a poste- 

 rior, replace the corresponding groups. When typically arranged, the anterior cells 

 communicate with the middle meatus by means of apertures that open into the 

 upper part of the infundibulum. The middle cells also open into the middle meat 



