MEDIAN SAGITTAL SECTION OF THE SKULL. 185 



angles, respectively ; the superior angle is cut off by the cribriform plate ; whilst the 

 sides correspond to the frontal and nasal bones anteriorly, and the sphenoidal 

 conchas, sphenoidal process of the palate, and the ala of the vomer posteriorly. 

 The cavity is therefore deep towards its middle, but gradually becomes shallower in 

 front and behind where the piriform aperture and choana are situated. The 

 piriforni opening of the nose, which is of half-heart shape, is larger than that of 

 the choanse (O.T. posterior nares), and is directed forwards and downwards ; the 

 choanas are of rhomboidal form, and slope backwards and downwards. The inferior 

 meatus is the channel which is overhung by the inferior concha, and its floor is 

 formed by the side- to-side concavity of the upper surface of the hard palate. Open- 

 ing into it above, under cover of the anterior part of the inferior concha, is the canal 

 for the naso-lacrimal duct ; whilst its floor is pierced in front near the middle 

 line by the canalis incisivus. The middle meatus is the hollow between the 

 middle and inferior conchas; it slopes from above downwards and backwards, 

 and is overhung by the free curved edge of the middle conchas, beneath which 

 there is a passage called the infundibulum, leading upwards and forwards to open 

 superiorly into the frontal sinus, as well as into some of the anterior ethmoidal cells. 

 Under cover of the centre of the middle concha and continuous with the 

 infundibulum in front there is a curved groove, the hiatus semilunaris, into 

 which open one or more orifices from the maxillary sinus. Above this groove 

 there is a rounded eminence, the bulla ethmoidalis, overlying the middle 

 ethmoidal cells, which usually open on its surface. The superior meatus, about 

 half the length of the middle meatus, is placed between the superior and middle 

 conchas in the posterior and upper part of the cavity ; it receives the openings of 

 the posterior ethmoidal cells. Near its posterior extremity the spheno - palatine 

 foramen pierces its lateral wall, and brings it in relation with the pterygo- 

 palatine fossa. The sphenoidal sinus opens on the roof of the nose, above the 

 level of the superior conchas, into a depression called the spheno-ethmoidal recess. 



Septum Nasi. If the opposite half of the section in which the osseous nasal 

 septum is retained be now studied, it will be seen to be formed by the crests of 

 the maxillary and palate bones below, on which rests the vomer, the posterior 

 border of which being free, forms the posterior edge of the nasal septum, which 

 slopes obliquely upwards and backwards towards the inferior surface of the body of 

 the sphenoid. Here the vomer articulates with the rostrum of the sphenoid. In 

 front of this the vomer articulates with the perpendicular part of the ethmoid, and 

 between them anteriorly there is an angular recess into which the cartilaginous 

 septum fits. Superiorly and anteriorly the osseous septum is completed by the 

 articulation of the perpendicular part of the ethmoid with the nasal spine of 

 the frontal, together with the nasal crest formed by the union of the nasal bones ; 

 whilst posteriorly and superiorly the perpendicular plate of the ethmoid articulates 

 with the median sphenoidal crest of the sphenoid. In most instances the osseous 

 septum is not perfectly vertical, but is deflected towards one or other side. 



Air-sinuses in Connexion with the Nasal Cavities. Connected with the 

 nasal cavities are a number of air-sinuses. These are found within the body of 

 the sphenoid, the labyrinth of the ethmoid, the orbital process of the palate bone, 

 the body of the maxilla, and the superciliary arch of the frontal bone. 



The sphenoidal sinus, of variable size, occupies the interior of the body of the 

 sphenoid. In some cases it extends towards the roots of the pterygoid processes. 

 In front it is formed in part by the absorption of the sphenoidal conchas and 

 is divided up into two cavities by a sagittally placed partition, which, however, 

 is frequently displaced to one or other side. It opens anteriorly into the roof 

 of the nose in the region of the spheno-ethmoidal recess. 



The ethmoidal sinuses are placed between the lateral aspects of the upper part 

 of the nasal cavities, and the cavities of the orbits, from which they are separated 

 by thin and papery walls. These air-spaces are completed by the articulation of 

 the ethmoid with the maxilla, lacrimal, frontal, sphenoid, and palate bones, and 

 are divided into three groups an anterior, middle, and posterior. The latter 

 communicates with the superior meatus ; the anterior and middle open either 

 independently or in conjunction with the infundibulum into the middle meatus. 



