3 i4 HEAD AND NECK 



of the nasal cavity. It is placed posterior to the vestibule, 

 and the free border of the inferior concha turns downwards 

 and limits it anteriorly (Fig. 117). On that account, and 

 because its floor slopes downwards and backwards, the in- 

 ferior meatus is more accessible to the current of expired 

 air than to the current of inspired air. In the anterior part 

 of the inferior meatus will be found the opening of the naso- 

 lacrimal duct, which conveys the tears to the nasal cavity 

 (Fig. 117). 



Dissection. To bring the aperture of the naso-lacrimal 

 canal into view, remove a small portion of the anterior part of 

 the inferior concha with the scissors (Fig. 117). 



The orifice of the naso-lacrimal duct varies in form, 

 according to the manner in which the mucous membrane is 

 arranged around it. Sometimes it is wide, patent, and circular ; 

 at other times the mucous membrane is prolonged over 

 the opening, reducing its size and acting as a flap valve to 

 the orifice. In some cases, indeed, the orifice may be so 

 minute that it is difficult to find. Its continuity with 

 the lacrimal sac should in all cases be established by passing 

 a probe, from above downwards, through the naso-lacrimal 

 canal (Fig. 9). 



The space above and behind the superior concha is termed 

 the recessus spheno-ethmoidalis^ and in its posterior part is the 

 aperture of the sphenoidal air sinus (Fig. 117). The orifice 

 of the sphenoidal air sinus may be circular or slit-like, accord- 

 ing to the manner in which the mucous membrane is disposed 

 around it. 



The term meatus communis is applied to the narrow cleft- 

 like portion of the nasal cavity which extends from the roof 

 to the floor between the septum medially and the conchse 

 laterally ; and the part of the cavity which lies posterior 

 to the conchal region, and between it and the choanae, 

 is the naso-pharyngeal meatus (Fig. 1 1 6, i 7 ). 



Muco - periosteum of the Lateral Walls of the Nasal 

 Cavities. It has been noted that the vestibule is lined with 

 integument. The remainder of each lateral wall of the nasal 

 cavity is covered with mucous membrane which is so closely 

 blended with the subjacent periosteum that the two are 

 inseparable and form a muco-periosteum. A similar membrane 

 covers the roof and the floor. The muco-periosteum is 

 continuous, through the naso-lacrimal duct, with the ocular 



