Chap. XIII] 



RESPIRATORY SYSTEM 



233 



rest the passage of dust and other foreign substances carried in with 

 the inspired air. 



The nasal fossae are two irregularly wedge-shaped cavities, 

 separated from one another by a partition, or septum, the upper 

 part of which consists of the perpendicular plate of the ethmoid, 



Fig. 140. — Sagittal Section of the Face and Neck, showing the Fibst 

 Portions of the Respiratory and Alimentary Tracts. (Gerrish.) 



and of the vomer, and the lower part of cartilage. The turbinated 

 bones and turbinated processes of the ethmoid, which are ex- 

 ceedingly light and spongy, project into the nasal cavities, and 

 divide them into three incomplete passages from before back- 

 wards, — the superior, middle, and inferior meatus. The palate 

 and maxillae separate the nasal and mouth cavities, and the crib- 

 riform plate of the ethmoid forms the partition between the 

 cranial and nasal cavities. 



