CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 287 



and are therefore higher in their middle part than either at the 

 anterior or posterior end at the anterior and posterior nares. These 

 fossae communicate with the frontal sinuses, as has already heen 

 noted in describing the skull. The outer wall of each nasal fossa 

 exhibits the three prominences formed by the superior and inferior 

 mass of cells of the lateral ethmoid, between which is the upper 

 meatus as well as that formed by the maxillo-turbinal, above which 

 is the middle meatus, while the inferior meatus is below the maxillo- 

 turbinal. These parts are all invested with thick mucous membrane, 

 so that their projection inwards and antero-posteriorly is much more 

 marked in the living or freshly-killed animal than it is in the dry 

 skeleton. 



At the roof of the fossa are the openings into the sphenoidal 

 sinuses into which the mucous membrane is prolonged. 



In the inferior meatus is the lower termination of the lachrymal 

 canal. 



The mucous membrane which invests the nasal fossa3 is called 

 the Schneiderian (or pituitary) membrane. It is very vascular, and 

 is inseparably united with the periosteum and perichondrium of the 

 different parts. At the margins of the anterior nostrils it becomes 

 continuous with the external skin, while at the posterior nostrils it 

 becomes continuous with the mucous lining of the pharynx. 



The epithelium which coats the mucous membrane varies in 

 character in different places. That portion which lines the lower part 

 and front of the nose is lined with squamous epithelium. In the 

 respiratory part of the cavity (i.e., the maxillo-turbinal bones and 

 all the parts below them) it is columnar and ciliated. In the olfac- 

 tory part of the cavity (i.e., the two masses of the lateral ethmoid 

 and the upper part of the septum) the epithelium is columnar but 

 not ciliated. The mucous membrane of this special, olfactory part 

 is very thick, soft and pulpy. 



The SPECIAL ORGANS OF SMELL are certain peculiar bodies called 

 OLFACTORY CELLS, spindle-shaped, nucleated, and placed between 

 the columnar epithelial cells. Each such spindle-shaped cell sends 

 out towards the surface a rod-like process, provided with long, 

 slender hairlets which project slightly beyond the surface. At its 

 opposite pole the cell sends out a deep process, which appears to be 

 continuous with the ultimate ramifications of the olfactory nerves. 



Thus the ultimate organs of smell appear to be so many minute 

 rods proceeding from cells. 



The OLFACTORY NERVES come off from the under surface of the 

 flattened end of each olfactory bulb, and piercing the cribriform 

 plate are distributed to the mucous membrane which invests the two 

 lateral masses of the ethmoid and the upper part of the median 

 septum. The nerves ramify in a plexiform manner over these parts, 

 forming a fine net- work. They are entirely composed of pale fibres, 

 and are finely granular. 



The rest of the mucous membrane is provided with nerves which 

 are ramifications of the fifth pair of cranial nerves. 



