522 RESPIRATORY APPARATUS IN MAMMALIA. 



rich in follicles, and having longitudinal folds ; and an external, of a fibrous nature. 

 These membranes receive numerous vessels, as well as nerves emanating from a 

 long filament of the spheno-palatine ganglion, and which may be traced from 

 the external side of the canal to near the incisive foramen, where it is lost. Such 

 is the organ of Jacobson ; its uses are quite unknown. (It belongs to the 

 olfactory region, for between the ciliated cells of its mucous membrane are found 

 staff-like cells, which are connected with the ends of the olfactory nerve-filaments.) 



Extremities. — The anterior or itiferior extremity of the nasal fossa {regio vesti- 

 bularis) is formed by the nostril already described. The posterior or superior 

 extremity presents, above, a space occupied by the ethmoidal cells. Below and 

 behind, this extremity communicates with the pharyngeal cavity by a wide oval 

 opening, which is circumscribed by the vomer and palate bones : this is the 

 guttural opening of the nasal fossa (or posterior nares). 



Steucture. — The nasal fossse offer for study : 1. The hony framework 

 by which these cavities are formed. 2. The cartilaginous s&ptum separating 

 them. 3. The pituitary membrane — the mucous layer lining their walls. 



1. Bony Framework of the Nasal Fossse. — This comprises : 1. The 

 nasal, maxillary, frontal, and palate bones, which together form a vast irregular 

 wall circumscribing the nasal fossae. 2. The ethmoid bone, occupying the bottom 

 of this tubular cavity, and the turbinated bones applied against the lateral walls. 

 3. The vomer, placed in the median plane, and serving as a support for the 

 cartilaginous partition dividing this single cavity into two compartments. All 

 these bones having been already studied in detail, we confine ourselves to their 

 simple enumeration. 



2. Middle Septum of the Nose {septum nasi) (Fig. 307, 4), — Formed of 

 cartilage susceptible of ossification, this partition is nothing more than the per- 

 pendicular lamina of the ethmoid bone prolonged to the extremity of the nose. 

 Its elongated form permits us to recognize in it two faces, two borders, and two 

 extremities. The faces are channeled by a multitude of furrows, which lodge 

 the anastomosing vessels of the magnificent venous plexus of the pituitary 

 membrane. 



The superior border, united to the frontal bone and median suture of the 

 nasal bones, expands to the right and left on the inner faces of these, in forming 

 two laminae, thin at their free margin, the section of which is represented in 

 Fig. 307. These laminsB are wide enough in front to project beyond the nasal 

 spine. The inferior border is received into the mortice of the vomer. 



The posterior extremity is continued without any precise limitation, by the 

 perpendicular lamina of the ethmoid bone. 



The anterior extremity, a little wider, supports the cartilages of the nostrils. 

 It is joined, below, to the premaxillary bones, and is spread out on the incisive 

 openings in a thick layer which exactly closes them. 



This septum is covered by a thick perichondrium, which adheres intimately 

 to the pituitary membrane. 



3. Pituitary Membrane. — This membrane — also designated the olfactory 

 mucous mombrane, and Schneiderian membrane — is continuous with the skin lining 

 the inner face .of the alse of the nose. Examined on the internal wall of the 

 nasal fossae, the pituitary membrane is seen to cover the cartilaginous septum 

 forming this wall, then spreads over the floor as well as the roof of the cavity, 

 reaching the outer wall, which it also covers in enveloping the external surface 

 of the turbinated bones^ and is insinuated, by the middle meatus, into the cells 



