444 THE RESPIRATOBT AFPASATUS IN MAMMALIA. 



The structure of this duct resembles that of the excretory ducts of glands ; 

 its walls are evidently composed of two tunics— an internal or mucous, 

 very rich in follicles, and having longitudinal folds, and an external, of a 

 fibrous nature. These membranes receive numerous vessels, as well as 

 nervous divisions 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. 



Extremities. — The anterior or inferior extremity of the nasal fossa is 

 formed by the nostril already described. The posterior or superior extremity 

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

 tliis extremity communicates with the pharyngeal cavity by a wide oval 

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

 guttural opening of the nasal fossa. 



Steuctueb. — The nasal fosste offer for study in their organisation : 

 1, The hony framework by which these cavities are formed ; 2, The 

 cartilaginous septum separating them ; 3, The pituitary membrane — the 

 mucous layer covering their walls. 



1. Bony Feambwokk op the Nasal FossiE. — This comprises : 1, The 

 nasal, maxillary, frontal, and palate hones, which together form a vast irre- 

 gular tube circumscribing the nasal fossse ; 2, The ethmoid bone, occupying 

 the bottom of this tubular cavity and the turbinated bonea applied against 

 the latei'al 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 op thk Nose ("Fig. 223, 4). — Formed of cartilage 

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

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

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

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

 lodge the anastomosing divisions 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 laminse, thin at their free margin, the section of which is 

 represented in Fig. 223. These laminse are wide enough in front to project 

 buyond 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. PiTuiTAEY Membeane. — This membrane, also designated the olfactory 

 mucous membrane, and Schneiderian membrane, is continuous with the cutaneous 

 integument covering the inner face of the alse of the nose. Considered at 

 first on the internal wall of the nasal fossas, the pituitary membrane is seen 

 to cover the cartilaginous septum forming this wall, then to be spread 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 



