THE NASAL CAVITIES. 521 



narrowest. The middle, comprised between the two tui'binated bones, presents, 

 on arriving near the ethmoidal cells, the orifice that brings all the sinuses into 

 commmiication with the nasal fossa. This orifice is ordinarily narrow and 

 curved ; but we have seen it sometimes converted into a foramen sufiiciently 

 wide to permit the introduction of a finger-end. It is also by this meatus that 

 the inferior compartment of the turbmated bones opens into the nasal fossa, these 

 two bones being each rolled in a contrary direction. The inferior meatus, situated 

 under the maxillary turbinated bone, is not distinct from the floor of the nasal 

 cavity (see Fig. 307 for the arrangement of the turbinated bones and the 

 meatuses on the external wall of the nose). 



Roof or arch. — This is formed by the nasal bone, and is only a nan'ow channel, 

 confounded, as has been said, with the superior meatus. 



Floor.— Wider, but not so long as the roof, which is opposite to it, and from 

 which it is distant by the height of the cartilaginous septum, the floor is concave 



Fig. 308. 



LONGITUDINAL MEDIAN SECTION OF THE HEAD AND UPPER PART OF THE NECK. 



1, 1, Atlas; 2, 2, dentata ; 3, trachea; 4, right stylo-thyroideus ; 5, guttural pouch ; 6, stylo.. 

 pharyngeus; 8, palato-pharyngeus ; 9, sphenoidal sinus; 10, cranial cavity; 11, occiput; 12, 

 parietal protuberance; 13, frontal sinus; 14, ethmoidal turbinated bone; 15, premaxillary turbi- 

 nated bone; 16, entrance to nostril; 18, pharyngeal cavity; 19, inferior maxilla; 20, premaxilla; 

 21, hard palate. 



from one wall to the other, and rests on the palatine arch, which separates .the 

 mouth from the nasal cavities. 



In front of this nasal region is remarked the canal or or gem. of Jacohson — a 

 short duct terminating in a cul-de-sac in the middle of the cartilaginous substance 

 which closes the incisive foramen. • At the bottom of this ctd-de-sac opens a 

 second canal, longer, wider, and more remarkable, but which has not yet been 

 described. (It has been described by Steno, and is named " Steno's canal") It 

 has sometimes the diameter of a writing-quill, commences by a cul-de-sac at the 

 level of the second molar tooth, and accompanies the inferior border of the vomer 

 from behind to before, where it is enveloped in a kind of cartilaginous sheath — 

 a dependency of the nasal septum ; it terminates, as we have said, after a course 

 of about 5 inches. 



The structure of Jacobson's organ resembles that of the excretory duct of 

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



