The =septum nasi= is incomplete in the macerated skull. It is formed by the perpendicular plate of the ethmoid behind, and the vomer below. In the fresh state it is completed by a plate of cartilage. The =superior meatus= (Meatus nasi superior) is a narrow passage between the roof and the superior turbinal bone. It ends at the cribriform plate of the ethmoid. The =middle meatus= (Meatus nasi medius) is the space between the two turbinal bones. In its posterior part is the very narrow opening into the maxillary sinus. The =inferior meatus= (Meatus nasi inferior) is the channel along the floor which is overhung by the inferior turbinal bone. It is much the largest and is the direct path between the anterior and posterior nares. The external aperture is bounded by the nasal bones and the premaxillæ. [Illustration: FIG. 37.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION PASSES THROUGH THE ANTERIOR END OF THE FACIAL CREST, AND BETWEEN THE THIRD AND FOURTH CHEEK TEETH. _a_, Superior, _b_, inferior turbinal bone; _c_, _d_, cavities of _a_ and _b_; _e_, common meatus; _f_, _g_, _h_, superior, middle, inferior meatus; _i_, _k_, passages to cavities of turbinal bones; _l_, naso-lacrimal duct; _m_, infraorbital canal; _n_, anterior end of maxillary sinus; _o_, septal cartilage. (After Ellenberger, in Leisering’s Atlas.) ] [Illustration: FIG. 38.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION IS CUT ABOUT HALFWAY BETWEEN THE ORBIT AND THE ANTERIOR END OF THE FACIAL CREST, AND PASSES BETWEEN THE FIFTH AND SIXTH CHEEK TEETH. _a_, Superior, _b_, inferior turbinal bone; _c_, _d_, cavities of _a_ and _b_; _e_, common meatus; _f_, superior, _g_, middle, _h_, inferior meatus; _i_, placed over ridge in maxillary sinus; _k_, communication between outer and inner (turbinal) part of maxillary sinus; _l_, naso-maxillary opening; _m_, naso-lacrimal canal; _n_, infraorbital canal. (After Ellenberger, in Leisering’s Atlas.) ] The posterior extremity or fundus is separated from the cranial cavity by the cribriform plate of the ethmoid, and is largely occupied by the lateral masses of that bone. THE PARANASAL SINUSES Connected directly or indirectly with the nasal cavity, of which they are diverticula, are four pairs of air-sinuses (Sinus paranasales), viz., maxillary, frontal, sphenopalatine, and ethmoidal.