520 RESPIRATORY APPARATUS IN MAMMALIA. 



separating them varies, in proportion as it is measured at the turbinated bones or 

 at the meatuses. 



Inner wall. — This is fonned by the nasal septum, and is perfectly smooth. 



Outer vmlL — This is chiefly constituted by the supermaxillary bone ; it is 

 very rugged, and is divided into three meatuses, or passages, by the tui'binated 

 bones — the iiTegular columns applied against the inner face of the before-mentioned 

 bone. 



The turbinated hones have already been described (p. 73), and we will only 

 now refer to the principal features in their arrangement. Each is a bony plate 

 rolled upon itself (Fig. 307, 2, 3), and divided, internally, into two sections, the 

 superior of which forms part of the sinus, and the inferior belongs to the nasal fossa ; 

 they are continued, inferiorly, by a fibro-cartilaginous framework, which prolongs 



Fig. 307. 



TRANSVERSE SECTION OF THE HEAD OF AN OLD HORSE, SHOWING THE ARRANGEMENT OF THE 

 NASAL CAVITIES AND MOUTH. 



I, 1, Nasal fossae ; 2, superior turbinated bone , 3, inferior ditto ; 4, median septum of the nose ; 5, 

 central part of the buccal cavity (drawn more spacious than it really is when the two jaws are 

 brought together) ; 6, 6, lateral portions of the same ; 7, section of the tongue, showing it filling 

 the lingual canal. 



their nasal section to the external orifice of the nose. The flexible appendage of 

 the ethmoidal turbinated bone is usually single, sometimes double, and disappears 

 before reaching the alfe of the nose. That of the premaxillary turbinated bone is 

 always bifurcated, and its antero-superior branch is directly continued by the 

 superior extremity of the internal wing of the nostril. 



The meatuses are distinguished into superior, middle, and inferior, or into 

 anterior, middle, and posterior, as the head is inspected in a vertical or horizontal 

 position. The superior passes along the corresponding border of the ethmoidal 

 turbinated bone, and is confounded with the roof of the nasal cavity ; it is pro- 

 longed, behind, to near the cribriform plate of the ethmoid bone, and is the 



